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Supersymmetry: Experimental Status

This talk presents results from the CMS and ATLAS Collaborations from searches for physics beyond the Standard Model motivated by supersymmetry from Run 1 of the LHC. Representative searches are described to illustrate the diverse nature of the search program in both background estimation techniques and final state topologies. The status of preparation for Run 2 searches at 13 TeV is also presented.


INTRODUCTION
The Standard Model of particle physics accurately describes the interactions of all known fundamental particles in the universe, and has remained the prevailing paradigm in the field for over forty years. Despite its success, the Standard Model remains an incomplete theory of fundamental particles and interactions. It does not include a description of gravity, nor does it explain the compelling astronomical evidence for dark matter. Of the proposed extensions to the Standard Model, supersymmetry (SUSY) has remained among the most popular for decades. It provides exactly the needed compensation to stabilize the Higgs mass, while additionally providing an ideal candidate for dark matter with a stable weakly interacting lightest supersymmetric particle (LSP).
The CMS [1] and ATLAS [2] experiments at the CERN Large Hadron Collider are general purpose detectors built to explore the fundamental nature of the universe. Among the results from the two experiments are many searches for supersymmetry, which have thus far yielded null results [3,4]. The search programs in both experiments are based on a wide arrange of techniques to measure standard model background contributions as well as a diverse range of possible final states. In this talk, a sample of results are shown to illustrate 1 techniques deployed in these searches. By no means are all relevant results discussed or presented.
In the first section, a series of general searches in different final states are described. The second section contains a discussion of more targeted searches focused on dedicated final state topologies, while the third section discussed difficult to reach signatures. The forth section attempts to put the full set of searches performed into a global context, while the final section shows progress toward new searches in the LHC Run 2 with 13 TeV proton-proton collisions.

INCLUSIVE SEARCHES
Unlike the Standard Model Higgs boson, supersymmetry has many free parameters, which can give rise to a great variety of signatures. Further, the unknown mass spectrum can also give rise to a great variety of production cross sections and final state kinematics. With such a broad range of possible signatures, a fruitful class of supersymmetry searches is performed with inclusive sensitivity. Here I describe four such examples in different final states.
The classic jets plus missing energy signature is searched for in a three dimensional binned analysis taking advantage of sensitivity in different bins of missing energy (MET), the sum of jet transverse momenta (HT), and the number of jets tagged as bottom quarks [5]. Events are selected by removing those with an identified electron or muon and then requiring at least three jets and at least one tagged b-quark jet. The main Standard Model backgrounds derive from tt, W + jets, Z + jets, and QCD multijet events. The contribution from each category of background is measured from data control samples to minimize the reliance on accurate simulation. In particular, single lepton events are used to predict the tt and W + jets backgrounds, and dilepton events are used to predict the Z + jets backgrounds with Z decays to neutrinos. The QCD multijet contribution is predicted by utilizing a kinematic sideband enriched in QCD events where a jet and MET are aligned.
No significant excess of events above the Standard Model predictions is observed. Fig. 1 (left) shows the data compared to the expected Standard Model contribution after selecting events with at least 3 b-quark jets. The search results are interpreted in several benchmark SUSY models, including gluino pair production with each gluino decaying to two b-quarks and the LSP. Fig. 1 (right) shows that such models are excluded for gluino masses as high as around 1.2 TeV.
A similarly broad search was performed with complementary events selected with exactly one muon or electron in [6]. Minimal requirements on MET and the transverse mass (MT) of the lepton and MET are used to select SUSYlike events. Sensitivity to a variety of models is obtained by classifying search regions into large and small jet multiplicity. Backgrounds arise predominately from W + jets and tt events. The size of the Standard Model contributions are predicted by identifying data control regions enriched in each background. MC simulation is then used with the overall normalization taken from the data 95% CL upper limit on cross section (pb) Figure 1: MET distribution in hadronic search with at least 3 b-jets (left) and limits from the search for gluino to bbLSP production (right). Taken from Ref. [5].
control region to predict the background in each signal region.
The MET distribution for a signal region with five or more jets is show in Fig. 2 (left) with data compared to the background estimation. No significant signal is observed in any of the search regions and 95% CL upper limits are set on a variety of simplified models. Fig. 2 (right) shows the limits for gluino pair production with both gluinos decaying into two top quarks with limits reaching beyond 1.3 TeV for light LSPs.

JHEP04(2015)116
Events / 50 GeV  Figure 8. Distribution of the missing transverse momentum E miss T in the 3-jet (top) and 5-jet (bottom) tt (left) and W +jets (right) control regions used in the hard single-lepton channel. The upper E miss T cut, indicated by the arrow, is not applied in these distributions. The purity in the background of interest is 66% (72%) for the 3-jet tt (W ) control region and 81% (45%) for the 5-jet tt (W ) control region; the 5-jet W control region is cross-contaminated by tt events at the level of 40%. The "Data/SM" plots show the ratio of data to the summed Standard Model expectation, which is derived from the fit described in section 9. The uncertainty band on the Standard Model expectation shown here combines the statistical uncertainty on the simulated event samples with the relevant systematic uncertainties (see text). The last bin includes the overflow. The "Top Quarks" label includes all top-quark-related backgrounds, while "V+jets" includes W +jets, Z+jets and other Drell-Yan backgrounds such as Z → τ + τ − and γ * /Z outside the Z pole region. For illustration, the expected signal distributions are shown for gluino pair production with mg = 1025 GeV, mχ± All limits at 95% CL Figure 18. 95% CL exclusion limit from the hard single-lepton channel in the (mg,mt (χ 0 1 ) ) plane for the simplified model with gluino-mediated top squark production, where the top squark is assumed to decay exclusively viat → cχ 0 1 (top) or where the gluinos are assumed to decay exclusively through a virtual top squark,g → ttχ 0 1 (bottom) . The dark blue dashed line shows the expected limits at 95% CL, with the light (yellow) bands indicating the ±1σ variation on the median expected limit due to the experimental and background-only theory uncertainties. The observed nominal limit is shown by a solid dark red line, with the dark red dotted lines indicating the ±1σ variation on this limit due to the theoretical scale and PDF uncertainties on the signal cross section.

ATLAS
-45 - Figure 2: MET distribution for the W + jets control region for the inclusive 1L search (left) and results from the search for gluino to ttLSP production (right). Taken from Ref. [6].
Next, a search was performed based on events with two electrons or muons with the same electric charge [7]. Such events are rare in the Standard Model, but can occur readily in many new physics signatures. The leptons are required to be well isolated to select prompt leptons from W or Z decays and remove those associated with jets, for example from semi-leptonic b decays. The main backgrounds arise from events with a non-prompt lepton that mistakenly passes the isolation criteria in addition to another prompt lepton or from events with two true prompt leptons arising from such rare processes as diboson production. The background from non-prompt leptons is determined by measuring the so-called "fake rate" of the likelihood of a non-prompt lepton to pass the isolation criteria in a data control sample, as shown in Fig. 3 (left). The contributions from rare backgrounds are taken from simulation. As with the other inclusive searches, signal regions are defined in a number of bins of MET, HT, and number of b-tagged jets to ensure sensitivity to a variety of possible signal models and parameter space. No significant excess of events is observed above the expected Standard Model backgrounds. Fig. 3 (right) shows the observed limit for sbottom pair production with each sbottom decaying to t, W , and the LSP.
95% CL upper limit on cross section (fb) Figure 3: Tight to loose isolation ratio used to measure non-prompt lepton fake rate (left) and results from same-sign dilepton search for sbottom pair production (right). Taken from Ref. [7]. The final inclusive search described in this talk is based on events with two high p T photons [8]. Such a signature is common in gauge mediated (GMSB) SUSY scenarios where the LSP is a gravitino and the lightest neutralino decays into a photon and the gravitino. Sensitivity to strong and electroweak production is achieved with search bins in low and high MET and jet multiplicity. The main backgrounds arise from combinatorial diphoton production and from photon + jets events where one of the jets is mistaken as an isolated photon. The missing energy in such background events is generally the result of mismeasured jets. Since the MET does not come from the photons themselves, data control samples composed of events from the photon isolation sidebands can be used to measure the expected MET shape. The MET shape is then normalized to the low MET region in the true two photon sample to predict the background in the signal region. Fig. 4 (left) shows the MET distribution for a signal region with MET > 200 GeV where the signal extends to higher MET than the remaining backgrounds. No significant excess of signal events is observed. Fig. 4 (right) shows the upper limits on gluino production in a GMSB scenario with a bino-like neutralino. < 800 GeV. Combinations of gluino and bino mass are excluded at 95% CL in the area below the unbroken curve. The observed limits are exhibited for the nominal SUSY model cross-section expectation, as well as for a SUSY cross section increased and decreased by one standard deviation of the cross-section systematic uncertainty. Also shown is the expected limit, as well as the ±1 and ±2 standard-deviation ranges of the expected limit. 30 Figure 4: MET distribution for inclusive diphoton search signal region (left) and results for gluino production GMSB model (right). Taken from Ref. [8].

TARGETED SEARCHES
In addition to generic inclusive searches, some SUSY signatures are sufficiently well motivated to demand dedicated searches targeting a more specific model. Here two such targeted searches are described. The first is for stau pair production, while the second targets direct stop production.
Direct stau pair production is well motivated, in particular by its potential connection to cosmological scenarios to describe the early evolution of the universe. While generic dilepton searches are often sensitive to stau production through the stau decays to electrons or muons, a dedicated search is required to capture sensitivity to hadronic stau decays which have the largest branching fraction. In [9] events with two hadronic tau candidates with opposite charge are selected. Z boson candidates are vetoed to reject Z to τ τ events, and events with a b-tagged jet are rejected to remove tt events. The remaining background is dominated by QCD multijet events. To select signal from this background, a multivariate boosted decision tree (BDT) is trained and only events with high BDT score are retained. After such selection, the main backgrounds remaining are W + jets and diboson events. The W + jets background is measured by identifying a data control sample enriched in W + jets and normalizing the MC simulation prediction to the yield in this control sample.  Another very well motivated SUSY scenario is direct stop production, as the stop plays a key role in the cancelation of quadratic divergences to the Higgs mass from top quark loops. The search in [10] utilizes the all hadronic final state to target stop pair production with stop to top, LSP decays with both tops decaying hadronically. The analysis uses a customized jet algorithm to identify two hadronic top decay candidates. Events with an isolated electron, muon, or tau are removed. The most significant background arrises from tt events with MET from a leptonic W decay where the charged lepton is lost. The separation of signal and background is achieved with a BDT trained to select signal events. Kinematic variables such as the angle between the MET and the jets in the subleading top candidate as shown in Fig. 6 (left) are used in the BDT. After the selection, MC simulation is used to estimate the total background contribution in each signal region. The MC is corrected to achieve good agreement with data in several key kinematic distributions and the background prediction is validated in the BDT sidebands. No significant excess of data over background is observed and limits are set on direct stop production. As shown in Fig. 6 (right) stop masses up to around 800 GeV are excluded for light LSPs.

EXPLORING GAPS
As more and more SUSY searches have yielded null results, an increasing effort has been placed on considering where a signal may yet be hiding in space accessible with current LHC data. This section describes four such searches that Events / 0. One such gap in SUSY sensitivity occurs when the stop has a mass very close to that of the top and LSP is very light. In this scenario, stop pair production looks very similar kinematically to top pair production and the signal can be very difficult to dig out. One approach is to use a precision measurement of the top cross section and compare it to the theoretically predicted cross section from the Standard Model. If excess events exist, they could be from the presence of stops. Additionally, the spin correlations of the scalar stops are somewhat different from that of the spin 1/2 tops. The search in [11] exploits this difference to gain sensitivity to stop production in this difficult region. Dileptonic tt events are used to compare the observed angular difference between the leptons with that expected from tt and stop pair production, as shown in Fig. 7 (left). No deviation from the expected Standard Model distribution is observed and limits are set on stop pair production, as shown in Fig. 7 (right).
Another difficult to access region occurs when SUSY particle masses are nearly degenerate. These so called "compressed" spectra can result in SUSY decays with little missing energy if the LSP is close in mass the parent particle. As in direct dark matter searches with the monojet topology, compressed SUSY can be searched for in events where the SUSY system recoils against an ISR jet. The compressed spectrum then produces missing energy when it is boosted. Such a technique is employed in [12] where the ISR jet and missing energy are searched for in combination with one or two low p T leptons, which can originate from stop or chargino decays. The resulting lepton p T spectrum is soft, as shown in Fig. 8 (left) for compressed decays. After selecting only events with low p T isolated leptons much of the background is removed and sensitivity to this difficult region is obtained, as shown in Fig. 8 (right). Reconstructed distribution for the sum of the three dilepton channels. The prediction for background (blue histogram) plus SM tt production (solid black histogram) and background plus tt prediction with no spin correlation (dashed black histogram) is compared to the data and to the result of the fit to the data (red dashed histogram) with the orange band representing the total systematic uncertainty on f SM . Both the SM tt and the no spin correlation tt predictions are normalized to the NNLO cross section including next-to-next-to-leadinglogarithm corrections [71, 72] (the theory uncertainty of 7% on this cross section is not displayed). The prediction fort1t1 production (mt 1 = 180 GeV and m˜ 0 1 = 1 GeV) normalized to the NLO cross section including next-to-leading-logarithm corrections [101] plus SM tt production plus background is also shown (solid green histogram). The lower plot shows those distributions (except for background only) divided by the SM tt plus background prediction.
The uncertainty due to the parton shower and hadronization model is determined by comparing two tt samples generated by ALPGEN, one interfaced with PYTHIA and the other one interfaced with HERWIG. The uncertainty on the amount of initial-and final-state radiation (ISR/FSR) in the simulated tt sample is assessed by comparing ALPGEN events, showered with PYTHIA, with varied amounts of ISR and FSR. As in Ref.
[42], the size of the variation is compatible with the recent measurements of additional jet activity in tt events [103]. The W t normalization is varied within the theoretical uncertainties of the cross-section calculation [86], and the sensitivity to the interference between W t production and tt production at NLO is studied by comparing the predictions of POWHEG-BOX with the diagram-removal (baseline) and diagram-subtraction schemes [85, 104]. As in Ref.
[42], the uncertainty due to the top quark mass is evaluated but not included in the systematic uncertainties, since it would have no significant Figure 7: Angle between two leptons in dileptonic tt cross section measurement compared to SUSY stop signal (left) and resulting exclusion limit for stop production (right). Taken from Ref. [11].   Another alternative to ISR to boost the compressed SUSY spectrum is vector boson fusion (VBF). The pair of VBF jets serves the same purpose of providing a boost to the SUSY system, which would otherwise have very low MET. The search in [13] exploits the VBF topology to search for compressed SUSY with complementary sensitivity to the ISR searches. As an additional discriminating variable, the mass of the VBF dijet system can be utilized to select high mass events more typical of signal. Figure 10 (left) shows the dijet mass distribution for background compared to signal. The observed distribution is consistent with the Standard Model expectation and no evidence for SUSY is found. Figure 10 (right) shows the search results interpreted as limits on compressed sbottom pair production as well as direct dark matter production.

THE BROAD PICTURE
With the plethora of possible SUSY signatures and searches performed at the LHC, it is important to put the entirety of the search program together to assess where things stand. Many different searches can be sensitive to the same model. When mutually exclusive final states provide complementary sensitivity, a combination of the results of the different relevant searches can extend the overall reach. For example, in Fig. 11 (left) the results from searches using five different final states are shown along with the combination of the five searches, which extends the sensitivity beyond any of the individual searches alone. Alternatively, different searches can be designed to be sensitive to different regions of parameter space for a given model. When the exclusion regions for each in-  dividual search are overlaid, the total exclusion can show significant coverage. For example, Fig. 11 (right) shows the exclusions from eight different searches targeting direct stop production. In total, they exclude a very significant region of the plane. Such summary plots also serve to highlight regions where gaps exist in the current sensitivity and can motivate future efforts.  Figure 11: Summary plots of various search results from gluino-mediated stop production (left) from [3] and direct stop production (right) from [4].
An alternative approach to assess the overall state of the SUSY search program is to consider full SUSY models. A popular approach is to utilize the parameterized minimal supersymmetric standard model (pMSSM) which parameterizes SUSY with 19 free parameters after making several experimentally well motivated assumptions. Many SUSY signal points are then generated based on scanning the 19 parameters to provide a set of possible SUSY mass spectra. In [14] and [15] scans of pMSSM points are compared to a variety of results from the CMS and ATLAS collaborations, respectively. The points are then classified into those which are excluded by at least one of the searches and those that remain viable. The fraction of excluded points for gluino and slepton production from [15], for example, are show in Fig. 12. As expected, the lower mass points are more likely to be excluded and the results generally compare well to the simplified model results. However, certain of the pMSSM points that remain allowed can be studied in further detail to understand how to better design searches to capture sensitivity to these points in future searches.  Table 3. In both cases, the solid white lines overlaid are observed simplified-model limits from the 0-lepton + 2-6 jets + E miss T search [57] at 95% CL. In theg-˜ 0 1 case, the simplified-model limit is set assuming direct production of gluino pairs and that the squarks are decoupled, with gluino decaying to quarks and a neutralino,g ! q + q +˜ 0 1 . In theq-˜ 0 1 plane, both lines are drawn assuming directly produced first/second-generation squark pairs, with each squark decaying to a quark and a neutralino, q ! q +˜ 0 1 . The solid line corresponds to the case where all eight squarks from the first two generations are assumed to be degenerate. The dashed line has the squark production cross-section scaled down by a factor of four to emulate the e↵ect of only two of those eight squarks being kinematically accessible.  There is a closer correspondence between the pMSSM sensitivity and that of an alternative simplified  Table 1, projected onto the˜-˜ 0 1 mass plane, where˜is the lightest slepton of the first two generations. The white line reflects one of the simplified-model limits (Figure 8(c) of Ref.
[53]) made for direct slepton pair production assuming that left-and right-handed selectrons (or smuons) are mass-degenerate and that each decays via˜± !`± +˜ 0 1 . The colour scale is as described in Figure 3. reduced sensitivity is found in the pMSSM when compared to the more-constrained simplified model.

685
This can be understood by recognising that this particular model presupposes that the left-and right-686 handed selectrons and smuons are all mass degenerate, and that each has a 100% branching ratio to a 687 lepton and a LSP. Breaking these assumptions reduces the number of signal events, and hence allows 688 more models to evade detection.

689
When the assumption of degenerate left-and right-handed states is dropped from the simplified model,  Table 1, projected onto the plane of LSP and lightest stau mass. It can be seen that the Run 1 sensitivity 697 to staus is limited, with large fractions of model points surviving even at the lowest stau masses. This is 698 largely because it is di cult to trigger on events resulting from direct stau production, and backgrounds 699 to stau searches are much larger than for the equivalent search for sleptons of the first two generations.

700
No definitive lower bound can be placed on the stau mass by ATLAS after Run 1.

PREPARATION FOR 13 TeV
The year 2015 saw the restart of the LHC after "long shutdown 1" in 2013-2014. The shutdown allowed for the successful retuning of the LHC to achieve a record collision energy of √ s = 13 TeV. At the time of the LHCP conference, each experiment had collected a few dozen pb −1 worth of 13 TeV data, which was used to commission the 13 TeV SUSY searches [16,17,18]. In this section, results of these commissioning exercises are shown. Figure 13 shows the trigger efficiencies as measured in 13 TeV data for triggers based on HT and MET. Such triggers are utilized for hadronic SUSY searches. Figure 14 shows distributions of sensitive SUSY variables in single lepton control samples compared to MC simulation. In the left plot, a sample with no b-tagged jets is selected to test the modeling of the W + jets background, while in the right plot, a single muon sample is selected and visible energy templates are used to predicted the hadronic tau background. In both cases, the 13 TeV is is observed to be in good agreement with expectation from MC. HT800 trigger provides common sample for high mass hadronic gluino search HT350_MET100 trigger targets lower mass, e.g., compressed models Measure rates and efficiencies with 50 ns data.

2
Jets and E T miss calculated using full particle flow objects after a fast pre-filter using calorimeter only.  Another important background is W or top with hadronic τ decays.
Measure with muon control sample by emulating τ jet from muon.  Figure 14: Early 13 TeV data commissioning plots for lost lepton (left) and hadronic tau (right) backgrounds for hadronic SUSY searches. Taken from Ref. [16].

All-hadronic search in H T and H T miss
In Fig. 15 (left) dilepton control sample events are plotted compared to simulation to test the understanding of Z + jets background prediction techniques. In Fig. 15 (right), the distribution of MET/ √ HT is plotted comparing 13 TeV data and MC simulation for multijet events, showing good understanding of the QCD multijet background prediction methods. Figure 16 shows 13 TeV data commissioning results for the same-sign dilepton search. In the left plot, the isolation distribution for identified muons is plotted, showing good agreement between data and simulation. In the right plot, the di-electron mass is shown for same-sign and opposite-sign dilepton events, which is a key ingredient in measuring the rate of wrong charge assignment in electrons.
In summary, the 13 TeV data are remarkably well understood only a short period after data taking. Both experiments are well on track for producing SUSY search results when a sufficient amount of 13 TeV is available. First results are expected based on the full 2015 dataset.  in control regions CRW (left) and CRT (right), after requiring an isolated electron or muon with p T > 25 GeV, at least two jets with p T > 100, 60 GeV respectively, E miss T > 100 GeV and 30 GeV < m T (`, E miss T ) < 100 GeV. The total MC background expectation is normalised to data. In the lower panels the ratio of data to total MC background expectation is presented. The contribution from the multi-jet production is found to be negligible. Only statistical uncertainties are shown, without any experimental or theoretical systematic uncertainties. in control region CRZ after requiring two isolated leptons of opposite sign and identical flavour with p T > 25 GeV, at least two jets with p T > 100, 60 GeV respectively, E miss T > 100 GeV and 66 GeV < m``< 116 GeV. The total MC background expectation is normalised to data. In the lower panels the ratio of data to total MC background expectation is presented. The contribution from the multi-jet production is found to be negligible. Only statistical uncertainties are shown, without any experimental or theoretical systematic uncertainties.
Measure the lepton fake rate as a function of kinematics using observables such as isolation.