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Snowmass Cosmic Frontiers 6 (CF6) Working Group Summary --The Bright Side of the Cosmic Frontier: Cosmic Probes of Fundamental Physics

Report of the CF6 Working Group at Snowmass 2013. Topics addressed include ultra-high energy cosmic rays, neutrinos, gamma rays, baryogenesis, and experiments probing the fundamental nature of spacetime.


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• Determine the origin of the highest energy particles in the universe and understand the acceleration processes at work throughout the Universe.
• Measure particle cross sections at energies unattainable in Earth-bound accelerators.
• Measure the highest energy neutrinos that arise from interactions of the ultra-high-energy cosmic rays with the microwave background radiation.
• Measure the extragalactic background light between 1 and 100 microns to understand the star formation history of the Universe.
• Measure the mass hierarchy of the neutrinos.
• Search for physics beyond the standard model encoded in cosmic messengers as they cross the Universe.
• Understand the origins of the matter antimatter asymmetry of the Universe.
• Probe the fundamental nature of spacetime.
In many of these areas future progress will depend upon either the detailed understanding of particle acceleration in the universe or the development of methods for controlling systematic errors introduced by our lack of understanding of these processes. High-resolution gamma ray measurements (spectral, angular, and temporal) of many objects and classes of objects are needed to find the source invariant physics that is the signal for physics beyond the standard model. Such measurements in conjunction with measurements at other wavelengths and with measurements of cosmic rays, neutrinos, and gravity waves will enable us to understand Nature's particle accelerators. Recommendations: • Significant U.S. participation in the CTA project [28]. U.S. scientists developed the imaging atmospheric Cherenkov technique. Continued leadership in this area is possible with the development of novel telescope designs. A U.S. proposal to more than double the number of mid-scale telescopes would result in a sensitivity gain of 2-3 significantly improving the prospects for the indirect detection of dark matter, understanding particle acceleration processes, and searching for other signatures of physics beyond the standard model.
• Simultaneous operation of Fermi, HAWC, and VERITAS. Understanding particle acceleration and separating astrophysical processes from physics beyond the standard model requires observations over a broad energy range. The above three instruments will provide simultaneous coverage from 30 MeV to 100 TeV. HAWC and VERITAS will simultaneously view the same sky enabling prompt follow up observations of transient phenomena.
• Construction of the PINGU neutrino detector [29] at the South Pole. U.S. scientists have been leaders in the field of high-energy neutrino observations. PINGU, by densely instrumenting a portion of the IceCube Deep Core array, will lower the energy threshold for neutrinos to a few GeV. This will allow for a measurement of the neutrino mass hierarchy using atmospheric neutrinos.
• Continued operation of the Auger and TA air shower arrays with upgrades to enhance the determination of the composition and interactions of cosmic rays near the energy of the GZK suppression, and flight of the JEM-EUSO mission [30,31] to extend observations of the cosmic ray spectrum and anisotropy well beyond the GZK region.
Extragalactic background light. In addition to advancing our understanding of particle acceleration and astrophysical backgrounds for dark matter searches, the intense gamma-ray beams generated by AGN and gamma-ray bursts can be used to probe the intervening space and search for physics beyond the standard model. Some areas that can be studied include: measuring the extragalactic background light (EBL), using the EBL to measure the intergalactic magnetic fields, and searching for axion-like particles (ALPs) (note, ALPs are not the QCD axion). The EBL pervades the universe and is the sum of all the light generated by stars and the re-radiation of this light in the infrared band by dust [72,73,74] and is therefore sensitive to the star formation history of the universe. The production of electron-positron pairs from photon-photon scattering of the EBL with high-energy gamma rays leads to an energy dependent attenuation length for high-energy gamma rays [75]. At the same time, cosmic rays (if they are accelerated by AGN) will interact with EBL and CMB along the line of sight and generate secondary gamma rays at relatively close distances of the observer [76]. This absorption, with the inclusion of secondary gamma rays, can be used to measure or constrain the EBL [73,74]. Lower limits on the EBL can be established from galaxy counts [72]. A signature of new physics would be an inconsistency between these lower limits and the measurements of the TeV spectra from AGN. Such a discrepancy could be explained either by the secondary production of gamma rays from cosmic rays produced at AGN or by photon-alp mixing mediated by the intergalactic magnetic fields.
Intergalactic magnetic fields. The origin of the Galactic magnetic fields remains a mystery. While astrophysical dynamos can efficiently amplify pre-existing magnetic fields, the generation of a magnetic field is difficult [77]. The strength of the magnetic fields in the voids between galaxy clusters should be similar to the primordial magnetic field. Measurement of AGN spectra and time delays in the GeV-TeV region have been used to set both lower and upper bounds on the strength of the IGMF [78,79]. Current bounds are model dependent and span a large range of values for the magnetic field [80]. Improved measurements of the EBL, the measurement of variability from distant AGN, and improved determinations of the energy spectra (and understanding of the intrinsic AGN spectra) are needed to significantly improve these limits.
Tests of Lorentz invariance violation. Experimentally probing Planck-scale physics, where quantum gravitational effects become large, is challenging. A unique signature of such affects would be the violation of Lorentz invariance (a natural though not necessary property of theories of quantum gravity) [81,82,83]. Short, intense pulses of gamma rays from distant objects such as gamma ray bursts and active galaxies, provide a laboratory to search for small, energy dependent, differences in the speed of light. Current limits have reached the Planck scale if the energy dependence of the violation is linear [84] and 6.4 × 10 10 GeV if the violation is quadratic [85] with energy. Future instruments could improve upon these limits by at least a factor of 10 and 50 (respectively) and significantly increase the sample size used to search for these affects.
TeV Gamma-Ray Instruments. Ground-based TeV instruments fall into two classes: extensive air shower (EAS) arrays, capable of simultaneously viewing the entire overhead sky, and imaging atmospheric Cherenkov telescopes (IACTs), pointed instruments with high sensitivity and resolution. Current IACTs are VERITAS, MAGIC, and HESS, while the HAWC (under construction) and Tibet arrays are the only operating EAS arrays. The next generation of IACT, known as CTA, will consist of a large array of IACTs with roughly an order of magnitude greater sensitivity than current instruments. It is expected to discover over 1000 sources in the TeV band [28,86]. The U.S. portion of the collaboration is proposing to more than double the number of mid-sized telescopes to over 50 using a novel optical design that will improve the sensitivity of CTA by a factor of 2-3 (a result of improved angular resolution of the new telescopes and an increase in the number of telescopes).

Baryogenesis
According to standard cosmology, the current preponderance of matter arises during the very early universe from an asymmetry of about one part per hundred million between the densities of quarks and antiquarks. This asymmetry must have been created after inflation due to some physical process known as baryogenesis. Baryogenesis requires extending the standard model of particle physics, via some new physics which must couple to standard model particles and which must be important during or after the end of inflation. Constraints on the inflation scale and on the reheat temperature at the end of inflation give an upper bound on the relevant energy scale for baryogenesis. The new physics must violate CP (symmetry between matter and antimatter) as the CP violation in the standard model is insufficient [87]. There are a very large number of theoretical baryogenesis proposals. Some well-motivated possibilities are: Leptogenesis Theoretically, baryon number violation at high temperatures is rapid in the standard model via nonperturbative electroweak processes known as sphalerons. Sphalerons conserve the difference between baryon and lepton numbers, leading to the idea of leptogenesis [88]. The decay of very heavy neutrinos in the early universe could occur in a CP violating way, creating a lepton asymmetry that the sphalerons convert into a baryon asymmetry. Whether and how leptogenesis could occur can be clarified by intensity frontier experiments (CP violation and lepton number violation in neutrino physics) [89], cosmic frontier experiments (most leptogenesis proposals require a high inflation scale which can be constrained in cosmic microwave background measurements), and energy frontier experiments [90] (because whether or not other new physics exists has important implications for leptogenesis).

Affleck-Dine baryogenesis
In supersymmetric theories condensates of the scalar partners of quarks and leptons have relatively low energy density and are likely to be present at the end of inflation [91]. The subsequent evolution and decay of the condensates can produce the baryon asymmetry, and in some models, the dark matter [92,93]. The dark matter could be macroscopic lumps of scalar quarks or leptons known as Q-balls, which have unusual detection signatures in cosmic frontier experiments [94]. A critical test of this theory is the search for supersymmetry, primarily via collider experiments. Instruments such as HAWC and IceCube 1 Executive Summary 7 will be sensitive to extremely low fluxes of Q-balls -over two orders of magnitude lower than current limits. While the non-observation of Q-balls can not rule out the Affleck-Dine mechanism, the observation of a Q-ball would have a profound impact on our understanding of the universe.
Electroweak baryogenesis New non-standard model particles that are coupled to the Higgs boson can provide a first order phase transition for electroweak symmetry breaking, which proceeds via nucleation and expansion of bubbles of broken phase. CP violating interactions of particles with the expanding bubble walls can lead to a CP violating particle density in the symmetric phase, which can be converted by sphalerons into the baryon asymmetry [95,96]. The baryons then enter the bubbles of broken phase where the sphaleron rate is negligible. This scenario can be definitively tested via the search for new particles in high-energy colliders, and via nonvanishing electric dipole moments for the neutron and for atoms. Non-standard CP violation in D and B physics may appear in some models. A non standard Higgs self-coupling is a generic consequence. The first order phase transition in the early universe can show up via relic gravitational waves.
Other experiments that can shed light on baryogenesis include searches for baryon number violation. Neutron anti-neutron oscillations violate the difference between baryon and lepton numbers, and most leptogenesis scenarios will not work if such processes are too rapid. Proton decay would provide evidence for Grand Unification, which would imply the existence of heavy particles whose decay could be responsible for baryogenesis, and which is a feature of some leptogenesis and supersymmetric models.

Fundamental nature of spacetime
Quantum effects of space-time are predicted to originate at the Planck scale. In standard quantum field theory, their effects are strongly suppressed at experimentally accessible energies, so space-time is predicted to behave almost classically, for practical purposes, in particle experiments. However, new quantum effects of geometry originating at the Planck scale from geometrical degrees of freedom not included in standard field theory may have effects on macroscopic scales [97,98] that could be measured by laser interferometers such as the holometer [99]. In addition, cosmic particles of high energies can probe departures from Lorenz invariance [100] and the existence of extra-dimensions on scales above the LHC.

Overview of the Report
The Cosmic Frontier number 6, named Cosmic Probes of Fundamental Physics, was charged with summarizing current knowledge and identifying future opportunities (both experimental and theoretical) in the use of astrophysical probes of fundamental physics. Because of the breadth of this area of research CF6 has been subdivided into 3 main topical areas: • CF6-A Cosmic Rays, Gamma Rays and Neutrinos (conveners: Gus Sinnis, Tom Weiler) • CF6-B The Matter of the Cosmological Asymmetry (convener: Ann Nelson) • CF6-C Exploring the Basic Nature of Space and Time (conveners: Aaron Chou, Craig Hogan) We received 14 whitepapers addressing the current challenges and future opportunities in each of these fields as they relate to the other frontiers of high energy physics. Cosmic rays, gamma rays, and neutrinos from astrophysical sources can be used to probe fundamental symmetries, particle interactions at energies not attainable on Earth, and understand the nature of particle acceleration in the cosmos. CF6-A encompasses the following topical areas:  [101]; the first detection of 10 TeV to PeV neutrinos by Icecube [102]; and the precise positron fraction measurement of the Alpha Magnetic Spectrometer [103].

CF6-B The Matter of the Cosmological Asymmetry
The cosmological asymmetry between baryons and anti-baryons is one of the strongest pieces of evidence for physics beyond the standard model. Ultra-high energy cosmic rays (UHECRs), now commonly taken to be CRs with energies > 6 × 10 19 eV, were first reported just over 50 years ago by John Linsley [104]. These are the only particles with energies exceeding those available at terrestrial accelerators. The Large Hadron Collider (LHC) will reach an equivalent fixedtarget energy of 10 17 eV, whereas UHECRs have been observed with energies in excess of 10 20 eV. With UHECRs one can conduct particle physics measurements up to two orders of magnitude higher in the lab frame, or one order of magnitude higher in the center-of-mass frame, than the LHC energy reach. As discussed in more detail below, the properties of UHECR air showers appear to be inconsistent with models which are tuned to accelerator measurements; one possible explanation is that new physics intervenes at energies beyond the LHC reach. UHECR experiments are the only way to access this energy range and make detailed measurements of air showers in order to address this question. It is worth noting that cosmic ray experiments have already yielded particle physics results at energies far exceeding those accessible to the LHC, one of the latest being a measurement of the p-air cross-section at √ s = 57 TeV [40], a result which excludes some hadronic models' extrapolations beyond LHC energies.
The two largest currently operating UHECR observatories are the Pierre Auger Observatory in the Southern hemisphere, covering an area of 3000 km 2 , and the Telescope Array (TA) in the Northern hemisphere, covering about 700 km 2 . Both observatories employ hybrid detection techniques, sampling cosmic ray air shower particles as they arrive at the Earth's surface and also detecting the fluorescence light produced when UHECR air showers excite atmospheric nitrogen, for the ∼ 10% of events arriving on dark, moonless nights. Both Auger and TA feature "low energy" extensions, which will provide an overlap with the LHC energy regime, while also allowing measurements in the galactic-to-extragalactic transition region.
The most important result so far from the present generation of observatories is the conclusive evidence that the UHECR flux drops precipitously at high energy. The discovery of a suppression at the end of the cosmic ray spectrum was first reported by HiRes and Auger [7,8] and later confirmed by TA [9]; by now the significance is well in excess of 20σ compared to a continuous power law extrapolation beyond the ankle feature [10]. This suppression is consistent with the GZK prediction that interactions with cosmic background photons will rapidly degrade UHECR energies [5]. Intriguingly, however, there are also indications that the source of the suppression may be more complex than originally anticipated.
Lower energy observations of the elongation rate (the rate of change with energy of the mean depth-of-showermaximum, X max ) [41,105,106,107], indicate that the composition becomes lighter as energy increases toward ∼ 10 18.3 eV, fueling a widespread supposition that extragalactic cosmic rays are primarily protons. At the highest energies however the situation remains ambiguous. HiRes and TA observe an elongation rate consistent with a light, unchanging composition, supporting the model in which the highest energy cosmic rays are protons and the energy spectrum is shaped by interactions with the CMBR [108,45]. However the Auger Observatory's data exhibits a decreasing elongation rate as well as a decreasing spread in X max with increasing energy. Interpreted with present shower simulations, this implies that the composition is becoming gradually heavier beginning around 5 × 10 18 eV [109,44]. If true, this would have important implications for the astrophysics of the sources. A trend toward heavier composition could reflect the endpoint of cosmic acceleration, with heavier nuclei dominating the composition near the end of the spectrum -which coincidentally falls off near the expected GZK cutoff region [110]. In this scenario, the suppression would constitute an imprint of the accelerator characteristics rather than energy loss in transit. It is also possible that a mixed or heavy composition is emitted from the sources, and photodisintegration of nuclei and other GZK energy losses suppress the flux [108].
An alternative possibility for the origin of the break in the elongation rate could be even more interesting: this feature might arise from some change in the particle interactions at UHE not captured by event generators tuned to LHC and other accelerator data. Adding weight to this possibility are the Auger measurements, using three independent methods, showing that existing hadronic interaction models do not simultaneously fit all shower observables. For example, the actual hadronic muon content of UHE air showers measured in hybrid events is a factor 1.3 − 1.6 larger than predicted by models tuned to LHC data [45], even allowing for a mixed composition. Thus a critical step required to fully understand X max observations is to identify and correct the deficiencies in current shower models. This is a strong motivation for upgrading present-generation detectors to enable full understanding of the hadronic interactions involved in air shower development. Fortunately, the information which will be accessible in shower observations -including the correlation between X max and the ground signal in individual hybrid events [111], the comparison between X max and X µ max (the atmospheric depth where muon production is maximum), the dependence of ground signal on zenith angle, and other detailed shower observations -is so rich and multifaceted that it will enable composition and particle physics to be disentangled [111].
An additional intriguing twist in the present observational situation is that the HiRes and TA results are consistent with a proton dominated flux everywhere above the ankle [112,113], although with present statistics the TA and Auger elongation rates are consistent within errors [114]. Since the sources seen by the HiRes and TA in the Northern hemisphere may not the same sources as seen by the Auger Observatory in the South, the composition need not be the same. When TA statistics are sufficient to make a clear determination as to whether the elongation rate observed in the North is the same as recorded by the Auger observatory in the South, it will be of great consequence for astrophysics even without knowing exactly how to translate from elongation rate to composition. If the composition (elongation rate) in North and South are not the same, it will mean i) that there are at least two source types, one accelerating primarily protons and another accelerating a mixed composition, and ii) that in at least one hemisphere, the UHECRs are produced mainly by one or a small number of sources. This is a strong motivation for increasing the aperture of TA as described in Section 2.5.1.
Another major result of the present generation of observatories is the search for anisotropy in the distribution of arrival directions. Around 10 18 eV, Auger has provided a strong upper limit on the dipole anisotropy [115,116] which is almost sufficient to rule out a Galactic origin assuming these cosmic rays are indeed predominantly protons and making reasonable assumptions about the Galactic magnetic field (GMF). When the TA and Auger data are combined, the limit will be even stronger or a signal will be found [117].
As the energy increases, evidence for anisotropy mounts. Auger has reported a notable correlation of cosmic ray arrival directions with nearby galaxies of the Veron-Cetty and Veron catalog of Active Galactic Nuclei (AGN) [47]. With more data accumulated, the central value of the correlation fraction has decreased but the significance has remained at the 3-sigma level [49,50]. The HiRes experiment did not observe such a correlation [118], but the most recent results from TA [51,52] show a degree of correlation compatible with that seen by Auger in its full data set, and with a similar pre-trial significance. Furthermore, TA finds a significant correlation between the highest energy events' pointing directions and the local large-scale structure of the universe [119].
While indications of anisotropy are becoming stronger, a completely clear picture is thus far elusive, especially regarding the identity of the sources themselves. Perhaps a clear picture should not be expected, given the possibility of multiple types of sources and that fact that the composition in the South could be mixed or become heavy at the highest energies, while at the same time the flux could be more proton-dominated in the North. Adding to the difficulty of comparing correlation results of Northern and Southern hemisphere observatories is the fact that the magnitude and directions of magnetic deflections and the degree of multipleimaging are expected to vary quite strongly across the sky [120]. Fortunately astrophysical observations and theoretical effort are rapidly improving GMF models [121], so that the back-tracking to correct for deflections should become feasible, to some extent, on the time-scale of the next generation of experiments.

Astrophysical Particle Acceleration and Gamma Ray Observations
In the very-high-energy (VHE) band (defined here as gamma rays between approximately 50 GeV and 100 TeV) one sees only non-thermal radiation, mostly from the acceleration of charged particles by the most extreme objects in the universe (some non-thermal radiation may be due to the decay or annihilation of fundamental particles). Within our Galaxy these objects include supernova remnants, rapidly spinning neutron stars, stellar mass black holes, and x-ray binary systems (composed of a black hole or neutron star orbited by another star). Extragalactic objects that accelerate particles include supermassive black holes, gamma-ray bursts, starburst galaxies, and galaxy clusters. Many of these objects have extreme gravitational and magnetic fields, which may play a central role in the acceleration of particles in relativistic jets. By studying these sources we can discover the sources of the Galactic and extragalactic cosmic rays, study physics in extreme environments, and search for signatures of physics beyond the Standard Model.
In the past decade we have discovered that particle acceleration is ubiquitous in the universe. As VHE telescopes have improved in sensitivity we have found more objects and more classes of objects where particle acceleration occurs. From both gamma-ray and cosmic-ray observations we know that somewhere in the universe there are objects capable of accelerating fundamental particles to over 10 20 eV. Understanding these extreme environments and how they accelerate particles is of fundamental interest. In addition, these high-energy particle beams, visible from cosmologically interesting distances, allow us to probe fundamental physics at scales and in ways that are not possible in earth-bound particle accelerators. In most cases the astrophysical signatures of the particle acceleration processes are encoded and multiplexed with signatures from physics beyond the Standard Model. Thus, to extract information related to Beyond the Standard Model Physics it is necessary to understand the particle acceleration mechanisms and any imprints on the gamma-ray energy spectrum that may arise from the local environment.
Galactic Sources of VHE Gamma Rays Cosmic rays were discovered over one hundred years ago [122]. Cosmic rays with energies below about 10 17 eV are likely accelerated within the Galaxy. Because protons and other cosmic-ray nuclei are charged they bend in the Galactic magnetic field and do not point to their origin. Any site of cosmic rays should also produce gamma rays. Recent gamma-ray measurements by Fermi and VERITAS have conclusively shown that supernovae are sites of cosmic-ray acceleration [1,3], solving a century old problem.
The recent observations of an increasing positron:electron ratio above 100 MeV [11,123] may be attributed to the annihilation of dark matter particles with mass ∼100 GeV. However, before attributing this signature to the dark matter it is important to understand any potential astrophysical backgrounds that may contribute to (or even dominate) the positron fraction at high energies. Pulsar wind nebulae (PWN) are a potential source of high-energy positrons that could mimic the signature from dark matter decay. Thus it is important to understand the energy spectrum of PWN and the propagation of cosmic-ray electrons and positrons to understand the origin of the PAMELA signal. At VHE energies PWN dominate the Galactic sources with over 30 discovered to date [17]. PWN accelerate electrons and positrons in equal numbers at the termination shock of the pulsar wind. These electrons and positrons are then trapped by the magnetic field of the nebula and produce gamma rays via inverse Compton interactions with the CMB and low energy photons in the nebula. As the nebula expands, the magnetic field weakens and eventually the electrons and positrons are released to propagate through the Galaxy. Vela X-1 and Geminga are two nearby pulsars that may contribute significantly to the local positron/electron ratio. Recent measurements of TeV gamma rays from Geminga by the Milagro telescope [84] have been used to show that the local positron to electron ratio, based solely on Geminga as a source, is consistent with the observations of PAMELA [13]. Data at higher energies will be needed to distinguish a dark matter source (where one expects the ratio to fall above the dark matter mass) from PWN.
Extragalactic Sources of VHE Gamma Rays The predominate class of extragalactic object detected in the VHE energy band are active galactic nuclei (AGN), accretion powered super-massive black holes with relativistic jets emitted along their rotation axes. VHE gamma rays have also been detect from starburst galaxies [124], presumably from cosmic-ray generation and interactions in those galaxies. Galaxy clusters and gamma-ray bursts are other important potential sources of VHE gamma rays that so far have eluded detection in this energy band. Here we discuss these sources and their implications for particle acceleration and fundamental physics (details on the fundamental physics can be found in other sections of this chapter).
VHE observations of galaxy clusters may provide the best sensitivity to the annihilation or decay of dark matter. However, there are several astrophysical mechanisms by which galaxy clusters can produce VHE gamma rays. Galaxy clusters may contain active galaxies, known sources of VHE gamma rays, structure formation shocks can accelerate injected electrons and protons, and cosmic rays accelerated within the cluster will produce gamma rays through the decay of neutral pions when they interact with the intracluster medium. While the non-observation of VHE emission from a galaxy cluster can be used to set stringent limits on the presence of WIMP-like dark matter, the detection of a dark matter signal will rely on our understanding of astrophysical production of gamma rays within galaxy clusters.
AGN have proved to be prolific sources of VHE gamma rays. The observed high-energy emission is strongly dependent on the viewing angle with respect to the jet, as the bulk of the VHE gamma rays are produced by processes within the jets. AGN provide an excellent beam of photons with which to perform fundamental physics studies. If one can understand the inherent spectrum of these objects the observed spectrum at earth can be used to search for the existence of axion-like particles. Since active galaxies exhibit strong flaring behavior on short timescales they can be used to time the relative arrival time of photons of different energies to search for violations of Lorentz invariance. Since there are many active galaxies detected over a large range of redshifts when can use the redshift dependence of an observed signature to help disentangle the astrophysical phenomena from the fundamental physics. However, before extracting such fundamental physics one must understand the acceleration mechanism, the inherent spectrum of gamma rays, and the observed source-to-source variations.
Currently there is much that is not understood about particle acceleration in AGN. The standard model of acceleration in jets is based on the first-order Fermi mechanism, where particles are accelerated via reflections from fast moving shocks. However, the importance of magnetic reconnection as an acceleration mechanism in AGN is not understood and depends on the relative energy carried by particles or an electromagnetic field. The nature of the particles accelerated by jets is also not known. Most observations are consistent with the acceleration of electrons and the subsequent production of gamma rays via inverse Compton scattering from lower energy photons. However, hadronic models can not be ruled out and are required if the ultra-highenergy cosmic rays are produced in AGN.
Gamma-ray bursts (GRBs) are the most violent objects in the universe, emitting over 10 52 ergs in gamma rays. There are believed to be two classes of GRBs: the first whose progenitors are massive stars undergoing core collapse supernovae and the other merger events of neutrons stars or neutron star black hole systems. In both cases it is believed that accretion onto a black hole powers highly relativistic jets where particles Community Planning Study: Snowmass 2013 2 CF6-A: Cosmic Rays, Gamma Rays, and Neutrinos 13 are accelerated to high energies. In some GRBs a solar mass of material may be accelerated to bulk velocities approaching the speed of light (Γ ∼ 1000). The time duration of GRBs spans the range from a few milliseconds to 1000 seconds and in many cases fine substructure is present in the gamma-ray arrival times. Energy dependent time lags in the GRB onset and the structure within a burst can be used to search for small effects due to violation of Lorentz invariance. As with AGN it is necessary to understand the inherent time evolution of the GRB energy spectrum to extract the fundamental physics. To date the highest energy gamma ray detected from a GRB had an energy at earth of 94 GeV [125], which corresponds to an energy of 125 GeV at the GRB's redshift of 0.34. No convincing detection of a GRB has been made by a ground-based VHE instrument, though the next generation of wide-field instrument, HAWC, should have sufficient sensitivity to detect the more energetic GRBs [126].
Detecting an axion signature in the energy spectrum of AGN requires knowledge of the inherent AGN spectrum and the density of the extragalactic background light (as a function of redshift). To attribute any observed energy dependent time delay of photons from distant sources such as AGN or GRBs requires knowledge of the time evolution of the gamma-ray spectra from these objects. While the current generation of VHE instruments has made enormous progress in detecting new sources and classes of sources, the next generation of instruments must have the precision and sensitivity to disentangle the astrophysical processes from the fundamental physics processes. This will require excellent energy and angler resolution, multiple source detections with a large span of redshifts, and multi-wavelength and multi-messenger campaigns to truly understand the astrophysical process of particle acceleration in these extreme objects.

GZK Neutrinos
Neutrinos result from the cascade of decays following cosmic ray interactions with the microwave background. The processes involved in the production of these neutrinos are well-known low-energy particle physics boosted by the large Lorentz factor of the cosmci ray-CMB frame, and thus predictions of their fluxes are on a firm footing. The flux of these neutrinos depends on the composition of the highest energy cosmic rays and on the evolution of the cosmic ray sources with redshift. Measurement of the GZK neutrino flux would help disentangle the effenct of cosmc ray interactions, composition, and source evolution, and provides a unique beam of high energy neutrinos to probe neutrino cross sections at EeV energies.

Supernova Burst Neutrinos
The measurement of the time evolution of the neutrino energy and flavor spectrum from supernovae can revolutionize our understanding of neutrino properties, supernova physics, and discover or tightly constrain non-standard interactions. Collective oscillations of neutrinos leaving the neutron star surface imprint distinct signatures on the time evolution of the neutrino spectrum that depend, in a dramatic fashion, on the neutrino mass hierarchy and the mixing angle θ 13 [60,61,62]. With θ 13 now measured to be non-zero [127] the measurement of the electron neutrino energy spectrum from a supernova burst can be used to determine the neutrino mass hierarchy. Figure 1 shows the resulting neutrino and antineutrino energy spectra for the normal and inverted hierarchies, before and after the spectral swapping from collective oscillations. Note that the change in the anti-neutrino energy spectrum (between the normal and inverted hierarchies) is much less dramatic than for the neutrino spectrum. A liquid argon detector such as LBNE is most sensitive to the electron neutrinos from a supernova burst measure the neutrino through the reaction ν e + 40 Ar → e − + 40 K * and therefore best suited to measure the mass hierarchy of the neutrinos with supernova neutrinos. However, since the cosmic-ray backgrounds at the surface are quite large, the detector must be located underground to perform this physics. In addition, with the detection of 1000 or more neutrinos from a supernova one could temporally resolve the energy and flavor spectrum of the neutrinos and glean information about the supernova process and the formation of the remnant neutron star. For nearby Galactic supernovae, large detectors such as IceCube complement low-background detectors by providing data with high temporal resolution [128].

Atmospheric Neutrinos
In addition to neutrinos from supernova bursts, nature provides a steady source of neutrinos that can be used to determine the neutrino mass hierarchy. The atmospheric neutrinos generated by extensive air showers travel through the earth. A detector sensitive to these neutrinos can measure neutrinos of with different energies that have traversed a range of baselines, thus probing a large space of L/E. The baselines available are approximately the diameter of the earth (∼12700 km), in principle enabling sensitive searches for matter effects and sensitivity to the neutrino mass hierarchy. The Precision IceCube Next Generation Upgrade (PINGU) experiment [29], would provide an additional 20-40 strings (each with 60-100 photomultiplier tubes) to further "infill" the DeepCore array [18] within IceCube. With a string spacing of 20-25 meters (compared to 73 m spacing within DeepCore and 125 m spacing for IceCube), the energy threshold for neutrinos of would be ∼5 GeV. of PINGU are key to understanding the sensitivity to the neutrino mass hierarchy. The studies presented in [29] use algorithms developed for DeepCore to estimate these resolutions and the curves in Figure 2 are based on those algorithms.

Hadronic Interactions
Ultra high energy cosmic ray measurements can be used to constrain hadronlic interaction cross sections and mutliplicities at energies far beyond those accessible using terrestrial accelerators. While the composition of the primary cosmic ray beam remains an open question, even a small admixture of protons allows extraction of the proton-air cross section from the tail of the distribution of the depth of shower maximum X max toward large atmospheric depths. For example, Auger data has been used to perform a measurement of the p-air cross-section at √ s = 57 TeV [40], a result which excludes some hadronic models' extrapolations beyond LHC energies.
In addition, detailed analysis of the spectrum and anisotropy of cosmic rays together with X max and other observables that are diagnostics of composition and hadron interactions may ultimately allow the composition and interactions of cosmic rays to be disentangled. Recent developments in this area are discussed below in [cross reference to cosmic ray origins section].

Neutrino Cross Sections at Extremely High Energies
Large neutrino telescopes can measure the neutrino-nucleon cross-section by studying neutrino absorption in the Earth [33,34,35]. At high energies, this cross-section is sensitive to new physics. In particular, if there arXiv:1304.4891v1 [astro-ph.HE] 17 Ap ditional rolled-up dimensions, then the cross-section will increase sharply at an energy corresponding to the inverse size of the extra dimension(s). Figure 1 shows the neutrino-nucleon cross-sections calculated for the Standard Model, plus several models with additional dimensions [3]. Other types of new physics can produce similar effects. For example, the presence of leptoquarks could produce a similar increase in the cross-section [4]. Neutrino absorption becomes an effective technique for measuring the cross-section at neutrino energies above about 50 TeV, the energy at which absorption (assuming the Standard Model cross-sections) begins to reduce the flux of vertically upward-going neutrinos, altering the zenith angle distribution. Figure 2 shows the expected zenith angle distribution for neutrinos with energies between 10 13 and 10 21 eV. Lower energy neutrinos can be used as a normalization, to check the angular acceptance Bjorken−x and Q 2 values beyond t data, so extrapolations are required t sections. LHC data can be used to c distributions, but, even at current ex Cube, the neutrino energies are 100 are accessible at accelerators. So, su possible, especially for neutrino-philic Two classes of analyses are foreseen IceCube [6] and proposed Km3Net [7 detectors can study the absorption o trinos. These are a well understood s is that the flux drops rapidly with e quate statistics are only available in ergy range (perhaps up to 100 TeV). I neutrinos is high enough, they may e be extended slightly. These analyses sensitive to the composition of the Ea ever, the core composition is already and a very large data set is needed t geophysical measurements. The curr ties are less important to this analy cross-section uncertainties.
In the longer term, the proposed ANNA [9] radio Cherenkov detector neutrino flux at much higher energi These detectors observe the radio p tromagnetic and hadronic showers pr interactions in Antarctic ice. They ical neutrinos for the measurement. are based on Greissen-Zatsepin-Kum nos, produced when cosmic microwa diation photons excite cosmic-ray pr above 4 × 10 19 eV to a ∆ + resonanc peak around 10 19 eV [10]. This flux is should be present as long as these h rays are protons. For heavier nuclei, cleon is lower, and the neutrino produ At these energies, for even the Sta sections, the Earth is almost complet trinos, so the cross-section measurem servation of neutrinos near the horiz the angular distribution for neutrino gies. At energies above 10 17 eV, the Figure 3. The neutrino interaction cross-section for the standard model, plus several models that incorporate additional dimensions. [36] are additional rolled-up dimensions, then the cross-section will increase sharply at an energy corresponding to the inverse size of the extra dimension(s). Figure 3 shows the neutrino-nucleon cross-sections calculated for the Standard Model, plus several models with additional dimensions [36]. Other types of new physics can produce similar effects. For example, the presence of leptoquarks could produce a similar increase in the cross-section [37].
Neutrino absorption becomes an effective technique for measuring the cross section at neutrino energies above about 50 TeV, the energy at which absorption (assuming the Standard Model cross-sections) begins to reduce the flux of vertically upward-going neutrinos, altering the zenith angle distribution. Figure 4 shows the expected zenith angle distribution for neutrinos with energies between 10 13 and 10 21 eV. Lower energy neutrinos can be used as a normalization, to check the angular acceptance of the detector, and to calibrate for the small zenith angle dependence in the atmospheric neutrino flux.
At neutrino energies much above 10 17 eV, the cross sections depend significantly on parton distributions at Bjorken x and Q 2 values beyond the reach of HERA data, so extrapolations are required to predict the cross sections. LHC data can be used to contrain the parton distributions, but, even at current experiments like IceCube, the neutrino energies are 100 times higher than are accessible at accelerators. So, suprises are certainly possible, especially for new physics with large couplings to the neutrino sector.

Violation of Lorentz Invariance
The development of theories that merge general relativity and quantum mechanics -quantum gravity, typically lead to the violation of invariance principles that have been sacrosanct in physics. The simple fact that quantum mechanics requires a minimum length scale (typically taken to be the Planck length (1.6×10 −35 m or 1.2×10 19 GeV) that is independent of reference frame, is in itself a violation of Lorentz invariance which is scale independent [81]. Probing physics at the Planck scale is difficult or impossible in earth-bound experiments, however nature has provided a mechanism by which we may probe certain aspects few degrees below the horizon, so this measurement requires good angular resolution. ARA simulations predict a mean resolution of about 6 • [8], while ARIANNA sim-ulations give a mean zenith angl a similar azimuthal resolution. to observe of order 10 events tector (37 ARA stations, 900 A GZK neutrino energy spectrum eV, corresponding to a ν-nucleo of 140 TeV. Because these analyses meas ance, they cannot easily different and neutral current cross-sectio neutrino survives, but at a lower must make some assumptions a sections; one can assume that th linearly from the standard-mod CERN's Large Hadron Collid ilar measurements, at, by 2015, mass energies up to 14 TeV, a energy accessible with GZK ne have a much higher luminosity, s sections, will have higher sensitiv ios where the cross-section rises philic new physics, UHE neutrin We thank the organizers of for the opportunity to present would like to acknowledge supp University and CCAPP, and th Foundation under grant PHY-1 edges support from the U.S. Nat under grant 0653266 and the U. under contract number DE-AC-  [36] of Lorentz invariance violation to extraordinary precision. In addition to quantum gravity, other motivations for considering violations of Lorentz invariance include the need for a high-energy cutoff to control divergences in quantum field theory [82], and to develop a consistent theory of black holes [83]. One possible manifestation of the breaking of Lorentz invariance is a vacuum dispersion relation, an energy dependence to the velocity of light. The energy dependence of the speed of light can be probed to high accuracy by measuring the time of high-energy, short pulses of light that have travelled cosmological distances. Gamma-ray bursts and flares from active galaxies provide an excellent laboratory for such an investigation. (For a review of other manifestations of a violation of Lorentz invariance see [129].) When considering a vacuum dispersion relation for electromagnetic radiation one typically performs a simple expansion with linear and quadratic terms: where ζ n s is either zero or one. The linear and quadratic terms are often treated independently (i.e. one of the ζ s n is zero. The linear term violates CPT invariance [130,131],while the quadratic term preserves CPT invariance. The above dispersion relation then leads to an energy dependent delay [100]: The dispersion relation may have a directional dependence [132], which argues for a sample of bursts and flares large enough to probe the dispersion relation in several directions. The best limits on the linear term come from Fermi LAT observations of distant gamma-ray bursts and have probed to energies above the Planck scale [84]. The best limits on the quadratic term (where energy reach is more important than distance) are derived from observations of flaring active galaxies by imaging atmospheric Cherenkov telescopes (E QG > 6 × 10 11 GeV [133]. Current limits on both the linear and quadratic terms are given in Table 0-1. Future experiments such as CTA and HAWC are expected to improve upon these limits and increase the sample size of bursts and flares to probe any directional dependence to the vacuum dispersion relation. For example if HAWC observed a single gamma-ray burst at a redshift of 1 and could time the arrival of the high energy photons (>100 GeV) to one second, the limit on E QG from that single observation would be 4.9 × 10 19 GeV for the linear term and 1.5 × 10 11 GeV for the quadratic term, roughly a factor of 2 higher than the best current limits [136]. The improvement from CTA is more dramatic. CTA will have roughly an order of magnitude greater sensitivity than VERITAS or H.E.S.S. Therefore CTA should improve the current limits on the quadratic term by a factor of 50 and be capable of probing to orders of magnitude above the Planck energy in the linear term [137]. If AGN flares exhibit greater variability than current instruments can resolve, the limits from CTA will be even more sensitive.

Antiparticles and antinuclei
Modern cosmological models strongly favor a universe in which an early baryon asymmetry develops followed by annihilation, leading to a negligible abundance of antiparticles in the present-day universe. Antiparticles and anti-nuclei can nevertheless be produced in energetic astrophysical processes and through decays.
Electrons and Positrons Cosmic ray electrons are produced and accelerated in various astrophysical contexts and are considered primary cosmic rays, making up about 1% of the total cosmic ray flux. No primary sources of cosmic ray positrons are known. However, positrons may also be injected by β + decays, from the magnetospheres of compact objects due to pair production, and as the result of primary cosmic ray interactions with the interstellar medium during the propagation. As a result, the number of cosmic ray positrons compared to the number of electrons is expected to decrease with energy. Observations by PAMELA [138], Fermi [139], and AMS-02 [140] have shown that the positron fraction instead rises with energies above 10 GeV. The high statistics positron fraction measurement by AMS-02 shows a steady increase up to 350 GeV, with a decreasing slope above 250 GeV. AMS-02 will expand the measurements to the TeV energy scale with 2% energy resolution and unprecedented high statistics. These measurement could be a hint for a possible primary positron component as the kinematic characteristics of such a new primary process are most likely different from the production mechanisms of secondary positrons and could materialize in the form of an excess on top of the conventional diffuse positron spectrum. The interpretation will be tightly constrained by how the positron fraction continues at higher energies. Possible explanations span a wide range where dark matter particle self-annihilations or decay [141,142,143,144,145] and so far unknown astrophysical sources like nearby pulsars [146,147] are the most popular ones. Studying the anisotropy level of the positron flux helps to distinguish between diffuse and more concentrated sources. The AMS-02 limit on the dipole anisotropy of the positron fraction is currently 3.6% at the 95% confidence level [140]. However, a ∼10 times lower level of anisotropy is expected from nearby pulsars [146].
Antiprotons The conventional astrophysical source of antiprotons is the production of antiproton-proton pairs in high energy collisions of cosmic rays with the interstellar medium. Low energy antiprotons can only be produced in high energy collisions, since sufficient energy must be available in the CM frame to allow antiprotons produced in the backward (anti-beam) direction to appear to be nearly at rest in the lab frame. These high energy collisions are rare due to the falling cosmic ray spectrum, so the flux of antiprotons below a GeV is kinematically suppressed.
The best measurements of the cosmic ray antiproton flux are from BESS [148,149] and PAMELA [150].
Antiprotons are a powerful tool to constrain dark matter annihilation and decay models as the astrophysical production has only small uncertainties and the antiproton propagation is better under control than for positrons. However, dark matter annihilation (decay) fluxes from different channels (modes) are very similar in shape and, in addition, also very similar in shape to the models for astrophysical production. This makes the interpretation challenging, but upcoming AMS-02 antiproton results have the potential to further constrain dark matter properties [151]. The existing measurements do not require to introduce an additional primary antiproton component, which has to be taken into account for the interpretation of the positron fraction.
Antideuterons Secondary antideuterons, like antiprotons, are produced when cosmic ray protons or antiprotons interact with the interstellar medium, but the production threshold for this reaction is higher for antideuterons than antiprotons. Collision kinematics also disfavor the formation of low energy antideuterons in these interactions. Moreover the steep energy spectrum of cosmic rays means there are fewer particles with sufficient energy to produce secondary antideuterons, and those that are produced will have relatively large kinetic energy. As a consequence, a low energy search for primary antideuterons has very low background. Figure 5 shows the expected antideuteron flux from secondary and tertiary interactions as well as several dark matter models. The different boxes demonstrate the antideuteron flux limits of BESS [155] and the sensitivity reaches of AMS and GAPS [156,157,158], which reach for the first time the sensitivity to probe predictions of well-motivated models. GAPS is a dedicated low energy antideuteron balloon experiment and had a successful prototype flight in 2012 [159,160]. AMS and GAPS have mostly complementary kinetic energy ranges but also some overlap in the most interesting low energy region. Another very important virtue comes from the different detection techniques. AMS identifies particles by analyzing the event signatures of different subsequent subdetectors and a strong magnetic field and GAPS by slowing down antideuterons with kinetic energies below 300 MeV, creating exotic atoms inside the target material, and analyzing the decay structure. The combination of AMS and GAPS allows the study of both a large energy range and independent experimental confirmation, which is crucial for a rare event search like the hunt for cosmic-ray antideuterons.  [152,153] and secondary/tertiary background flux from cosmic ray interactions with the interstellar medium (blue line) [154]. Antideuteron limits from BESS [155] and sensitivities for the running AMS [156,157] and the planned GAPS experiments [158] are also shown.

Primordial Black Holes
Primordial density fluctuations can lead to the formation of black holes, the typical black hole mass will be of order the horizon mass (or smaller) at the time of formation (M ≈ 10 15 (t/10 − 23s)g) [161]. Therefore the mass spectrum of PBHs can span the range from supermassive black holes to Planck mass black holes. Hawking demonstrated that black holes radiate energy and have a temperature that is proportional to the inverse of their mass (T ≈ 1/M 13 GeV, where M 13 is the black hole mass in units of 10 13 g [162]. Primordial black holes with an initial mass of less than 10 15 g would have evaporated completely. Such black holes would have been formed in the first 10 −23 seconds after the Big Bang. The cosmological consequences of primordial black holes are many, their existence (or absence) and mass spectrum probe the density perturbations at early times and very small scales [163,164]. They can seed dark matter clumping, forming ultra compact minihalos and primordial stellar mass size black holes (formed during the QCD phase transition about 10 microseconds after the Big Bang [165]) may make up the dark matter [166,167].
The luminosity of a black hole is inversely proportional to the square of the black hole mass and the proportionality constant is related to the number of available degrees of freedom. The emission mechanism can be thought of as the creation of particle anti-particle pair creation at the event horizon, with one of the particles being absorbed and the other emitted. All fundamental particles will be produced at the event horizon if their mass is less than the black hole temperature. Therefore the time evolution of the emission spectrum of a PBH is sensitive to physics at energy scales not attainable on earth. The observed emission spectrum can be found by convolving the emitted particle spectrum with particle fragmentation functions to derive the gamma-ray and neutrino luminosity functions [168]. Black holes with an initial mass of roughly 5 × 10 14 g would now be in their final stages of evaporation, emitting GeV-TeV particles.

21
The discovery of a primordial black hole would have an enormous impact on science -informing us of the existence of particle states at extremely high energies (well above those available at the LHC) and the spectrum of the primordial density fluctuations at extremely small scales (roughly 30 orders of magnitude smaller than the scales probed by the CMB measurements).
Experimentally there are several ways to search for the existence of PBHs. Indirect techniques rely on detecting the cumulative emission of particles (typically anti-protons or 100 MeV gamma rays) by black holes that have or are in the process of evaporating. Direct detection techniques rely on the detection of a black hole in the final stages of evaporation. Understanding the relationship between the two techniques is complex as it depends upon the mass spectrum of primordial black holes and their clustering (direct techniques are sensitive to relatively local black hole evaporation). While the indirect methods have essentially reached the limits of their sensitivity (the actual measured anti-proton and diffuse 100 MeV gamma-ray flux is used to set limits on the number of primordial black holes that could have evaporated in the past), direct techniques hold the promise of significantly improving upon the current limits.
The diffuse gamma-ray flux measured by EGRET has been used to set an upper limit of Ω pbh < [169] on the contribution to the mass density of the universe from primordial black holes of mass less than 10 15 g. If PBHs cluster in our Galaxy as ordinary matter the offset of the Sun from th Galactic Center would lead to an anisotropy in the fraction of the diffuse gamma-ray background that is due to emission from PBHs. Wright [170] found such an anisotropy in the gamma-ray background measured by EGRET, however at a level much smaller than expected if PBHs clump as luminous matter. From the measured anisotropy one can set an upper limit on the rate density of evaporating PBHs in the Galactic halo of < 0.42pc −3 yr −1 [170].
The evaporation of PBHs would produce equal numbers of protons and antiprotons (and similarly for anti deuterons and deuterons). A PBH signature would manifest itself as an increase in the antiproton to proton fraction at low energies (below ∼1 GeV, where the effects of solar modulation are important). The absence of such an increase and the measured value of the antiproton flux by the BESS-Polar II instrument sets an upper limit to the local (within a ∼few kpc [171]) rate density of evaporating PBHs of 1.2 × 10 −3 pc −3 yr −1 [172]. Anti-deuteron data from AMS and or GAPS could improve upon this limit [173].
Experiments sensitive to the final stages of the PBH evaporation process, IACTS and EAS arrays, probe the very local distribution of PBHs with mass near 5 × 10 14 g. Figure 6 shows the current limits and the expected sensitivity of future instruments (HAWC and CTA). Note that the CTA limit is an estimate based upon the current limit established by the H.E.S.S. collaboration, the increased sensitivity of CTA compared to H.E.S.S. (10x sensitivity allows one to probe a volume 10 3/2 greater), and the larger field-of-view of CTA relative to H.E.S.S. CTA could improve upon current limits by 2 orders of magnitude.

The Extragalactic Background Light and the Search for Axion-like Particles
The extragalactic background light (EBL) is the sum over the history of the universe of the ultraviolet (UV) through far infrared (IR) photon fields and their evolution. The light sources are predominantly stars and starlight reradiated in the infrared (IR) band by dust. The EBL thus contains an imprint of the past history of the universe including galaxy and star formation and any other sources of radiation. In principle it is possible to calculate the EBL spectrum from first principles -initial mass functions of stars, stellar evolution theory, and the effect of dust. Alternatively one can sum the radiation fields from observed galaxy counts. A direct measurement of the EBL could then be compared to these estimates. If the measured EBL is higher than expectations this would be a signature of the injection of a new radiation source, for example the decay of weakly interacting particles or an early population of massive stars. If the measured EBL (as measured by the resultant opacity of the universe at VHE energies, see below) is lower than expected, this The sensitivity of CTA has been estimated see text for details. References for figure: HAWC [174], Whipple [175], VERITAS [176], CYGNUS [177], Tibet [178], and H.E.S.S. [179].
would be a strong indication that the VHE photons are mixing with an axion-like particle via interactions with intergalactic magnetic fields or that distant high-energy gamma rays are a by-product of the production of the ultra-high-energy cosmic rays. This uncertainty is borne out by calculations based on observational uncertainties in determinations of galaxy luminosity densities [72]. Just as the CMB is responsible for the absorption of ultra-high-energy protons [5,6] and photons above 100 TeV, the EBL interacts with gamma rays with energies between ∼100 GeV and ∼100 TeV through the resonant production of electron-positron pairs, resulting in an energy and distance dependent attenuation of VHE gamma rays from distant sources. Direct measurements of the EBL are difficult due to sources of foreground light. If one understands the inherent energy spectra of AGN then the observed spectra at earth in the VHE regime can be used to measure the spectrum of the EBL. To date the best upper bounds on the EBL spectral energy distribution (SED) are derived from observations of AGN spectra. These limits are near the lower bounds established by the contribution to the EBL from galaxies. Figure 7 shows the current limits and measurements of the spectral energy distribution of the EBL [180].
As can be seen from the figure there is a large uncertainty in our knowledge of the SED of the EBL. The sensitivity of CTA to distant AGN should dramatically improve our knowledge of the EBL and therefore our understanding of the history of star and galaxy formation in the universe.
Axion-like Particles Axions were first postulated in [181] to solve the strong CP problem (the fact that CP symmetry is apparently conserved in QCD). Such axions couple to the electromagnetic field through a term in the Lagrangian of the form g aγ E · Ba, where a is the axion field and g aγ is the axion photon coupling, which for traditional axions is inversely proportional to the axion mass. Axion-like particles (ALPs) have a similar coupling to the electromagnetic field, however they do not have the same relationship between coupling strength and mass. Axions and axion-like particles are a leading dark matter candidate and searches for them are discussed in CF-3 of this document. A common feature of axion/ALP searches is exploitation of the axion/ALP photon coupling in the presence of a large magnetic field. In the cosmic arena  [73]. Other legend references can be found in [74]. This figure was taken from [180].
gamma rays emitted by distant sources such as AGN can mix with ALPs via the intergalactic magnetic fields (or stronger local fields near the acceleration region). A fraction of these ALPs then reconvert to gamma rays before reaching the earth. While this would lead to a decrease in the gamma-ray flux reaching the earth in the absence of the EBL there can be an increased gamma ray flux at earth compared to the EBL absorbed expectations. The spectrum measured at earth will depend upon the intrinsic AGN spectrum, the EBL spectral energy distribution, and the intergalactic magnetic fields. These measurements can provide the most sensitive searches for axion-like particles with extremely low masses.
We are now entering an era where there significant number of AGN have been observed over a broad energy range from 100 MeV -few TeV. A detailed analysis of 50 AGN spectra (out to a redshift of 0.5) measured in the TeV region has found evidence for a suppression of EBL absorption at the 4.2 standard deviation level [182]. The measured spectra are consistent with expectations based on photon-ALP mixing, with an IGMF of 1 nG and a photon-ALP coupling strength of g aγ = 10 −11 GeV − 1 (below the upper bound from CAST [183]). Though given the large range of IGMF/g aγ parameter space available it is difficult to draw strong conclusions based upon this analysis.
Another approach is to search for irregularities in AGN spectra, expected if there is a photon-ALP coupling due to magnetic field irregularities in the IGMF. The H.E.S.S. collaboration, finding no such irregularities has set limits on the photon-alp coupling of 2 × 10 −11 GeV −1 in the mass range of near 20 neV [184]. This limit is the most restrictive in this mass range.
At present the situation is in undecided and it should be noted that if AGN produce ultra-high-energy protons, interactions of these protons with the CMB will produce secondary gamma rays that can also lead to an enhanced flux of VHE gamma rays (over that expected with EBL absorption) [185,186,187,188,80,189,190,191,192,193,194,195]. The observation of a single flare at energies above ∼1 TeV from a distant AGN (z > 0.15) could negate the UHECR interpretation. What is needed are more measurements spanning a larger redshift range, with greater sensitivity and energy resolution from 100 GeV to 10 TeV. CTA should detect at least an order of magnitude more AGN and HAWC can monitor every AGN in its field of view for flaring activity -for detailed followup observations by CTA. Figure 8 shows the current limits on g aγ as a function of axion mass along with an estimate of the expected improvement in sensitivity from CTA [196]. The line marked "WD cooling hint" comes from the observation that the cooling rate white dwarf stars is faster than expected, possibly indicating another energy loss mechanism (such as ALPs) [197]. Not shown in the figure is a limit on the coupling for axion masses less than ∼16 meV around 1.3 × 10 −12 to 2.2 × 10 −12 from the duration of the neutrino burst from SN1987a (an additional energy loss mechanism would have shortened the duration of the neutrino burst) [197]. The expected sensitivity of CTA could probe the region of the white dwarf cooling hint, which is below the bound from SN1987a.

Intergalactic Magnetic Fields
Intergalactic magnetic fields (IGMFs) offer a new window on the early-universe cosmology and new physics. The magnetic fields deep in the voids between galaxies, where no significant star formation or gas convection could take place, are the closest approximation to primordial seed magnetic fields left over from the Big Bang. These fields are vestiges of some non-thermal events in the early cosmological history. They could have been produced at the end of inflation from inflaton dynamics, or in the course of a cosmological phase transition, or due to some other processes in the early universe that involve new physics [77]. The strength of IGMFs is still poorly understood [198,199]. Until recently, only the upper limits of 10 −9 G were inferred from the observational data [200]. Theoretical models assuming the dynamo origin of galactic magnetic fields require primordial seed fields of B > 10 −30 G [201], which can be considered a theoretical lower limit.
With the advent of a new generation of gamma-ray instruments, one can measure the values of IGMFs deep in the voids, along the line of sight to distant gamma-ray sources, such as blazars. One can measure IGMFs using time delays [78], or by searching extended halos around the point objects [76,79]. Recent analyses of spectra 25 of distant blazars produced both lower and upper bounds: 0.01 fG < B < 30 fG on Mpc scales [188]. The narrowing of the range of primordial fields to a vicinity of a femtogauss [188,187,202,185,186] has already stimulated a number of studies in primordial magnetogenesis [77,203,204,205,206,207,208,209,210].
Future observations of distant blazars with CTA will allow mapping out of IGMFs, as well as a determination of both the average strengths and the correlation length distributions of these fields. These observations can also measure the presence of helical magnetic fields [211], predicted in scenarios in which cosmic baryogenesis and magneto-genesis occur concurrently during a cosmological phase transition [212,213,214,215,216]. In addition to providing a clear signal for a primordial origin of the IGMF, helicity is also an important factor in the evolution of magnetic fields enabling their growth to astrophysically relevant scales at the present epoch.
This will offer a new exciting possibility to test models of inflation and new physics via a new and unique window on the early universe.

Magnetic Monopoles
The possibility of magnetic monopoles goes back at at least to 1931 [217]. It is theoretically attractive, as it could explain the observed quantization of electrical charge. This argument also gives the 'natural' size of the magnetic charge. Monopoles appear naturally in many grand unified theories (GUTs). In most of these theories, monopoles have masses comparable to the unification scale, and so are far beyond the reach of current or planned accelerators. However, they might have been produced in the early universe [218]; since they do not decay, they may still be present.
Direct and indirect techniques have been used to search for monpoles. The indirect searches rely on how monpoles effect on various astrophysical phenomena. For example, the Parker bound [219] is based on the fact that a sufficient density of magnetic monopoles would short circuit the galaxys magnetic fields. From the existence of these fields, a flux limit is set at roughly one monopole per football field per year; this is roughly the upper limit for direct searches to be useful.
The required technique for direct searches depends on the monpole velocity. Heavy (GUTs scale) monopoles are expected to be slow, with velocities of order 10 −4 -10 −3 c. The most sensitive search for monpoles with this velocity was by the MACRO experiment, which set limits a factor of several below the Parker bound [220]. Newer experiments have set lower limits on non-relativistic monpooles, but only if they catalyze proton decay [221]. The catalysis of proton decay is not unexpected in GUTs theories, but the details are heavily theory-dependent. These experiments set two-dimensional limits, in terms of monopole flux catalysis cross-section. Assumptions are also required about the final state(s) of the decaying proton.
If monopoles have masses below the GUTs scale, than they could be accelerated to relativistic velocities. Then, they would emit Cherenkov radiation, and could be detector by neutrino telescopes. This signature has some similarities to that for nuclearites. Several neutrino detectors have searched for relativistic monopoles, with negative results [222,223,224,225]. These experiments mostly observe monopoles underground, so one must account for the energy loss before the monopole reaches the detector. They set limits ranging from of order 1/10 of the Parker limit (for slightly relativistic monpoles, observed in optical detectors, to limits several orders of magnitude tighter, for ultra-relativistic monpoles observed with radio detectors.
A future dedicated experiment for non-relativistic, non-catalyzing significantly larger than MACRO would be quite expensive, and, absent new findings, is probably not worthwhile. However, optical and radio Cherenkov detectors will continue to set tighter limits. Although the searches are quite speculative, a positive detection would drastically change our view of particle physics.

Q Balls
Understanding the origin of the matter anti-matter asymmetry in the universe is one of the most fundamental problems facing modern physics. Though there are several viable explanations, experimental evidence has been lacking. Leptogenesis is being probed by experiments such as the Long Baseline Neutrino Experiment (LBNE), which is searching for CP violation in the neutrino sector, and neutrinoless double beta decay experiments, sensitive to the Majorana nature of the neutrino. Other sources of baryonic CP violation are being probed with µ → eγ experiments and searches for neutron and electron dipole moments. An alternate scenario for baryogensis, the Affleck-Dine mechanism [226], active at the end of the inflationary epoch, can naturally generate a large baryon asymmetry -if supersymmetry is the correct theory of unification. A prediction of this theory is the formation of Bose condensates of the scaler field. The condensates of squarks would form Q-balls, potentially stable states (Q-balls with masses larger than about 10 15 GeV are stable) with large baryon number. Stable Q-balls are candidates for the dark matter in the universe. Reasonable values of Q-ball charge and the SUSY breaking scale put the detection of Q-balls within the reach of planned high-energy gamma-ray, cosmic-ray, and neutrino telescopes.
S. Coleman [227] first realized that supersymmetric theories have non-topological stable solutions which he called "Q-balls". A review of Q-balls and baryogenesis may be found in [228]. Here we give some of the important parameters for Q-balls: where M S is the SUSY breaking scale in TeV and Q is the baryon number (charge) of the Q-ball. Note that the cross section is purely geometrical and quite large. Since the mass grows slower than the charge Q-balls will be stable, when it is energetically impossible for them to decay into an equivalent number of protons, which occurs at Q > 5.0 × 10 14 (M S /T eV ) 4 [229]. Because of their high mass, the expected flux of Q-balls at earth would be low (if they comprise the dark matter in the universe): where we have assumed that ρ DM = 0.3 GeV/cm 3 and v = 300km sec −1 , the virial velocity. For stable Qballs and interesting values of the SUSY breaking scale Q-ball fluxes of ∼10 −16 or less may be expected. Such a low flux requires the large effective areas that are now common in cosmic-ray, gamma-ray, and neutrino experiments.
The interaction of a Q-ball with ordinary matter proceeds would leave a spectacular signal in the appropriate detector. As a Q-ball interacts with a proton, the proton is absorbed (increasing the baryon number of the Q-ball) and an anti-proton is emitted. Therefore at each interaction with a nucleus the energy released into the detector will be approximately 1 GeV/nucleon, most of which will be in the form of pions.
Currently the best limits on Q-balls have been set by the Super Kamiokande [230]  therefor mass and charge) due to the requirement that a Q-ball not interact in the veto region of the detector. Current neutrino detectors such as IceCube and future gamma-ray and cosmic-ray detectors such as HAWC and JEM-EUSO (note that JEM-EUSO has not published a sensitivity to Q-balls at the time of this report) should be able to significantly improve upon the current limits and extend the search to more theoretically interesting parameter space of large cross sections. Figure 9 shows the current limits with the expected sensitivity of HAWC, IceCube. The diagonal lines are theoretical expectations of for SUSY breaking scales of 1 and 10 TeV. In any of these detectors one should be able to determine the direction of a Q-ball. In this scenario, these instruments serve as directional direct dark matter detectors.

Strangelets
Quark nuggets, nuclearites, and strangelets are different names for lumps of a hypothetical phase of absolutely stable quark matter, named strange quark matter because it is made of about one-third up, down, and strange quarks [234,235]. Whether strange quark matter (SQM) is absolutely stable is a question yet to be decided by experiment or astrophysical observation (see [236,237,238,239] for reviews). If stable, strange quark matter objects may exist with baryon numbers ranging from ordinary nuclei to about 2 × 10 57 corresponding to gravitational instability of strange stars [240,241].
Small SQM lumps with baryon number A < 10 7 are called strangelets [242]. Strangelets have an electron cloud neutralizing the slightly positive quark charge. They are unlikely to survive the early Universe, but may form at collisions of strange star binaries or may be accelerated off the surface of pulsars. Relics from the cosmological quark-hadron phase transition are usually called quark nuggets, while nuclearites are nuggets that hit the Earth and may leave detectable signatures as unusual meteor-events, earth-quakes, etched tracks in old mica, in meteorites and in cosmic-ray detectors [243].
Current searches for SQM states have excluded quark nuggets as dark matter candidates with 3 × 10 7 < A < 5 × 10 25 , but a lower flux of relics or strange star collision debris has not been ruled out. Several experiments have searched for strangelets in cosmic rays with some interesting events not claimed as discoveries but interpreted as flux limits reviewed here [244,245]. More recently the Lunar Soil Strangelet Search (LSSS) has reported new strangelet limits [246] in a sample of 15 grams of lunar soil from Apollo 11. The Alpha Magnetic Spectrometer (AMS) is currently searching for stragelets from the International Space Station [247]. AMS is uniquely suited to discover extreme rigidity strangelets and should be able to probe a wide mass range. AMS recently released their first results [12] and have presented accurate measurements of cosmic ray electrons, positrons, protons, and nuclei at the International Cosmic Ray Conference 2013. No results from the strangelet searches has been announced to date.

UHECR Experiments
Auger The Pierre Auger collaboration is currently planning an upgrade of the existing 3000km 2 hybrid observatory located near Malargüe, Argentina. The current observatory is comprised of an array of 1600 12 ton water Cherenkov surface detectors, most of which are spaced on a 1.5 km hexagonal grid, together with air fluorescence detectors distributed at four sites overlooking the surface detector array. The objectives of the planned upgrade are to elucidate the origin of the flux suppression at the highest energies, measure the composition of UHECRs up to highest energies with sufficient resolution to detect a 10% proton component, provide composition-tagging to facilitate anisotropy studies, and study hadronic interactions at center-ofmass energies an order-of-magnitude greater than at the LHC.
To address these objectives, the Auger upgrade focuses on enhancing the ability of the detector to better separately determine the lateral distribution function of the muon and electromagnetic (EM) components of showers, and to extend these measurements to smaller core distances (and large shower particle fluxes) than is possible with the existing system. The upgrades also would improve measurement not only of the depth of shower maximum for the EM component of the shower (as is done at present using the fluorescence detectors in hybrid events) but also the depth at which muon production reaches its maximum, using the surface detector alone.
An upgrade of the electronics of the surface detectors is the first step in separating the muon and EM components of the showers on an event by event basis [3]. It includes faster timing of surface detector signals, improving significantly the ability to distinguish close-in-time muon pulses across the entire array.
The updated electronics will also facilitate the addition of dedicated muon detectors over all or part of the surface detector array to further improve the muon/EM separation and reduce model-dependent systematic errors. This gives rise to a 'boot-strap' approach, where the model-independent, direct muon determination abilities of the upgraded detector are used to validate and refine the more indirect detection methods and analyses.
Distinct strategic options appear in how to use muon identification and/or separation of the muonic and EM components of the signal, to test and improve the hadronic physics modeling and to assign a compositionprobability to each event. The most direct method is to simply separate the muonic and EM signals in each tank, and use timing and geometry to infer the location of peak muon production point in the development of the air shower [4]. A second approach is to fit the total signal in every tank, including some elements of the FADC timing, to a superposition of templates of EM and muonic components. This gives a more accurate energy and angular reconstruction than the traditional method, and at the same time gives X max with remarkable accuracy , about 30 g cm −2 .
The additional muon identification technologies under study and prototyping include (1) segmentation of the interiors of surface detectors, to separate penetrating muons from the lower-energy electromagnetic component, and (2) placement of external particle detectors (such as RPCs or scintillators) with the existing Auger detectors.
In addition, it is planned to extend fluorescence light measurements into twilight by running with reduced gain, thereby increasing the duty cycle for the highest energy showers by up to a factor of two. This does not involve a hardware upgrade but rather is an operational change in the existing detector.
It is planned to operate the upgraded Pierre Auger Observatory from 2015 to 2023, which would approximately double the data set which will have been collected prior to implementation of the upgrade.
Telescope Array The Telescope Array (TA) collaboration has several upgrades in progress and planned for the near future. The TA Low Energy (TALE) extension will extend the observatory's reach in energy spectrum and composition studies into the 10 16 eV decade, enabling TA to probe the "second knee" region and the galactic-to-extragalactic transition. An additional decade downward in energy will be achieved via the Non-Imaging CHErenkov (NICHE) array, bridging the gap between Telescope Array and experiments operating in the knee regime. At the highest energies, the TA×4 ("TA times four") detector will greatly enhance Northern Hemisphere statistics, of particular importance in arrival direction anisotropy and composition studies.
TALE While TA has been able to extend analysis down to about 10 18 eV, this is insufficient to fully observe the galactic-to-extragalactic transition. In addition, it is optimal to observe cosmic rays from LHC energies through the second knee and up to the GZK cutoff with one well cross-calibrated detector. TALE, the low energy extension to the Telescope Array, is designed to lower the energy threshold to about 10 16.5 eV. To do this, an additional ten fluorescence telescopes were installed, viewing up to 57 degrees in elevation angle. Installation of a new graded array of about 100 scintillator detectors is currently in progress. This extension will enable the Telescope Array to measure the energy and composition of cosmic rays to much lower energies while cross calibrated with the detectors of the main Telescope Array. By pushing the energy threshold down to 10 16.5 eV, the TA collaboration hopes to sort out the galactic and extragalactic contributions to the cosmic ray flux.
NICHE Co-sited with TA/TALE, the Non-Imaging CHErenkov Array (NICHE) will measure the flux and nuclear composition of cosmic rays from below 10 16 eV to 10 18 eV in its initial deployment. Furthermore, the low-energy threshold can be significantly decreased below the cosmic ray knee via counter redeployment or by including additional counters. NICHE uses easily deployable detectors -consisting of a single phototube and Winston cone -to measure the amplitude and time-spread of the air-shower Cherenkov signal to achieve an event-by-event measurement of X max and energy, each with excellent resolution. NICHE will have sufficient area and angular acceptance to have significant overlap with the TA/TALE detectors to allow for energy cross-calibration. Simulated NICHE performance has shown that the array has the ability to distinguish between several different composition models as well as measure the end of Galactic cosmic ray spectrum.
TA×4 The Telescope Array (TA) collaboration is proposing to build TA×4. This is a project to expand the TA surface detector by a factor of four, bringing the total instrumented area to 3000 km 2 . The plan is to build 500 more scintillation counters and deploy them in an array of 2.08 km spacing. This array, plus the existing TA SD would reach the design size. There is plenty of room at the TA site to expand on the northern, western, and southern sides of the TA SD. The new array would need a fluorescence detector overlooking it to set the energy scale, so the TA×4 plan includes a fluorescence detector of 10 telescopes. These will be reconditioned HiRes telescopes.
The aim of the design is to collect data for anisotropy studies at the highest energies. An anisotropy signal due to the local large scale structure of the universe (the local 250 Mpc) really should be present at energies larger than about 57 EeV, where the extragalactic and galactic magnetic fields have an effect on cosmic rays trajectories smaller than the size of the local large scale structures. Telescope Array has already seen hints of such structure. If successful, the TA×4 project will take 3 years to acquire the funds, build and deploy the detectors. Including the current 5 TA-years of data, 3 years of operation of TA×4 would yield 20 TA-years of data. These data will be sufficient to determine unambiguously whether an LSS anisotropy exists. If the LSS signal really comes from the TA hot spot, 3 years of TA×4 data will clarify this signal and yield a 5σ observation.
JEM-EUSO JEM-EUSO will be the first space observatory for the study of extreme energy cosmic rays with energies of ∼ 10 20 eV. The Extreme Universe Space Observatory (EUSO) to be accommodated on the Japanese Experiment Module (JEM) of the International Space Station (ISS) will look down towards the Earth and use the atmosphere as a giant detector [30,31]. The 2.5 m ultraviolet (UV) telescope with a 60 o field of view (FOV) will observe the fluorescence signal produced by the extensive air-showers (EAS) generated by extremely energetic cosmic rays (EECRs) that enter the Earths atmosphere.
The main objective of JEM-EUSO is to identify the sources of the highest energy cosmic rays and thus begin particle astronomy. JEM-EUSO will significantly increase the worldwide data collection of particles at extreme energies providing clear anisotropy signals for the identification of the first sources of extragalactic cosmic rays and the measurement of the energy spectrum beyond the Greisen-Zatsepin-Kuzmin (GZK) feature. Identifying the sources will solve a longstanding mystery and further the study of particle interactions with center of mass energies beyond 100 TeV.
JEM-EUSO is also sensitive to very low fluxes of extremely high-energy neutrinos that may be produced if cosmic accelerators reach higher energies than those observed thus far. The observation of extremely energetic neutrinos would make possible studies of neutrino interactions with center of mass above 100 TeV.
JEM-EUSO will also contribute to the investigation of phenomena intrinsic to the Earths atmosphere or induced by the flux of meteoroids and strangelets (or nucleorites) incoming from space.
A worldwide collaborating of 75 research groups from 13 countries is designing JEM-EUSO to operate for more than 3 years onboard the ISS which orbits around the Earth every 91 minutes at an altitude of about 400 km. JEM-EUSO will image light from the isotropic nitrogen fluorescence excited by the EAS, and the forward-beamed Cherenkov radiation reflected from the Earths surface or cloud tops. The highly-pixelized high-speed JEM-EUSO camera will capture the time development of an EAS to determine the energy and arrival direction of the EECR. The cameras focal plane is covered by MAPMTs with 3 × 10 5 pixels, each less than 3 mm, giving a 0.07 o resolution per pixel; a pixel covers about 0.5 km on the surface of the Earth. The characteristics of the EAS can be used to determine the original direction, energy, and nature of the EECR. JEM-EUSO will be able to discriminate between hadron, gamma ray, and neutrino initiated showers. Radio Detection of Cosmic Rays Detection of cosmic ray air showers using radio techniques has undergone a resurgnece of interest in recent years. Detectors have been operated in coincidence with air shower arrays at Auger [248,249], Telescope Array [250], and Kascade-Grande [251,252]. Cosmic ray radio emission has also been detected by the ANITA balloon-borne interferometer [57]. The dominant mechanisms include geosynchrotron emission and Cherenkov emission, and result in polarized emission beamed near the shower axis. The intensity and lateral distribution of the radio signal can be used to infer shower energy and depth of maximum [252]. In addition detectors being planned in conjunction with air shower arrays, there are prospects for radio detection using large interferometric array such as LOFAR [253,254] and SKA, as well as from space [255].

Radar Detection of Cosmic Ray Showers
Radar is a another candidate remote sensing technique with a potential to achieve a 100% duty cycle. The Telescope Array RAdar (TARA) project in Utah is designed to test the idea that ionization produced by extensive air showers should scatter RF radiation.
TARA employs two analog television transmitters, with a combined output power of 40 kW, broadcasting a 54.1 MHz (low-VHF) signal over the Telescope Array surface detector. The signal is enhanced by use of a high gain (over 20 dBi) phased array of Yagi antennas which boosts the equivalent isotropic radiated power to over 8 MW.
Due to the high velocity of the air shower ionization front, the RF scattered off of an extensive air shower will be characterized by a Doppler shift of several tens of Megahertz. To detect such signals, TARA employs a 250 MS/s receiver along with an onboard FPGA to allow smart triggering at a level well below that of galactic sky noise.
TARA commissioning is currently underway, with preliminary results anticipated in Fall of 2013.

Future Gamma-Ray Experiments
At energies above 100 GeV the flux of gamma rays from astrophysical objects is sufficiently small that detection using a space-based instrument becomes prohibitively expensive. Ground-based VHE gamma-ray instruments are of two types: Imaging Atmospheric Cherenkov Telescopes (IACTs), that use large mirrors to image the Cherenkov light generated in the atmosphere by extensive air showers and Extensive Air Shower (EAS) arrays, that directly detect particles that reach the ground -predominantly gamma rays, electrons and positrons. The characteristics of the two types of instruments complement each other and together they provide excellent coverage of a broad array of astrophysical objects. IACTs have significantly better instantaneous sensitivity, better energy resolution, and better angular resolution. However, they can only view a small portion of the sky at any one time and they can only operate on clear moonless nights (though progress has been made in extending operations into the lunar cycle). (Current instruments have a fieldof-view of ∼5milli-sr and the planned CTA will have a field-of-view of ∼15 milli-sr.) In contrast, EAS arrays can operate continuously and are sensitive to air showers from the entire overhead sky. Because the energy threshold of EAS arrays depends upon the atmospheric depth and therefore the zenith angle of the primary gamma ray, field-of-view is typically considered to be 2 sr. In IACTs the rejection of the cosmic-ray background is accomplished through a combination of image analysis and angular resolution. In EAS arrays background rejection is accomplished through a combination of muon detection and angular resolution.
Given the above characteristics IACTs are to: • perform detailed morphological studies (spectrally resolved) of Galactic sources for multi wavelength studies -required to understand acceleration mechanisms • extend ground-based measurements to relatively low energies (below 50 GeV) -required to probe the pulsar mechanism, provide overlap with space-based instruments, and for sensitivity to lower has WIMPs • observe the fastest transients from known sources -required to understand the acceleration mechanism and environment in active galaxies and provide the best limits on violation of Lorentz invariance • detect faint sources and thereby provide the best indirect limits on dark matter annihilation • resolve energy spectra of astrophysical sources -required for dark matter detection, understanding acceleration processes, and searching for evidence of axion-like particles Similarly EAS arrays: • monitor the sky and alert the more sensitive IACTs and other instruments operating at different wavelengths (and particle type) for followup observations of transient phenomena -critical to understanding complex astrophysical environments).
• detect prompt VHE emission from gamma-ray bursts • perform unbiased sky surveys to discover new sources and phenomena (including undetected high M/L dark matter sources) • study the highest energy gamma rays, where sensitivity is typically flux limited • study large sources such as galaxy clusters (for dark matter annihilation) and Galactic and extragalactic diffuse emission Given the complexity and diversity of sources of VHE gamma rays it is important to have both types of instruments available as well as instrument operating at lower wavelengths (100 MeV gamma rays, x-rays, radio, and optical) to glean the full benefit from any of the instruments. Single observations at a single wavelength are rarely (if ever) sufficient to properly understand astrophysical sources -a requirement if one is to extract fundamental physics from VHE gamma-ray observations. Therefore it is important to operate Fermi, VERITAS, and HAWC simultaneously for a period of several years. Results from such a period will inform us of the value of these observations and allow one to make physically motivated decisions about further operations.
VERITAS VERITAS (the Very Energetic Radiation Imaging Telescope Array System) is a ground-based gamma-ray instrument operating at the Fred Lawrence Whipple Observatory in southern Arizona, USA. It comprises an array of four 12m optical reflectors, which exploit the imaging atmospheric Cherenkov technique to measure emission from astrophysical sources in the ∼ 100 GeV to ∼ 30 TeV energy range. The array has been operating smoothly since 2007, recording around 1000 hours of data per year. The angular and energy resolution are energy dependent: at 1 TeV they are ∼ 0.1 • (68% containment radius) and ∼ 15%, respectively. The sensitivity of the array is sufficient to detect a source with 1% of the steady Crab Nebula flux in less than 25 hours, while the Crab itself is detected in a matter of seconds. VERITAS has recently completed a series of upgrades, which included relocating the original prototype telescope, installing a new trigger system, and upgrading the telescope cameras with more sensitive photomultiplier tubes. These upgrades combined have halved the time required to detect a typical gamma-ray source.
VERITAS has now detected 46 sources -around one-third of the known TeV catalog -and over half of these detections were new discoveries. The catalog comprises active galaxies (blazars and radio galaxies), Community Planning Study: Snowmass 2013 2 CF6-A: Cosmic Rays, Gamma Rays, and Neutrinos 33 a starburst galaxy, a pulsar, many pulsar wind nebulae, binary systems, supernova remnants, and sources whose nature remains unknown. Core science goals include the study of cosmic ray particle acceleration, both within our Galaxy (e.g. in supernova remnants) and externally (e.g. in AGN jets), searching for the gamma-ray signature of dark matter annihilation and primordial black hole evaporation, and constraining fundamental physical effects such as Lorentz invariance violation. The attenuation and cascading of TeV gamma-ray photons from distant sources can also be used to measure or constrain the extragalactic infra-red background light, and the extragalactic magnetic field strength. All of these studies are greatly enhanced by contemporaneous overlap with complementary gamma-ray instruments including the Fermi gamma-ray space telescope and HAWC.
Cherenkov Telescope Array The Cherenkov Telescope Array (CTA) is a concept for a ground-based observatory [28] for very high-energy (VHE, 30 GeV -300 TeV) γ rays. It will use imaging atmospheric Cherenkov telescopes (IACTs) deployed over ≥10 km 2 to detect flashes of Cherenkov light from air showers initiated by γ rays, a technique pioneered at Whipple Observatory in the US. Current IACTs have cameras consisting of ∼ thousand fast photomultiplier tubes, and their effective collecting areas typically reach ∼ 0.1 km 2 . The current generation, namely H.E.S.S., MAGIC, and VERITAS, have up to 5 IACTs with separations of ∼100 m to record multiple views of each shower. CTA will be an array of ∼ one hundred IACTs to increase the collection area and the number of recorded images of each γ-ray shower.
The current concept for CTA consists of subarrays with telescopes of 3 different sizes. The CTA-US consortium is working towards contributing mid-sized telescopes (MSTs, 9 -12 m) to CTA (e.g., 36 MSTs were recommended by Astro2010 [256] and PASAG [257]) to optimize the performance of CTA in the 80 GeV -5 TeV regime. The U.S. groups are currently designing and building a prototype telescope to test the feasibility of an innovative, Schwarzschild-Couder telescope (SCT) design [258,259,260] that will allow much smaller, less expensive cameras, yet providing ∼ 10 4 pixels, better angular resolution and a much larger field of view than conventional Davies-Cotton designs. An important aspect of the prototype program is the design and testing of a new optical system, mirror technologies and camera electronics for the necessary advances in performance, reliability and lowered cost.
This work is a collaboration between national laboratories, universities and industry and consists of ∼ 20 research groups in the US. The US groups have received 5M$ through an NSF-MRI program to construct a prototype telescope between 2012-2015 with the goal to get a realistic estimate of construction costs and performance.
Overall, CTA will (a) provide an order of magnitude better sensitivity for deep observations (∼ 10 −3 Crab nebula flux); (b) have a much greater detection area (∼ 1 km 2 ), and hence detection rates, for transient phenomena; (c) improve the angular resolution (0.02 • at 1 TeV) to resolve cosmic accelerators; (d) provide uniform energy coverage from ∼ 30 GeV to beyond 100 TeV photon energy; and (e) enhance the sky survey capability, monitoring capability, and flexibility of operation relative to current IACTs. These improvements will provide a dramatic step in exploring non-thermal processes in our Universe. Figure 10 shows simulation results of the flux sensitivity for several possible telescope configurations: the baseline array [261] (solid green line) using up to 25 mid-sized telescopes (MSTs) and the large-sized and small-sized telescopes to cover the low energies (E ≤ 0.1 TeV) and high energies (E ≥ 10 TeV). Furthermore we show the effect when adding the US contribution of mid-sized telescopes (recommended by Astro2010 and PASAG), amounting to either 61 DC-MSTs or 61 SC-MSTs.
The addition of the US telescopes would bring the sensitivity to the ∼ 10 −3 Crab level, and improve the sensitivity up to a factor of 3 in the 80 GeV -5 TeV regime. This large improvement is due to increasing the MST array to its optimal size, where a much larger fraction of events fall within the array (contained events) for excellent event reconstruction, and the better resolution SC telescope compared to the DC design. Figure 11 shows the results of a simulation of the angular resolution for several telescope configurations for CTA as a function of energy. The black dashed line shows the CTA requirement for angular resolution [262], the green line corresponds to the baseline array. The blue line and the red line show the angular resolution with and without the US contribution of more than doubling the number of mid-sized telescopes, the 61 SC-MSTs or 61 DC-MSTs, respectively. For comparison we also show the angular resolution of Fermi (orange line). It should be emphasized that the use of the SC design provides a substantial improvement in angular resolution, which not only translates into a better sensitivity for point sources, but also potentially provides new physics capabilities through high-resolution imaging in the TeV regime.
The High Altitude Water Cherenkov Experiment The High Altitude Water Cherenkov Experiment (HAWC) is a wide field-of-view (>2 sr), high duty-cycle (> 95%) TeV gamma-ray experiment currently under construction at the vulcan Sierra Negra in Mexico. HAWC will be composed of 300 large water Cherenkov detectors (WCDs) (7m diameter by 4.3 meters high), instrumented with 4 photomultiplier tubes (PMTs). Three of the PMTs are 8" Hamamatsu R5912 (reused from the Milagro experiment) and the 4 th central PMT is a high quantum efficiency 10" R7081-MOD Hamamatsu PMT. The WCDs will be placed in a close-packed array covering a total area of approximately 20,000 m 2 . The array will begin operations with 100 WCDs in August of 2013 and the full array should begin operations approximately one year later. HAWC builds upon the success of the Milagro experiment, which demonstrated the advantages of using water Cherenkov technology for wide-field ground-based gamma-ray instruments (dense sampling of the air shower particles on the ground and sensitivity to the gamma rays in an air shower) that lead to a dramatically lower energy threshold over previous generation of instruments based upon a sparse array of plastic scintillators. In five years of operation HAWC will survey 8 sr of the sky with a sensitivity roughly 15 times greater than that of Milagro and well matched to the sensitivity of current IACTs.
The sensitivity of HAWC to point sources of TeV gamma rays will be roughly 15 times that of Milagro, enabling HAWC to detect the Crab Nebula in a single transit (compared to 3-4 months for Milagro). As an all-sky instrument with a low energy threshold below 100 GeV, HAWC is well suited to detect transient phenomena in the VHE sky. Flares from active galaxies and gamma-ray bursts are prime scientific targets for HAWC [126]. Extrapolations from current detections of GRBs by the Fermi LAT indicate that HAWC should detect about 1.6 GRBs each year [263]. The detection of a GRB at >100 GeV would enable HAWC to probe for violations of Lorentz Invariance with a sensitivity about a factor of two beyond current limits. Flares (or lack of flares) from distant AGN will directly test the UHECR origin of the highest energy gamma rays from these objects, which is critical to understanding the sources of UHECRs and the role of axion-like particles in the propagation of VHE gamma rays. The final state emission from a primordial black hole will have a signature that is similar too -but distinct from -a gamma-ray burst and HAWC will be sensitive to such emission over a volume roughly 2 orders of magnitude larger than has been probed to date. HAWC's sensitivity to the annihilation of dark matter particles peaks in the dark matter mass range above 10 TeV and the large field-of-view gives HAWC unique sensitivity to baryon-poor dwarf galaxies for high-mass dark matter.
LHAASO The LHAASO (Large High Altitude Air Shower Observatory) is an ambitious project based upon a combination of water Cherenkov technology, scintillation detectors, and air Cherenkov technology. LHAASO will consist of a ∼90,000m 2 water Cherenkov detector surrounded by 5100 scintillation detectors distributed over an area of ∼1km s with 43,000 m 2 of buried muon detectors. In addition 24 air fluorescence/Cherenkov telescopes will be located onsite. At an altitude of ∼4300m, it is expected that LHAASO will have somewhat better sensitivity than HAWC at low energies (<10 TeV), with significantly improved sensitivity at higher energies. This project recently received approval from the Chinese government and the completion of construction is expected in 2018.
A Future Wide-Field High-Duty Cycle Gamma-Ray Experiment HAWC was designed and built based on the results from the Milagro experiment. Similarly, the design of a future wide-field high duty-cycle experiment will be based upon the results from HAWC (or LHAASO). There are two distinct paths for a future instrument: significantly higher sensitivity to higher energy gamma rays, in excess of 100 TeV or significantly reducing the useful energy threshold. If the HAWC data shows that exciting physics is to be found at the highest energies (cosmic-ray origins, Galactic gamma-ray sources), then a plan to increase the collecting area at the highest energies would be recommended. Such an upgrade could be performed at the existing HAWC site or at a new location, perhaps in the Southern hemisphere to provide an alert system for the CTA South. On the other hand, if extragalactic phenomena, especially transient events such as flares from active galaxies to gamma-ray bursts, yield a rich source of information on particle acceleration, ultra-high-energy cosmic rays, and tests of fundamental physics, a detector with a significantly lower energy threshold would be recommended. Such an instrument would require the highest altitude site attainable, and thus would naturally be placed in the Southern hemisphere. Within the Chajnantor plateau in Chili it seems feasible to site such an instrument at ∼6km above sea level.

Neutrino Experiments
IceCube and KM3NeT IceCube is a 1 km 3 neutrino observatory located at the South Pole [19]. Completed in December, 2010, it instruments 1 km 3 of Antarctica ice with 5,160 optical sensors, mounted at depths between 1450 and 2450 m, on 86 vertical strings which are emplaced in holes drilled in the icecap. These sensors observe the Cherenkov radiation from the charged particles which are produced when highenergy neutrinos interact in the Antarctic ice. 78 of the strings are arranged on a 125 m triangular grid; this array has an energy threshold of about 100 GeV. The remaining 8 strings form a denser subarray called "DeepCore" [18]; these strings have smaller spacings, with most of DOMs on the bottom 350 m of the strings. DeepCore has an energy threshold of about 10 GeV.
A surface array, called IceTop, completes the installation [264]. It comprises 162 ice filled tanks. The array is sensitive to cosmic-ray air showers with energy above about 100 TeV. One key feature of IceTop is its altitude; at 2735 m above sea level, so IceTop is relatively near shower maximum for the showers of greatest interest (above 1 PeV); this reduces its sensitivity to many systematic errors, such as hadronic interaction models.
IceCube observes about 200 neutrino interactions per day, mostly produced in cosmic-ray air showers. It has measured the ν µ spectrum from energies of 100 GeV up to 1 PeV, the atmospheric ν e spectrum from energies of 80 GeV up to 6 TeV, and has set limits on ν τ [265]. Using DeepCore, it has observed atmospheric neutrino oscillations, studying neutrinos with energies in the 10-60 GeV region [266].
IceCube has searched for astrophysical neutrinos in many channels, including searches for point sources [267], episodic sources, GRBs [268] and diffuse searches. The point source searches have not observed any excesses over atmospheric backgrounds, but the diffuse searches have seen a clear excess of expectations, Community Planning Study: Snowmass 2013 2 CF6-A: Cosmic Rays, Gamma Rays, and Neutrinos 37 most notably "Bert" and "Ernie," two neutrino events with energies above 1 PeV [4]. A systematic study found 28 contained events, with energies above 60 TeV, over an expected background of about 12 events; this is roughly a 4σ excess [269].
IceCube has also studied cosmic-ray air showers, including measurements of the energy spectrum, composition (combining IceTop air shower data with buried measurements of TeV muon fluxes) and anisotropy [270]. Surprisingly, the anisotropy persists up to energies of at least 400 TeV; this challenges our models of cosmicray production and propagation [271]. It also studies high transverse momentum muons produced in air showers, establishing the connection between cosmic-ray physics and perturbative quantum chromodynamics [272].
IceCube also searches for a variety of beyond-standard model phenomena: neutrinos from WIMP annihilation in the Sun, the Earth, the galactic halo or nearby dwarf galaxies, searches for magnetic monopoles and upward-going particle pairs; the latter is expected many variants of supersymmetry with a high mass scale.
A consortium of European institutions are proposing to build the KM3NeT detector [273]. This 5-6 km 3 detector would be located in the Mediterranean sea, where it would have a good (neutrino) view of the galactic center. Seawater has a somewhat longer optical scattering length than Antarctic ice, so KM3NeT should also have somewhat better angular resolution than IceCube.
PINGU and other high-density detectors PINGU(Precision IceCube Next Generation Upgrade) is a proposed high-density infill array within IceCube and DeepCore [274]. It will consist of 20 to 40 additional strings, with at least 60 optical modules per string, to observe neutrinos with energies down to a few GeV. The optical modules would be similar to those used in IceCube. Its main physics goal is to determine whether the neutrino mass hierarchy is 'normal' or 'inverted.' It is sensitive to the hierarchy because low energy electron neutrinos passing through dense matter (i.e. the Earths core and mantle) can resonantly oscillate into other flavors; the energy dependence of this resonant oscillation depends on the hierarchy.

ORCA (Oscillation Research with Cosmics in the Abyss)
[275]is a proposed high-density phototube array in the Meditranean. It would study a similar set of physics topics as PINGU.
Looking further ahead, MICA (Multi-Megaton Ice Cherenkov Array) would be an array with an even denser array of photosensors [276]. It will instrument a large volume (several megatons) with enough photosensor area to be search for proton decay and observe supernova neutrinos from other galaxies. The supernova search will benefit from both the large volume and the high photon sensor density to allow nearly background-free searches for neutrinos from supernovas in moderately nearby galaxies; the goal is a detector big enough to will collect a statistically meaningful number of supernovae. will have discovery potential for many data-driven neutrino flux models while ARA37 will either begin to collect a sample of neutrinos from those models, or reach even the more pessimistic models where the cosmic ray composition at the highest energies is mixed.

Radio Cherenkov Experiments
ARIANNA ARIANNA (Antarctic Ross Ice-Shelf Antenna Neutrino Array) is a surface array being deployed on the Ross Ice Shelf with a similar aim to reach the detector volume necessary to measure a sample of order 100 UHE neutrinos. ARIANNA aims to detect the impulsive radio Cherenkov signals from neutrino interactions both directly, and after reflections from the highly reflective ice-sea water boundary below the shelf. Together the direct and reflected signals complete the coverage of the upper sky. Initial results using a broadband pulser have verified the integrity of signals post-reflection. ARIANNA is on-track to complete a 7-station hexagonal detector unit in the 2013-2014 season and proposes to deploy a 960 station array. Although ARIANNA requires a large number of stations for the same detection volume compared to a deep array like ARA, a surface array is in principle simpler to deploy.
EVA The ExaVolt Antenna (EVA) is a next-generation balloon experiment that would turn the stadiumsized balloon itself into an enormous antenna, which would make EVA the world's largest airborne telescope. An impulsive signal from the ice below would be incident on ∼10 m high reflector region that would be affixed along the bulge of the balloon, and focused onto the receiver array suspended on the inside of the balloon. EVA takes advantage of the Super Pressure Balloon technology that is being developed by NASA, where the internal pressure of the balloon is higher than the outside pressure and the balloon holds its shape to ∼1% along any dimension. EVA expects to use a 18. The Cosmological Asymmetry between matter and anti-matter provides firm evidence for non-dark physics beyond the standard model. The current net density of baryons implies an asymmetry during the hot early universe between quarks and anti quarks, with about 10 8 + 1 quarks for every 10 8 anti-quarks. This asymmetry must have arisen after the end of inflation, due to some unknown mechanism called baryogenesis.
In 1967 Andrei Sakharov proposed that baryogenesis could arise from out of equilibrium new physics which violates baryon number conservation, and the C and CP symmetries between matter and antimatter [277]. At nonzero temperature baryon number violation in the standard model proceeds via the baryon number violating electroweak field configurations are known as sphalerons [278]. In 1985 Kuzmin, Rubakov and Shaposhnikov pointed out that sphaleron processes are sufficiently rapid at the high temperatures of the early universe to play a role in baryogenesis [279]. They also proposed that a strongly first order electroweak phase transition could provide the necessary departure from thermal equilibrium. However it is now known that the minimal standard model with a 125 GeV Higgs does not undergo a phase transition [280,281,282], and also does not have sufficient CP violation to produce an asymmetry of order 10 −8 [87]. However many extensions of the standard model do provide the necessary conditions for baryogenesis, as well as exciting opportunities for a wide variety of experiments. For recent reviews, see ref. [283,284]. Here we summarize the most well motivated possibilities.

Leptogenesis
The recent advent of the evidence of non-zero neutrino masses opens up the possibility of leptogenesis [88], generation of a net lepton number. As sphaleron processes conserve B − L, the difference between baryon number and lepton number, they tend to convert part of the primordial lepton number asymmetry into a baryon number asymmetry. The successful implementation of leptogenesis requires the existence of new CP violating phases in the lepton sector. In this scenario, the baryon asymmetry is related to the properties of the neutrinos. This subject is an example of the synergy between physics at the Cosmic and the Intensity Frontiers [285].

Standard Leptogenesis
Standard leptogenesis [88] is implemented within the seesaw mechanism for small neutrino masses in the presence of right-handed neutrinos. The full seesaw Lagrangian contains the Yukawa interactions for the neutrinos that in turn gives the Dirac mass terms for the neutrinos, as well as the lepton number violating Majorana mass terms, M R , for the right-handed neutrinos. At temperature T < M R , right-handed neutrinos, N , can generate a primordial lepton number asymmetry via out-of-equilibrium decays, N → H and N → H, where H is the SU(2) Higgs doublet and is the lepton doublet. The quantum interference between the tree-level and one-loop contributions to the decay can lead to a lepton number asymmetry.
The predictions for the baryon number asymmetry through standard leptogenesis depends on the coupling constants in the seesaw Lagrangian. Thus, by demanding that sufficient baryon number asymmetry to be generated, constraints [89] on neutrino parameters can be obtained. Specifically, the mass of the lightest right-handed neutrino is constrained to be M 1 > 3 × 10 9 GeV. One also obtains an upper bound on the light effective neutrino mass, m 1 < 0.12 eV, which is incompatible with the quasi-degenerate spectrum.
If the right-handed neutrinos are produced thermally, the lower bound on the lightest right-handed neutrino mass is then translated into a lower bound on the reheating temperature, M RH > M 1 > O(10 9 ) GeV. Such a high reheating temperature is problematic for many extensions of the Standard Model. For instance in many variants of supersymmetry, constraints from WMAP (for stable gravitino) and BBN (for unstable gravitino) typically require the reheating temperature to be several orders of magnitude lower than 10 9 GeV, incompatible with the condition for successful standard leptogenesis in supersymmetric models.

Alternative Realizations
To evade the gravitino over production problem, several scenarios have been proposed in which the conflicts between leptogenesis and gravitino over-production problem are overcome in different ways: • resonant enhancement in the self-energy diagrams due to near degenerate right-handed neutrino masses: in resonant leptogenesis [90], it has been shown that sufficient asymmetry can be generated even with TeV scale right-handed neutrino masses, leading to the possibility of testing this scenario at the collider experiments.
• relaxing the relation between the lepton number asymmetry and the right-handed neutrino mass: one example is the soft leptogenesis [286], where the asymmetry arises in mixing, instead of decay. In this case, the source of CP violation is the complex phases in the soft SUSY parameters.
• relaxing the relation between the reheating temperature and the right-handed neutrino mass: one realization of non-thermal leptogenesis is the production of the right-handed neutrinos by inflaton decay [287].

Dirac Leptogenesis
It was pointed out [288] that leptogenesis can be implemented even in the case when neutrinos are Dirac fermions which acquire small masses through highly suppressed Yukawa couplings without violating lepton number. The realization of this depends critically on the following three characteristics of the sphaleron effects: (i) only the left-handed particles couple to the sphalerons; (ii) the sphalerons change (B+L) but not (B-L); (iii) the sphaleron effects are in equilibrium for T > T EW . For the neutrinos, given that the neutrino Dirac mass is very tiny (m D < 10 keV), the left-right equilibration can occur at a much longer time scale compared to the electroweak epoch when the sphaleron washout is in effect. Suppose that some processes initially produce a negative lepton number (∆L L ), which is stored in the left-handed neutrinos, and a positive lepton number (∆L R ), which is stored in the right-handed neutrinos. Because sphalerons only couple to the left-handed particles, part of the negative lepton number stored in left-handed neutrinos get converted into a positive baryon number by the electroweak anomaly. This negative lepton number ∆L L with reduced magnitude eventually equilibrates with the positive lepton number, ∆L R when the temperature of the Universe drops to T T EW . Because the equilibrating processes conserve both the baryon number B and the lepton number L separately, they result in a Universe with a total positive baryon number and a total positive lepton number. And hence a net baryon number can be generated even with B = L = 0 initially. In the seesaw Lagrangian at high scale in the presence of three right-handed neutrinos, there are in total 6 mixing angles and 6 physical CP phases. On the other hand, the effective Lagrangian at low energy after integrating out the right-handed neutrinos, only three mixing angles and three physical CP phases remain. Given the presence of extra mixing angles and phases at high energy, it is generally impossible to connect leptogenesis (within the standard leptogenesis framework) and low energy CP violation processes in a model independent way. Nevertheless, this statement is weakened when the flavor effects, which are relevant if leptogenesis takes place at T < 10 12 GeV, are taken into account. On the other hand, within certain predictive models for neutrino masses, strong connections can be established even in the absence of flavor effects.

Community
Generally, two classes of models have been shown to exhibit possible connection between leptogenesis and CP violation in neutrino oscillation. These include: • models with rank-2 mass matrix: It has been shown that in a model with only two right-handed neutrinos with a rank-2 neutrino mass matrix, the sign of the baryon number asymmetry is related to the sign of CP violation in neutrino oscillation [289].
• models where CP violation comes from a single source: These include models with spontaneous CP violation, for example in minimal left-right symmetry model, there is only one physical CP phase in the lepton sector [290]. All leptonic CP violating processes (leptogenesis, neutrino oscillation, etc) are determined solely by this phase. Another example is a model with finite group family symmetry T with complex Clebsch-Gordan coefficients. CP violation in this model is due entirely to the complex CG coefficients. As the only non-vanishing leptonic phases are the low energy ones due to the symmetry of the model, there exists a strong connection between leptogenesis and low energy CP violating processes in this model [291].

Affleck-Dine Baryogenesis
In supersymmetric extensions of the Standard Model, there exist field configurations-condensates-of squark and slepton fields with very large expectation values and relatively low energy density compared with the thermal energy. In supersymmetric theories, at the exit from inflation, the observable universe would typically be in one of these configurations. Affleck and Dine showed that CP and baryon number violation at high energy would lead to net baryon production from the coherent evolution and subsequent decay of the condensates [91]. The Affleck-Dine scenario is consistent with a low reheat scale, and in some variants, can provide an explanation for dark matter as well. Decay of the condensates into baryons and WIMPS can provide an explanation for the similar cosmological densities of dark matter and baryons [292,293]. Another interesting possibility is that the condensates will fragments into stable lumps of matter with macroscopic amounts of baryon number and lower energy/baryon number than ordinary matter. For some parameters, these lumps, called Q-balls, are a viable dark matter candidate with unusual phenomenology [92,93,94].

Electroweak Baryogenesis
A strongly first order electroweak phase transition can occur in some extensions of the standard model, and provide the departure from thermal equilibrium necessary for baryogenesis. Such a phase transition proceeds via nucleation of bubbles of broken phase which expand to fill the entire universe. Inside the bubbles the Higgs expectation value is large, sphaleron transitions are suppressed, and baryon number is conserved, while the symmetric phase with no Higgs expectation value and unsuppressed sphalerons exists outside the bubbles. CP violating scattering of particles with the expanding bubble walls can lead to an CP asymmetric particle content in the symmetric phase, which will bias the sphalerons towards producing a net baryon number. This baryon number will then pass into the bubbles and, provided the sphalerons inside the bubble are suppressed by a large enough Higgs expectation value, survive until the present. For strongly first order phase transition, the effective potential at the critical temperature for the Higgs field must possess degenerate minima with a barrier between them. This barrier requires new bosons which are coupled to the Higgs field. Some well explored contenders for electroweak baryogenesis models are the MSSM [95], and generic two Higgs doublet models [96]. The MSSM is still a viable baryogenesis model provided the scalar partner of the right handed top quark is lighter than the top quark [294], and nonminimal supersymmetric theories are much less constrained [295]. In general two Higgs doublet models have a large parameter space [296,297], a significant portion of which remains viable for baryogenesis after the recent 126 GeV Higgs boson discovery [298]. Both the MSSM and two Higgs doublet models possess potential additional sources of CP violation and new contributions to Electric Dipole Moments (EDMs), which constrain the magnitude of the new CP violating phases. The theory connecting the new sources of CP violation with the total baryon number produced is very complicated and still possesses considerable sources of uncertainty, and is still being actively developed. See ref. [299] for a recent review. Eventually reliable theoretical computations of the baryon asymmetry will allow for predictions for EDMs and for new particle properties in electroweak baryogenesis models, but we are not quite there yet.

Other Baryogenesis mechanisms
Sakharov's original model, and subsequent baryogenesis models based on Grand Unified Theories, relied on the out of equilibrium decays of very heavy particles of mass of order 10 15 GeV. Such theories are mostly now inconsistent within the modern theory of inflation, as it is difficult to obtain a high enough reheat temperature to produce such particles.
A variety of other mechanisms for baryogenesis have been suggested, at energy scales ranging from just above the nucleosynthesis temperature of an MeV [300] to very high temperatures or high scale out of equilibrium processes occurring at the end of inflation. Some theories do not require baryon number violation at all, as the dark matter can carry an equal and opposite baryon number [301].

Experimental Signatures of Baryogenesis
The origin of the matter-anti-matter asymmetry in the universe is one of the most profound scientific questions of our time. A wide variety of experiments at all three frontiers can provide illumination.

Cosmic Frontier Experiments and Baryogenesis:
• Constraining the scale of inflation from the impact of tensor fluctuations on the CMB is important.
• Gravity wave experiments could provide evidence for a first order phase transition.
• Certain dark matter models are connected with baryogenesis and can provide unusual signatures. • CP violation in the neutrino sector would provide support for the leptogenesis scenario, and some models make for specific predictions.
• Evidence for or against neutrino Majorana masses would also impact leptogenesis theory.
• Electroweak baryogenesis scenarios provide strong motivation to search for EDMs, exotic CPV in meson physics, and rare decays.
• Proton d'qecay provides an important constraint on Grand Unified Model Building and on new sources of Baryon number violation.

Energy Frontier experiments and Baryogenesis:
• Collider tests of extended Higgs sectors and searches for new light scalars are important.
• Electroweak baryogenesis models typically have sizeable modifications of the triple Higgs self coupling [302,303] 4 CF6-C: Exploring the Basic Nature of Space and Time

Quantum Geometry and The Holographic Universe
New quantum degrees of freedom of space-time, originating at the Planck scale, could create a coherent indeterminacy and noise in the transverse position of massive bodies on macroscopic scales.
Quantum effects of space-time are predicted to originate at the Planck scale, ct P ≡ hG/c 3 = 1.616 × 10 −35 m. In standard quantum field theory, their effects are strongly suppressed at experimentally accessible energies, so space-time is predicted to behave almost classically, for practical purposes, in particle experiments. However, new quantum effects of geometry originating at the Planck scale-from geometrical degrees of freedom not included in standard field theory-may have effects on macroscopic scales that could be measured by laser interferometers.
The possibility of new quantum-geometrical degrees of freedom is suggested from several theoretical directions. Quantum physics is experimentally proven to violate the principle of locality on which classical space-time is based. Gravitational theory suggests that quantum states of space-time systems do not respect locality of the kind assumed by quantum field theory, and suggests that space-time and gravity are approximate statistical behaviors of a quantum system with a holographic information content, far less than that predicted by quantum field theory. [97,98] Quantum geometry could arise in Planck scale physics, but still produce a detectable displacement in a macroscopic experiment. [99] A typical uncertainty in wave mechanics, if information about transverse position is transmitted nonlocally with a bandwidth limit, is the scale familiar from diffraction-limited imaging: the geometric mean of inverse bandwidth and apparatus size. For separations on a laboratory scale, a Planck scale frequency limit leads to a transverse uncertainty in position on the order of attometers. Displacements of massive bodies of this order are detectable using laser interferometry.
No fundamental theory of quantum geometry exists, but a consistent effective theory, based on general properties of quantum mechanics and covariance, can be used to precisely predict a phenomenology on macroscopic scales. In particular, the theory precisely relates the number of geometrical position eigenstates to the amplitude of indeterminacy in transverse position at separation L, so it can be related to the holographic density of states predicted from gravitational theory. This hypothesis leads to an exact prediction for the variance in transverse position with no free parameters, [304] x 2 ⊥ = Lct P / √ 4π.
Planckian indeterminacy leads to a new form of noise in position with this displacement, on a timescale L/c. This form of indeterminacy would have escaped detection to date, and indeed is overwhelmed by standard quantum indeterminacy on the mass scale of elementary particles. However, it is detectable as a new source of quantum-geometrical noise in an interferometer that coherently measures the positions of massive bodies in two directions over a macroscopic volume. [305,99]

The Fermilab Holometer
An experiment is under development at Fermilab designed to detect or rule out a transverse position noise with Planck spectral density, using correlated signals from an adjacent pair of Michelson interferometers. A detection would open an experimental window on quantum space-time.
The Fermilab Holometer is an experiment (E-990) designed to detect or rule out quantum-geometrical noise with these properties.
[306] Much of the technology has been developed by LIGO and other projects to measure displacements due to gravitational radiation. The quantum-geometrical measurement however calls for application of the technology in a new experimental design. Measurements can be made at relatively high (MHz) frequencies, where environmental and gravitational noise sources are smaller, both shrinking and simplifying the layout. The experiment is designed to measure the specific and peculiarly quantummechanical signatures of the effect, such as nonlocal coherence and transverse nature of the indeterminacy, the frequency cross spectrum, and time-domain cross correlation function. It is anticipated that the experiment will be complete, and either detect or rule out this form of Planckian noise, within about two years.
If the noise is found not to exist, only a modest followup effort may be motivated to pursue the limits somewhat past the Planck scale for a conclusive result. If it is found, a significantly expanded experimental program can be pursued to obtain high precision results and map out the spatiotemporal properties of quantum geometry.

Torsion Balance Experiments
Another example of table top experiments with sensitivity to quantum gravitational effects are torsion balance experiments [307], which are sensitive to preferred frame effects, noncommutative geometry, new dimensions at short distances, and equivalence principle violation. CF34. What are the roles of cosmic-ray, gamma-ray, and neutrino experiments for particle physics? What future experiments are needed in these areas and why? Are there areas in which these can have a unique impact?
In this document we have discussed a broad range of fundamental physics that can be gleaned from cosmic rays, gamma rays, and neutrinos (dark matter, axions, primordial black holes, Q-balls, Lorentz invariance violation, intergalactic magnetic fields, particle interactions at high energies, neutrino mass hierarchy, etc.) that can not be reached through other methods.
CTA will usher in the era of precision VHE astrophysics and in conjunction with current instruments (Fermi and HAWC) will provide a view of the high-energy universe that will lead to an understanding of the astrophysical processes at work in these extreme objects and enable us to probe the laws of physics at energies, couplings, and mass scales that are beyond the reach of traditional high-energy physics experiments. Most importantly perhaps is the indirect detection of dark matter (see CF-2), where gamma-ray experiments have sensitivity to regions of parameter space not accessible to other techniques (accelerator and direct search techniques). The discovery of a primordial black hole or a Q-ball would would provide a wealth of data on the early universe and particle physics at energies not attainable in accelerators. Measurement of a vacuum dispersion relation for light would provide unique insight into the merging of quantum mechanics and general relativity. A measurement of the intergalactic magnetic fields would give insight into primordial magnetogenesis and the early universe processes, phase transitions or inflation dynamics that may have given rise to such a field.
PINGU will use the atmospheric neutrinos to measure the neutrino mass hierarchy. By using the atmospheric neutrinos generated by cosmic-ray interactions in the atmosphere a large range of L/E is available, enabling sensitive searches for matter effects on neutrino oscillations. At a relatively low cost, this can be used to make a definitive measurement of the neutrino mass hierarchy.
JEM-EUSO and a large aperture ground array Cosmic-ray experiments provide a window into particle interactions at the highest energies. While only large cross-section physics is accessible, given the low flux, well measured data with well-understood systematics, may offer channels to new physics. With much improved control over systematics and detector resolution and by combining various detectors (hybrid approach) current data has yielded interesting hints that we don't fully understand how to predict hadronic interactions at these energies. The measurement of total cross-section by HiRes and Auger at energies well beyond the LHC, while still crude, can be refined and be an important constraint on hadronic interaction models. A future large area surface detector will extend this measurement to higher energies, over an order of magnitude greater than achievable at the LHC.
From an astrophysics point of view, the next big question is the anisotropy of UHECR, i.e. the search for astrophysical sources. This is beginning to show interesting hints of association with nearby Large Scale Structure, but to really pin this down will require another order of magnitude or more of collecting area. JEM-EUSO, can achieve the required exposure. We know that UHECR originate within ∼100 Mpc of us. But until we can clearly state that the highest energy cosmic-ray flux anisotropy is understood from the point of view of astrophysical sources, the potential for new physics remains as strong as ever.
CF35. What will it take to identify the mechanism for baryogenesis or leptogenesis? Are there scenarios that could conceivably be considered to be established by experimental data in the next 20 years?
There are 3 well motivated scenarios for baryogenesis. Of these, electroweak baryogenesis is the one which is most likely to be definitively established or excluded within 20 years. In this scenario the baryon asymmetry of the universe can be related to the properties of new particles which couple to the Higgs boson, to properties of the Higgs boson, and to electric dipole moments (EDMs) of the neutron, the electron, and atoms. In particular difficult but doable theoretical calculations can relate the size and sign of a new CP violating phase to the baryon asymmetry and to EDMs.
It will be more difficult to establish the leptogenesis scenario, especially in the versions where it proceeds via the decay of very heavy (> 10 9 GeV) neutrinos. Some generic indications of this scenario are Majorana neutrinos, a light neutrino below 0.1 eV, and CP violation in neutrino oscillations, but verification of these features does not prove the theory is right, and the theory does not make a specific prediction for the phase which is observable in oscillations. However the heavy neutrino version requires a high inflation scale and high reheat temperature after inflation, which could be inconsistent with some kinds of new physics such as neutron-anti-neutron oscillations or supersymmetry with a gravitino in the mass range between a keV and 10 4 GeV, and so, depending on what other new physics is discovered, it could be excluded. Other leptogenesis scenarios involve new neutral leptons at or below the weak scale, or Dirac neutrinos, and have a restricted enough parameter space to be excluded or confirmed.
Affleck-Dine baryogenesis requires supersymmetry but does not necessarily require a specific SUSY spectrum, so definitively establishing or excluding this scenario is difficult. It is possible that this scenario can lead to the formation of stable Q-balls, an interesting dark matter candidate whose discovery would be strong evidence for the Affleck Dine scenario. Evidence of a high inflation scale from CMB B-modes would likely exclude this scenario, as fluctuations in the condensate would lead to isocurvature perturbations with an amplitude that has been ruled out by CMB measurements.
CF36. What are the leading prospects for detecting GZK neutrinos? What experimental program is required to do this in the next 5 years, 10 years, 20 years, and how important is this?
The prospects for detecting GZK neutrinos in the next decade are quite good. We know that there is a bottom to the flux of these neutrinos. Current efforts have ruled out the more optimistic scenarios and are now reaching into realistic parameter space. The next generation of instruments can improve upon current sensitivity by an order of magnitude giving sensitivity that is close to the lowest possible fluxes (an all iron composition). At the same time, the fraction of iron in the highest energy cosmic rays is lower than previously thought, significantly increasing the likelihood that the next generation of experiments will detect the GZK neutrinos.
How important is this? The GZK neutrinos provide a method to measure the neutrino cross section at a center-of-mass energy of 100 TeV! Observation of these interactions can provide information on beyond standard model physics that is many orders of magnitude beyond what is achievable in accelerator-based neutrino experiments. These observations will also enable us to understand cosmic accelerators.