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Resource Letter: The Standard Model and Beyond

This Resource Letter provides a guide to literature on the Standard Model of elementary particles and possible extensions. In the successful theory of quarks and leptons and their interactions, important questions remain, such as the mechanism of electroweak symmetry breaking, the origin of quark and lepton masses, the source of the baryon asymmetry of the Universe, and the makeup of its matter and energy density. References are cited for quarks and leptons, gauge theories, color and chromodynamics, weak interactions, electroweak unification, CP violation, dynamics of heavy quarks, Higgs bosons, precision electroweak measurements, supersymmetry, dynamical electroweak symmetry breaking, composite quarks and leptons, grand unification and extended gauge groups, string theories, large extra dimensions, neutrino masses, cosmic microwave background radiation, dark matter, dark energy, accelerator facilities, and non-accelerator experiments.


I. INTRODUCTION
The "Standard Model" of elementary particle physics encompasses the progress of the past half-century in understanding the weak, electromagnetic, and strong interactions. During this period tremendous strides were made in bringing quantum field theory to bear upon a wide variety of phenomena.
The arsenal of techniques for understanding the strong interactions in the 1960s included principles based on analyticity, unitarity, and symmetry. The successes of the emerging quark model often seemed mysterious. The ensuing decade yielded a theory of strong interactions, quantum chromodynamics (QCD), permitting calculations of a wide range of properties of the hadrons, or strongly interacting particles, and has been validated by the discovery of its force-carrier, the gluon.
In the 1960s the weak interactions were represented by a phenomenological fourfermion theory of no use for higher-order calculations. Attempts to describe weak interactions with heavy boson exchange bore fruit when these interactions were unified with electromagnetism and a suitable mechanism for generation of heavy boson mass was found. This electroweak theory has been spectacularly successful, leading to the prediction and observation of the W and Z bosons and to precision tests confirming the theory's validity in higher-order calculations.
This Resource Letter begins with sections devoted to the resources available for study of the Standard Model of particle physics and its extensions: periodicals (II), conference proceedings (III), texts and reviews (IV), historical references (V), popular literature (VI), Internet resources (VII), and a guide to Nobel prizes related to the subject (VIII).
A description of Standard Model research literature follows. In Section IX, based in part on [1], the ingredients of the standard model -the quarks and leptons and their interactions -are introduced, and QCD is discussed briefly. The unified theory of weak and electromagnetic interactions is described, its role in explaining CP violation is explained, and its missing piece -the Higgs boson -is mentioned.
Important questions remain that are not addressed in the Standard Model. These include the unification of the electroweak and strong interactions (possibly including gravity), the origin of quark and lepton masses, the source of the baryon asymmetry of the Universe, and the nature of its unseen matter and energy density. Some proposed Standard Model extensions devoted to these problems are noted in Section X. Concrete evidence for physics beyond the Standard Model, including neutrino masses, cosmic microwave background radiation, dark matter, and "dark energy," is described in Section XI. A variety of experimental methods are appropriate for probing these phenomena (Section XII). A brief summary (Section XIII) concludes.

II. PERIODICALS
The literature on the Standard Model of particle physics and its extensions is extensive and international, but a good sense of the field can be gained by perusing about a dozen main journals. Subsequent sections are devoted to other means of gaining information about this rapidly changing subject. years there occur both the International Symposium on Lepton and Photon Interactions at High Energies, of which the most recent was in Rome [3], and the International Europhysics Conference on High Energy Physics, most recently held in Budapest [4]. The locations of each of these conferences since 1990 are summarized in Table 1. A search of the SPIRES listing at the SLAC Library (see Sec. VII) is the easiest way to find the corresponding Proceedings.

A. Preprints
A comprehensive repository of preprints on experimental and theoretical particle physics may be found at http://arXiv.org/, including experimental papers at http://arXiv.org/archive/hep-ex, phenomenological papers (theory papers dealing with experiment) at http://arXiv.org/archive/hep-ph, and more abstract theoretical papers at http://arXiv.org/archive/hep-th. The SPIRES system at Stanford Linear Accelerator Center: http://www.slac.stanford.edu/spires/ lists a number of different categories, including books, conferences, experiments, preprints (SPIRES HEP), and even names and e-mail addresses of particle physicists.

B. Laboratories and accelerators
National and international high energy physics maintain extensive web pages with vast links to useful information. For a comprehensive listing, see http://www.nevis.columbia.edu/~quarknet/high energy physics links.htm. Some examples are given in Tables 2 and 3.  Group at Lawrence Berkeley National Laboratory, and Quarknet, a network for high school science teachers to involve them and their students in cutting-edge research in particle physics at http://quarknet.fnal.gov/. The IHEP laboratory in Russia hosts a chronology of particle physics discoveries: http://ontil.ihep.su/~ppds/discovery.html.

C. Popular article with extensive links
The Scientific American article on the future of particle physics by Steven Weinberg [132] appears on the web with a variety of links to other literature: http://www.sciam.com/issue.cfm?issueDate=Dec-99.

VIII. NOBEL PRIZES RELATED TO THE STANDARD MODEL
Some contributions in the past 45 years related to the formulation of the Standard Model that have been recognized by Nobel Prizes in Physics are summarized in Table  4. More information may be found on the web sites http://www.slac.stanford.edu/ library/nobel.html and http://www.nobel.se/physics/laureates. Many additional prizes were awarded for instrumentation or discoveries crucial to our present understanding of the Standard Model.

A. Quarks and leptons
The major ingredients of the Standard Model have been in place for some time, and can be gleaned from the popular article by Quigg [118]. The known building blocks of strongly interacting particles, the quarks [140, 141, 142], and the fundamental fermions lacking strong interactions, the leptons, are summarized in Table 5. The quark masses quoted there [73] are those for quarks probed at distances short compared with the characteristic size of strongly interacting particles. When regarded as constituents of strongly interacting particles, however, the u and d quarks act as quasi-particles with masses of about 0.  Table 5 fall into three "families." For evidence that all the existing families (at least those containing light neutrinos) may have been discovered, see [128].

B. Gauge theories
A theory of particles and their interactions permitting arbitrary changes of phase in the particle's quantum mechanical state is an Abelian local gauge theory such as electromagnetism. The term "Abelian" indicates that gauge (phase) transformations at a given space-time point commute with one another, while "local" stands for the freedom to make separate gauge transformations at each space-time point. The name "gauge" originated with Hermann Weyl [146].
Gauge transformations may be generalized to those that do not commute with one another at a given space-time point. The first such non-Abelian gauge theory was proposed by C. N. Yang and R. L. Mills [147], who used it to describe the strong interactions through self-interacting mesons of spin 1 carrying isosopic spin. The

C. Color and quantum chromodynamics
The quarks are distinguished from the leptons by possessing a three-fold charge known as "color" that enables them to interact strongly with one another [149,150,151]. We also speak of quark and lepton "flavor" when distinguishing the particles in Table 5 from one another. The evidence for color comes from several quarters.
1. Quark statistics. The ∆ ++ , a low-lying excited state of the nucleon, can be represented in the quark model as uuu, so it is totally symmetric in flavor. It has spin J = 3/2, a totally symmetric combination of the three J = 1/2 quark spins. As a ground state, its spatial wave function should be symmetric as well. While a state composed of fermions should be totally antisymmetric under the interchange of any two fermions, the state described so far is totally symmetric under the product of flavor, spin, and space interchanges. Color introduces an additional degree of freedom under which the interchange of two quarks can produce a minus sign.
2. Electron-positron annihilation to hadrons. The charges of all quarks that can be produced in pairs at a given center-of-mass energy is measured by the ratio 3. Neutral pion decay. The π 0 decay rate is governed by a quark loop diagram in which two photons are radiated by the quarks in The experimental rate is 7.8 ± 0.6 eV, in accord with experiment if S = 1 and N c = 3.
4. Triality. Quark composites appear only in multiples of three. Baryons are composed of qqq, while mesons are qq (with total quark number zero). This is compatible with our current understanding of QCD, in which only color-singlet states can appear in the spectrum.
A crucial feature of the QCD theory of strong interactions is its "asymptotic freedom," a weakening interaction strength at short distances permitting the interpretation of deep inelastic scattering experiments [96,152,153] in terms of quarks. This property was found to be characteristic of non-Abelian gauge theories such as color SU (3) [160]), but its significance for a strong-interaction theory was not realized then.
Direct evidence for the quanta of QCD, the gluons, was first presented in 1979 on the basis of extra "jets" of particles produced in electron-positron annihilations to hadrons. Normally one sees two clusters of energy associated with the fragmentation of each quark in e + e − → qq into hadrons. However, in some fraction of events an extra jet was seen, corresponding to the radiation of a gluon by one of the quarks. For a popular history of this discovery, containing further references, see [96].
The transformations that take one color of quark into another are those of the group SU(3). This group is called SU(3) color to distinguish it from the SU(3) flavor associated with the quarks u, d, and s. The charge-changing weak currents could be viewed as members of an SU(2) algebra [176,143]. However, the neutral member of this multiplet could not be identified with electric charge. Charged W ± bosons couple only to left-handed fermions, while the photon couples to both left and right-handed fermions. Moreover, a theory with only photons and charged weak bosons leads to unacceptable divergences in higherorder processes [18]. The neutral heavy Z boson can be arranged to cancel these divergences. It leads to neutral current interactions, in which (for example) an incident neutrino scatters inelastically on a hadronic target without changing its charge.
The discovery of neutral-current interactions of neutrinos [177,178,179,180] and other manifestations of the Z strikingly confirmed the new theory.
A key stumbling block to the construction of an electroweak theory applying to the quarks known at the time (u, d, and s) was the presence of flavor-changing neutral currents. The hypothesis of a fourth "charmed" quark c was an elegant way to avoid this problem [181]. The charmed quark also was crucial in avoiding "anomalies," effects due to triangle diagrams involving internal fermions and three external gauge bosons [182,183,184]. Evidence for charm was first found in 1974 in the form of the J/ψ particle [185, 186], a bound state of c andc. An earlier Resource Letter [75] deals with events leading up to this discovery, as well as early evidence for the fifth (b) quark to be mentioned below. The whole topic of electroweak unification is dealt with at an intermediate level in several references mentioned earlier (e.g., [14,18,24]).

F. CP violation
The symmetries of time reversal (T), charge conjugation (C), and space inversion or parity (P) have provided both clues and puzzles in our understanding of the fundamental interactions. The realization that the charge-changing weak interactions violated P and C maximally was central to the formulation of the V − A theory. The theory was constructed in 1957 to conserve the product CP, but the discovery in 1964 of the long-lived neutral kaon's decay to two pions (K L → ππ) [187] showed that even CP was not conserved. In 1973, Kobayashi and Maskawa (KM) [144] proposed that CP violation in the neutral kaon system could be explained in a model with three families of quarks. The quarks of the third family, now denoted by b for bottom and t for top, were subsequently discovered in 1977 [188, 189] and 1994 [190,191,192,193,194,195], respectively. Popular articles on these discoveries include one by Lederman [110] and Liss and Tipton [129].
An alternative theory of CP violation in the kaon system, proposed by Wolfenstein [196], involved a "superweak" CP-violating interaction mixing K 0 andK 0 , which would lead to identical CP violation in K L → π + π − and K L → π 0 π 0 . The discovery that this was not so (see [197,198] for the most recent published results, which are continually being updated in conference reports) disproved the superweak theory and displayed a "direct" form of CP violation with magnitude consistent with that predicted by the KM theory.
Decays of hadrons containing b quarks are further ground for testing the KM hypothesis and for displaying evidence for new physics beyond this "standard model" of CP violation. A meson containing ab quark will be known generically as a B meson. Electron-positron colliders have been constructed at SLAC (Stanford, CA) [199] and KEK (Tsukuba, Japan) [200] expressly to study B mesons; others at DESY (Hamburg, Germany) and Cornell (Ithaca, NY) [201] were fortunate in having just the right energy to produce B mesons in pairs. The BaBar detector at SLAC and the Belle detector at KEK have already produced a series of major results on B decays and CP violation [202,203]. Studies of particles containing b quarks also are expected to be an important part of the physics program at the Fermilab Tevatron [204] and the CERN Large Hadron Collider (LHC) [205]. with their spectroscopy and decays providing useful information on QCD at various distance scales. The states of light quarks bound to a single heavy quark have their own regularities. They are analogous to atoms in which the light quarks and gluons represent the "electronic" degrees of freedom, while the heavy quarks represent the nuclei. Thus, certain properties of these states are related in the same way that, for example, properties of hydrogen and deuterium are related. This "heavy quark symmetry" [55] has provided very useful guides to the properties of hadrons containing charm and beauty quarks, and permits more precise determinations of underlying weak couplings (such as elements of the Cabibbo-Koyayashi-Maskawa [CKM] matrix).

H. Higgs boson(s)
An unbroken SU(2) ⊗ U(1) theory involving the photon would require all fields to have zero mass, whereas the W ± and Z are massive. The symmetry-breaking that generates W and Z masses must not destroy the renormalizability of the theory. The Higgs mechanism achieves this goal at the price of introducing an additional degree of freedom correponding to a physical particle, the Higgs particle, which is the subject of intense searches [32,120,206,207]. Current 95% c.l. limits on a standard-model A similar problem is present whenever there is a large gap between the electroweak scale and any higher mass scale contributing to the Higgs boson mass. Supersymmetry solves this problem by introducing for each particle of spin J a superpartner of spin J ± 1/2 whose contribution to such loop diagrams cancels the original one in the limit of degenerate masses. Recent reviews of supersymmetry and its likely experimental signatures include [40, 62, 63, 64, 65, 66, 67], while earlier discussions are given by

C. Fermion mass and mixing patterns
The transitions between the (u, c, t) and (d, s, b) quarks owing to virtual W emission or absorption are described by the Cabibbo-Kobayashi-Maskawa (CKM) matrix mentioned in Sec. IX A. (For one parametrization of this matrix see [226].) The CKM matrix arises because the matrices that diagonalize the mass matrices of (u, c, t) and of (d, s, b) are not the same. A theory of quark masses would thus entail a specific form of the CKM matrix. For the corresponding matrix for leptons, see [227,228,229]. While a theory of quark and lepton masses still eludes us, attempts have been made to guess some of its general features [230,231,232,233,234,235].

D. Composite quarks and leptons
Families of quarks and leptons appear to be replicas of one another (see Table 5), aside from their differing masses and weak couplings. Attempts have been made to explain this regularity in terms of a composite structure, much as the periodic table of the elements reflects their underlying atomic structure. A set of guidelines for this program was laid down by 't Hooft

F. Strong CP problem and axions
In a non-abelian gauge theory such as SU(3) there can arise non-trivial gauge configurations that prevent terms in the Lagrangian proportional to Tr (G µνG µν ) from being ignored as pure divergences. Such terms can lead to strong CP violation. Their coefficient, a parameter conventionally called θ, must be of order 10 −10 or smaller in order not to conflict with limits on the electric dipole moment of the neutron [243]. Several proposals have been advanced for why θ is so small [40,244]. In one of the most interesting, θ is promoted to the status of a dynamical variable that can relax to a natural value of zero. As a consequence, there arises a nearly massless particle known as the axion, whose properties (and the search for which) are well-described in [40,244].

G. String theory
A truly unified theory of interactions must include gravity. The leading candidate for such a theory is string theory, which originated in pre-QCD attempts to explain the strong interactions [245,246,247,248] by replacing the space-time points of quantum field theories with extended objects ("strings"). In 1974 it was realized that string theories necessarily entailed a massless spin-2 particle, for which the graviton was an ideal candidate [249]. While it appeared that such theories required space-time to be 26-dimensional (or 10-dimensional in the presence of supersymmetry), these extra dimensions were interpreted in the 1980s as a source of the internal degrees of freedom characterizing particle quantum numbers (see. e.g., [250,251,252]). A typical scenario whereby string theory might yield predictions for the quark and lepton spectrum is described in [253].
Early results on string theory are

A. Neutrino masses
The ability of neutrinos of one species to undergo oscillations into another is an indication of non-zero and non-degenerate neutrino masses [57,58]. Several experiments find evidence for such oscillations. Reviews have appeared in [59,61,131]; the second of these also deals with precision electroweak tests using neutrinos.

Solar neutrinos:
Since the earliest attempts to detect neutrinos originating from the Sun in the mid-1960s, the flux has been less than predicted in the standard solar model [125]. Recent experiments at the Sudbury Neutrino Observatory (SNO) in Ontario [259,260] and the KamLAND experiment in Japan [261] strongly suggest that this deficit is due to oscillations of the electron neutrinos produced in the Sun into other species, most likely a combination of muon and tau neutrinos, induced by interaction with the Sun in a manner (now known as the MSW effect) first proposed by Mikheev and Smirnov [262] and Wolfenstein [263]. For reviews, see [35, 60].

Atmospheric neutrinos:
Neutrinos produced by the interactions of cosmic rays in the atmosphere are expected to be in the ratio ν µ : ν e = 2 : 1 (summing over neutrinos and antineutrinos) [264]. Instead, a ratio more like 1:1 is observed. This phenomenon has been traced to oscillations that are most likely ν µ → ν τ , as a result of definitive experiments performed by the Super-Kamiokande Collaboration in Japan [265,266]. The mixing appears to be close to maximal, in contrast to the small mixings of quarks described by off-diagonal elements of the CKM matrix.

B. Cosmic microwave background radiation
The 2.7 K radiation left over from the Big Bang contains a wealth of information about both the early Universe and particle physics. In particular, the spatial pattern of its fluctuations indicates that the Universe is exactly on the border between open and closed, and strongly supports the idea that the Universe underwent a period of exponential inflation early in its history [134,135,269,270,271]. For a review of the cosmological parameters, see [272].

C. Baryon asymmetry of the Universe
To explain why the visible Universe seems to contain so many more baryons than antibaryons, Sakharov [273] proposed shortly after the discovery of CP violation that three ingredients were needed: (1) CP (and C) violation; (2) baryon number violation, and (3) a period in which the Universe is not in thermal equilibrium. All of these conditions are expected to be satisfied in a wide range of theories, such as grand unified theories (Sec. XI.E) in which quarks and leptons, and the electroweak and strong interactions, are unified with one another [274]. However, details of the mechanism are not clear [112,113]. In some versions of the theory, for example, it is lepton number that is violated in the early stages of the Universe, giving rise to a lepton asymmetry that is then converted to a mixture of lepton and baryon asymmetry when the Universe has evolved further. For a recent review of this suggestion, see [275].

E. Dark energy
The Universe appears not only to be expanding, but its expansion appears to be speeding up. Evidence for this behavior comes from the study of distant supernovae, which furnish "standard candles" for a cosmological distance scale [130,272]. One interpretation is that a cosmological constant Λ (first proposed by Einstein shortly after he formulated the general theory of relativity) accounts for about 65% of the energy density of the Universe. This contribution is sometimes referred to as "dark energy," to distinguish it from the "dark matter" accounting for nearly all of the remaining energy density aside from a few-percent contribution from baryons [269,270]. An alternative suggestion is that the "dark energy" is due to a new field, dubbed "quintessence" [

XII. EXPERIMENTAL APPROACHES
The rise of the Standard Model would not have been possible without a variety of experimental facilities, including accelerators, detectors, and non-accelerator experiments. What follows is a brief description of some currently operating laboratories two, CLEO will return to the Υ(1S, 2S, 3S) resonances, after which it is planned to optimize CESR to run at the lower energies appropriate for charm production [289]. This will permit a return to many interesting questions with a vastly improved detector and statistical sample.

DESY (Germany)
A circular electron accelerator at the Deutsches Elektronen Synchrotron (DESY) laboratory was converted to an electron-positron collider (DORIS) whose experimental program paralleled that of CESR/CLEO for a number of years, yielding important information about Υ spectroscopy and B mesons, for example through work of the ARGUS Collaboration. Subsequent machines included the larger e + e − collider PETRA (maximum c.m. energy 46 GeV) and the currently operating HERA lepton-proton collider, which has studied both e − p and e + p interactions. HERA has extended information on deep inelastic lepton scattering to new kinematic regimes and provided important information on the gluon structure of the proton.

Fermilab (U.S.A.)
The Fermi National Accelerator Laboratory in Batavia, Illinois, U.S.A., began operation in 1972 as a proton accelerator with initial energy 200 GeV, rising to 400 GeV within a year. With the addition of a ring of superconducting magnets in 1983 it was converted to an energy of 800 GeV capable of providing protons to fixed targets and proton-antiproton collisions with a center-of-mass energy of 1.8 TeV [127,290]. Its energy has recently been upgraded to nearly 1 TeV per beam with the addition of a new 150-GeV proton ring called the Main Injector. Outstanding discoveries at Fermilab include those of the bottom quark in 1977 [188,189], the top quark in 1994 [190,191,192,193,194,195], and the tau neutrino in 2000 [291].

Frascati (Italy)
A major pioneer in the study of electron-positron collisions has been the Laboratori Nazionali di Frascati (INFN) near Rome, Italy. Starting in the early 1960s with the ADA collider and continuing through the ADONE storage ring, which begain operation in the late 1960s, the laboratory has now begun to operate a machine called DAΦNE (DAFNE), which seeks to produce kaons and other particles through the reaction e + e − → φ → . . . at a center-of-mass energy of 1.02 GeV.

KEK (Japan)
In the early 1970s, a 12-GeV proton synchrotron was constructed in Japan near Tokyo at the National Laboratory for High Energy Physics, for which KEK (Ko-Energi-Kenkyujo) is the acronym in Japanese. The next major project at KEK, the TRISTAN e + e − collider, attained a center-of-mass energy in excess of 60 GeV, the highest in the world for such a machine at its debut in 1986. Among the topics studied by TRISTAN included weak-electromagnetic interference through the processes e + e − → (γ * , Z * ) → . . ., where the asterisk denotes a virtual photon or Z. The latest project at KEK is the KEK-B e + e − collider, a lower-energy machine built in the TRISTAN tunnel, which is designed to produce pairs of B mesons with net motion on their center-of-mass by using unequal electron and positron energies. In this way the positions at which the B mesons decay can be spread out longitudinally, permitting easier study of time-dependences that are of particular interest in CP-violating processes. The Belle detector operating at KEK-B [200] is producing significant results on B decays, as mentioned above [203]), as is the BaBar detector operating at PEP-II (see the description of SLAC, below).

Novosibirsk (Russia)
A series of e + e − colliders has operated at the Budker Institute for High Energy Physics in Novosibirsk for a number of years. Indeed, work at this laboratory helped to pioneer the study of beam dynamics essential for achieving such collisions. These colliders performed important measurements at the center-of-mass energies of the Υ(9.46) and φ(1.02) resonances, where the numbers denote the mass in GeV/c 2 .

Protvino (Russia)
The largest accelerator at present in Russia is a 76-GeV proton synchrotron at Serphukhov (Protvino), which began operation in the early 1970s. It was the first to detect rising meson-baryon cross sections [292], followed soon by the observation of a similar effect in proton-proton collisions at the CERN ISR (see above).

SLAC (U.S.A.)
The early program of the 30-GeV 2-mile-long linear electron accelerator at the Stanford Linear Accelerator Center (SLAC) included the discovery of pointlike constituents inside the proton through deep inelastic scattering [96,152,153]. In the early 1970s the SPEAR electron-positron storage ring was constructed with maximum center-of-mass energy equal to 7.4 GeV. Late in 1973 this machine confirmed a surprising enhancement of the e + e − annihilation cross section starting at a c.m. energy of 4 GeV seen earlier at the Cambridge Electron Accelerator (CEA), and in 1974 was one of two sources of the discovery of the J/ψ particle [186], the other being a fixed-target experiment at Brookhaven National Laboratory [185] (see above). In the mid-1970s construction was begun on PEP, an electron-positron collider with c.m. energy of about 30 GeV, which performed studies of the elctroweak theory and was the first to measure the b quark lifetime. The energy of the LINAC was then raised to 50 GeV, both electrons and positrons were accelerated, and these were then bent in arcs to collide with one another at energies equal to or greater than the mass of the Z boson. This machine, the Stanford Linear Collider (SLC) [124], pioneered in precision studies of the Z boson through its Mark II and SLD detectors; its early measurement of the Z width was a piece of evidence for three families of quarks and leptons [128]. The latest SLAC project, the PEP-II asymmetric e + e − collider, has seen evidence for CP violation in B decays in its BaBar detector [199,202] (see also KEK-B and Belle, above), and has achieved record luminosity for any collider. By the middle of this decade both BaBar and Belle expect to have produced and recorded several hundred million BB pairs.

Thomas Jefferson National Laboratory (U.S.A.)
A moderate-energy (5.7-GeV) electron accelerator, this machine studies interactions with nuclei and the photoproduction and electroproduction of resonances con- . Whereas the focus of several laboratories initially had been the search for proton decay, it has now broadened to include the study of interactions of neutrinos from atmospheric cosmic rays, the Sun, and even supernovae, and the search for effects of dark matter.
The next stage of operation of detectors in the laboratories mentioned above includes the study of artificially produced neutrinos. The Fermilab accelerator will send neutrinos to the MINOS detector [296] in Soudan. The proton synchrotron at KEK in the K2K experiment [293], and later a machine known as the Japan Hadron Facility [297], will direct neutrinos to the SuperKamiokande detector in Kamioka. Finally, a detector known as KamLAND [298], also in the Kamioka mine, will be sensitive to neutrinos from reactors over a large portion of Japan, and has already reported its first results [261].
Some current and forthcoming detectors will also be sensitive to naturally occurring neutrinos. These include the Sudbury Neutrino Observatory in Ontario [299], the Borexino experiment [300] in Gran Sasso, and the SuperKamiokande detector mentioned above. At the South Pole a number of phototubes have been sunk deep into the ice in the AMANDA experiment [301], which is envisioned in the IceCube experiment [302] to expand to an effective volume of a cubic kilometer. The RICE experiment [303] seeks to study the low-frequency tail (at several hundred MHz) ofČerenkov emission by electrons produced by neutrinos, also in South Polar ice. A number of neutrino detectors are also deployed or planned deep underwater, e.g., in Lake Baikal

Atomic physics
A large accelerator is not always needed to study fundamental particle physics beyond the Standard Model. An example is the window on non-standard physics provided by atomic parity violation. (See the bibliography in [72].) Studies of weakelectromagnetic interfence in atoms such as Cs, Tl, and Pb are in principle sensitive to new interactions and extended gauge theories, particlarly if the effects of atomic physics can be separated from more fundamental effects.

C. Plans for future facilities
The particle physics community is developing a number of options to probe further beyond the Standard Model. These include a large linear e + e − collider, intense sources of neutrinos ("neutrino factories"), a muon collider, and a Very Large Hadron Collider

XIII. SUMMARY
The Standard Model of electroweak and strong interactions has been in place for nearly thirty years, but precise tests have entered a phase that permits glimpses of physics beyond this impressive structure, most likely associated with the yet-to-be discovered Higgs boson and certainly associated with new scales for neutrino masses. Studies of CP violation in decays of neutral kaons or B mesons are attaining impressive accuracy as well, and could yield cracks in the Standard Model at any time. It is time to ask what lies behind the pattern of fermion masses and mixings. This is an input to the Standard Model, characterized by many free parameters all of which await explanation.
Many avenues exist for exploration beyond the Standard Model, both theoretical and experimental. A lively dialogue between the two approaches must be maintained, with adequate support for each, if we are to take the next step in this exciting adventure.