Search for supersymmetry using final states with one lepton, jets, and missing transverse momentum with the ATLAS detector in sqrt{s} = 7 TeV pp

This Letter presents the first search for supersymmetry in final states containing one isolated electron or muon, jets, and missing transverse momentum from sqrt{s} = 7 TeV proton-proton collisions at the LHC. The data were recorded by the ATLAS experiment during 2010 and correspond to a total integrated luminosity of 35 pb-1. No excess above the standard model background expectation is observed. Limits are set on the parameters of the minimal supergravity framework, extending previous limits. For A_0 = 0 GeV, tan beta = 3, mu>0 and for equal squark and gluino masses, gluino masses below 700 GeV are excluded at 95% confidence level.

Many extensions of the standard model predict the existence of new colored particles, such as the squarks (q) and gluinos (g) of supersymmetric (SUSY) theories [1], which could be accessible at the LHC. The dominant SUSY production channels are squark-(anti)squark, squark-gluino, and gluino-gluino pair production. Squarks and gluinos are expected to decay to quarks and gluons and the SUSY partners of the gauge bosons (charginos,χ ± , and neutralinos,χ 0 ), leading to events with energetic jets. In R-parity conserving SUSY models [2], the lightest supersymmetric particle (LSP) is stable and escapes detection, giving rise to events with significant missing transverse momentum. In decay chains with charginos (q L → qχ ± ,g → qq ′χ ± ), chargino decay to the LSP can produce a high-momentum lepton. Currently, the most stringent limits on squark and gluino masses come from the LHC [3] and from the Tevatron [4][5][6].
This Letter reports on a search for events with exactly one isolated high-transverse momentum (p T ) electron or muon, at least three high-p T jets, and significant missing transverse momentum. An exact definition of the signal region will be given elsewhere in this Letter. From an experimental point of view, the requirement of an isolated high-p T lepton suppresses the QCD multijet background and facilitates triggering on interesting events. In addition to the signal region, three control regions are considered for the most important standard model backgrounds. A combined fit to the observed number of events in these four regions, together with an independent estimate of jets misidentified as leptons in QCD multijet events, is used to search for an excess of events in the signal region.
The analysis is sensitive to any new physics leading to such an excess, and is not optimized for any particular model of SUSY. The results are interpreted within the MSUGRA/CMSSM (minimal supergravity/constrained minimal supersymmetric standard model) framework [7,8] in terms of limits on the universal scalar and gaugino mass parameters m 0 and m 1/2 . These are presented for fixed values of the universal trilinear coupling parameter A 0 = 0 GeV, ratio of the vacuum expectation values of the two Higgs doublets tan β = 3, and Higgs mixing parameter µ > 0, in order to facilitate comparison with previous results.
The ATLAS detector [9] is a multipurpose particle physics apparatus with a forward-backward symmetric cylindrical geometry and near 4π coverage in solid angle [10]. The inner tracking detector (ID) consists of a silicon pixel detector, a silicon microstrip detector (SCT), and a transition radiation tracker (TRT). The ID is surrounded by a thin superconducting solenoid providing a 2 T magnetic field, and by high-granularity liquid-argon (LAr) sampling electromagnetic calorimeters. An ironscintillator tile calorimeter provides hadronic coverage in the central rapidity range. The end-cap and forward regions are instrumented with LAr calorimetry for both electromagnetic and hadronic measurements. The muon spectrometer (MS) surrounds the calorimeters and consists of three large superconducting toroids, a system of precision tracking chambers, and detectors for triggering.
The data used in this analysis were recorded in 2010 at the LHC at a center-of-mass energy of 7 TeV. Application of beam, detector, and data-quality requirements results in a total integrated luminosity of 35 pb −1 , with an estimated uncertainty of 11% [11]. The data have been selected with single lepton (e or µ) triggers. The detailed trigger requirements vary throughout the datataking period, but the thresholds are always low enough to ensure that leptons with p T > 20 GeV lie in the efficiency plateau.
Fully simulated Monte Carlo event samples are used to develop and validate the analysis procedure, compute detector acceptance and reconstruction efficiency, and aid in the background determination. Samples of events for background processes are generated as described in detail in Ref. [12]. For the major backgrounds, top quark pair and W +jets production, MC@NLO [13] v3.41 and ALPGEN [14] v2.13 are used. Further samples include QCD multijet events, single top production, diboson production, and Drell-Yan dilepton events.
Criteria for electron and muon identification closely follow those described in Ref. [21]. Electrons in the signal region are required to pass the "tight" selection criteria, with p T > 20 GeV and |η| < 2.47. Events are always vetoed if a "medium" electron is found in the electromagnetic calorimeter transition region, 1.37 < |η| < 1.52.
Muons are required to be identified either in both ID and MS systems (combined muons) or as a match between an extrapolated ID track and one or more segments in the MS. The ID track is required to have at least one pixel hit, more than five SCT hits, and a number of TRT hits that varies with η. For combined muons, a good match between ID and MS tracks is required, and the p T values measured by these two systems must be compatible within the resolution. The summed p T of other ID tracks within a distance ∆R = (∆η) 2 + (∆φ) 2 < 0.2 around the muon track is required to be less than 1.8 GeV. Only muons with p T > 20 GeV and |η| < 2.4 are considered. For the final selection, the distance between the z coordinate of the primary vertex and that of the extrapolated muon track at the point of closest approach to the primary vertex must be less than 10 mm.
Jets are reconstructed using the anti-k t jet clustering algorithm [22] with a radius parameter R = 0.4. The inputs to this algorithm are clusters of calorimeter cells seeded by cells with energy significantly above the measured noise. Jets are constructed by performing a four-vector sum over these clusters, treating each cluster as an (E, p) four-vector with zero mass. Jets are corrected for calorimeter non-compensation, upstream material and other effects using p T -and η-dependent calibration factors obtained from Monte Carlo and validated with extensive test-beam and collision-data studies [23]. Only jets with p T > 20 GeV and |η| < 2.5 are considered. If a jet and a "medium" electron are both identified within a distance ∆R < 0.2 of each other, the jet is discarded. Furthermore, identified "medium" electrons or muons are only considered if they satisfy ∆R > 0.4 with respect to the closest remaining jet. Events are discarded if they contain any jet failing basic quality selection criteria, which reject detector noise and non-collision backgrounds [24].
The calculation of the missing transverse momentum, E miss T , is based on the modulus of the vectorial sum of the p T of the reconstructed objects (jets with p T > 20 GeV, but over the full calorimeter coverage |η| < 4.9, and the selected lepton), any additional non-isolated muons and the calorimeter clusters not belonging to reconstructed objects.
Events are required to have at least one reconstructed primary vertex with at least five associated tracks. The selection criteria for signal and control regions are based on Monte Carlo studies prior to examining the data. The signal region is defined as follows. At least one identified electron or muon with p T > 20 GeV is required. Events are rejected if they contain a second identified lepton with p T > 20 GeV, because they are the subject of a future analysis. At least three jets with p T > 30 GeV are required, the leading one of which must have p T > 60 GeV. In order to reduce the background of events with fake E miss T from mismeasured jets, the missing transverse momentum vector E miss T is required not to point in the direction of any of the three leading jets: ∆φ(jet i , E miss T ) > 0. 2 (i = 1, 2, 3). The transverse mass between the lepton and the missing transverse momentum vector, , is required to be larger than 100 GeV. E miss T must exceed 125 GeV and must satisfy E miss T > 0.25 m eff , where the effective mass m eff is the scalar sum of the p T of the three leading jets, the p T of the lepton, and E miss T . Finally, a cut is applied on the effective mass: m eff > 500 GeV. The efficiency for the SUSY signal in the MSUGRA/CMSSM model defined earlier varies between 0.01% for m 1/2 = 100 GeV and 4% for m 1/2 = 350 GeV, with a smaller dependence on m 0 , for the electron channel and the muon channel separately. The inefficiency is dominated by the leptonic branching fractions in the SUSY signal.
Backgrounds from several standard model processes could contaminate the signal region. Top quark pair production and W +jets production backgrounds are estimated from a combined fit to the number of observed events in three control regions, using Monte Carlo simulations to derive the background in the signal region from the control regions. The background determination of QCD multijet production with a jet misidentified as an isolated lepton is purely data driven. Remaining backgrounds from other sources are estimated with simulations.
The three control regions have identical lepton and jet selection criteria as the signal region. The top control region is defined by a window in the two-dimensional plane of 30 GeV < E miss T < 80 GeV and 40 GeV < m T < 80 GeV and by requiring that at least one of the three leading jets is tagged as a b-quark jet. For the b-tagging, the secondary vertex algorithm SV0 [25] is used, which, for p T = 60 GeV jets, provides an efficiency of 50% for bquark jets and a mistag rate of 0.5% for light-quark jets. The W control region is defined by the same window in the E miss T − m T plane, but with the requirement that none of the three hardest jets is b-tagged. The QCD mul-tijet control region is defined by demanding low missing transverse momentum, E miss T < 40 GeV, and low transverse mass, m T < 40 GeV. This QCD control region is only used to estimate the QCD multijet background contribution to other background regions but not to the signal region. Instead, the electron and muon identification criteria are relaxed, obtaining a "loose" control sample that is dominated by QCD jets. A loose-tight matrix method, in close analogy to that described in Ref. [12], is then used to estimate the number of QCD multijet events with fake leptons in the signal region after final selection criteria: 0.0 +0.5 −0.0 in the muon channel and 0.0 +0.3 −0.0 in the electron channel.
Data are compared to expectations in Figure 1. The standard model backgrounds in the figure are normalized to the theoretical cross sections, except for the multijet background which is normalized to data in the QCD multijet control region. The data are in good agreement with the standard model expectations. After final selection, one event remains in the signal region in the electron channel and one event remains in the muon channel. Figure 1 also shows the expected distributions for the MSUGRA/CMSSM model point m 0 = 360 GeV and m 1/2 = 280 GeV.
A combined fit to the number of observed events in the signal and control regions is performed. The assumption that the Monte Carlo is able to predict the backgrounds in the signal region from the control regions is validated by checking additional control regions at low m T and at low E miss T . The defined control regions are not completely pure, and the combined fit takes the expected background cross-contaminations into account. The likelihood function of the fit can be written as: L(n|s, b, θ) = P S × P W × P T × P Q × C Syst , where n represents the number of observed events in data, s is the SUSY signal to be tested, b is the background, and θ represents the systematic uncertainties, which are treated as nuisance parameters with a Gaussian probability density function. The four P functions in the right hand side are Poisson probability distributions for event counts in the defined signal (S) and control regions (W, T, and Q for W , top pair and QCD multijets respectively), and C Syst represents the constraints on systematic uncertainties, including correlations.
The dominant sources of systematic uncertainties in the background estimates arise from Monte Carlo modeling of the shape of the E miss T and m T distributions in signal and control regions. These uncertainties are determined by variation of the Monte Carlo generator, as well as by variations of internal generator parameters. Finite statistics in the background control regions also contributes to the uncertainty. Experimental uncertainties are varied within their determined range and are dominated by the jet energy scale uncertainty [26], b-tagging uncertainties, and the uncertainty on the luminosity.
Systematic uncertainties on the SUSY signal are esti- mated by variation of the factorization and renormalization scales in Prospino and by including the parton density function (PDF) uncertainties using the eigenvector sets provided by CTEQ6 [27]. Uncertainties are calculated separately for the individual production processes. Within the relevant kinematic range, typical uncertainties resulting from scale variations are 10-16%, whereas PDF uncertainties vary from 5% forqq production to 15-30% forgg production.
The result of the combined fit to signal and control regions, leaving the number of signal events free in the signal region while not allowing for a signal contamination in the other regions, is shown in Table I. The observed number of events in data is consistent with the standard model expectation.
Limits are set on contributions of new physics to the signal region. These limits are obtained from a second combined fit to the four regions, this time allowing for a signal in all four regions, and leaving all nuisance parameters free. The limits are then derived from the profile likelihood ratio, Λ(s) = −2(ln L(n|s,b,θ) − ln L(n|ŝ,b,θ)), whereŝ,b andθ maximize the likelihood function andb andθ maximize the likelihood for a given choice of s. In the fit, s andŝ are constrained to be non-negative. The test statistic is Λ(s). The exclusion p-values are obtained from this using pseudo-experiments and the limits set are one-sided upper limits [28].
From the fit to a model with signal events only in the signal region, a 95% CL upper limit on the number of events from new physics in the signal region can be derived. This number is 2.2 in the electron channel and 2.5 in the muon channel. This corresponds to a 95% CL upper limit on the effective cross section for new processes in the signal region, including the effects of experimental acceptance and efficiency, of 0.065 pb for the electron channel and 0.073 pb for the muon channel.
Within the MSUGRA/CMSSM framework, the results are interpreted as limits in the m 0 −m 1/2 plane, as shown in Figure 2. For the model considered and for equal squark and gluino masses, gluino masses below 700 GeV are excluded at 95% CL. The limits depend only moderately on tan β.
In summary, the first ATLAS results on searches for supersymmetry with an isolated electron or muon, jets, and missing transverse momentum have been presented. In a data sample corresponding to 35 pb −1 , no significant deviations from the standard model expectation are observed. Limits on the cross section for new processes within the experimental acceptance and efficiency are set. For a chosen set of parameters within MSUGRA/CMSSM, and for equal squark and gluino masses, gluino masses below 700 GeV are excluded at 95% CL. These ATLAS results exceed previous limits set by other experiments [3][4][5][6].
We wish to thank CERN for the efficient commission-

ATLAS
Observed limit 95% CL Median expected limit =5, 2 fb β , tan q , g CDF FIG. 2: Observed and expected 95% CL exclusion limits, as well as the ±1σ variation on the expected limit, in the combined electron and muon channels. Also shown are the published limits from CMS [3], CDF [4], and D0 [5,6], and the results from the LEP experiments [29].
ing and operation of the LHC during this initial highenergy data-taking period as well as the support staff from our institutions without whom ATLAS could not be operated efficiently.