First ATLAS searches for signals of Supersymmetry in protonproton collisions at the LHC are presented. These searches are performed with the full data sample recorded in 2010, corresponding to an integrated luminosity of 35 

pb−1. Results for various channels with large missing transverse energy and different lepton and jet multiplicities are reported. A search for long-lived strongly interacting particles is also presented. Good consistency with the Standard Model prediction has been found in all channels; limits on squark and gluino masses are derived, which considerably improved previous results

Lari, T.

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DOI 10.1393/ncc/i2011-11090-9Colloquia: IFAE 2011IL NUOVO CIMENTO Vol. 34 C, N. 6 Novembre-Dicembre 2011Searches for supersymmetry with the ATLAS detectorT. Lari for the ATLAS CollaborationINFN, Sezione di Milano - Milano, Italy(ricevuto il 29 Luglio 2011; pubblicato online il 21 Dicembre 2011)Summary. — First ATLAS searches for signals of Supersymmetry in proton-proton collisions at the LHC are presented. These searches are performed withthe full data sample recorded in 2010, corresponding to an integrated luminosityof 35 pb−1. Results for various channels with large missing transverse energy anddifferent lepton and jet multiplicities are reported. A search for long-lived stronglyinteracting particles is also presented. Good consistency with the Standard Modelprediction has been found in all channels; limits on squark and gluino masses arederived, which considerably improved previous results.PACS 12.60.Jv – Supersymmetric models.1. – IntroductionSupersymmetry (SUSY) is one of the most promising candidates for extending theStandard Model. It is based on a new symmetry between fermions and bosons andtherefore predicts a large number of new particles [1]. These new particles need to beheavier than their Standard Model counterparts, since they have not been discovered yet.On the other hand, if they are not too heavy, and accessible with TeV scale energies,they would reduce the fine-tuning problem of the Standard Model. This argument makesthe search for SUSY particles an important part of the LHC physics program.In addition, a new quantum number called R-parity is introduced to avoid a fastproton decay. The conservation of this quantum number implies that the lightest SUSYparticle (LSP) is stable, providing a promising candidate to explain the nature of DarkMatter.The production at the LHC of scalar quarks and gluinos decaying (possibly withintermediate steps) into a weakly interacting LSP would be observed as an excess ofevents with large missing transverse energy ( EmissT ) and high transverse momentum (pT )jets. Additional particles, such as leptons or photons, might also be produced in theSUSY decay chains, depending on the mass pattern of the new particles.Since the SUSY breaking mechanism that determines this pattern is not known, theexperimental searches need to be quite generic. In the following searches for supersym-metry performed with the ATLAS detector [2] are presented for final states with largec© CERN on behalf of the ATLAS Collaboration under CC BY-NC 3.0 303304 T. LARI for the ATLAS COLLABORATIONEmissT and no (sect. 2), one (sect. 3) or two (sect. 4) leptons. In sect. 5 searches whichrequire b-jets to enhance the sensitivity to third generation squarks are presented. Fi-nally, in sect. 6 a search for the scenario in which gluinos or squark have a lifetime largerthan the time of flight through the detector are presented.These searches use the full 2010 data set. After application of beam, detector and dataquality requirements the integrated luminosity is 35 pb−1 with an uncertainty of 11%.2. – Searches with jets and missing transverse momentumThis analysis [3] defines several signal selections, which aim at maximal sensitivityfor different regions of the squark-gluino mass plane.All the selections require the presence of one jet with pT > 120GeV and EmissT >100GeV, which guarantees a trigger efficiency larger than 97%, and veto the presenceof an isolated electron or muon with pT > 20GeV. They differ for the number of jetswith pT > 40GeV (at least two or at least three) and for the cuts on the effective massand mT2. The former is the scalar sum of the selected jet pT and EmissT , while the latteris a generalization of transverse mass for two particles decaying into a jet and missingtransverse momentum [3].The two jet selections are optimal for squark pair production, followed by q˜ → qχ01,while the requirement of three jets is more suited to signal involving gluinos, for whichmore jets are expected from g˜ → qqχ01.The main backgrounds are multi-jet production (QCD), Z+jets, W+jets, and top pairproduction. The multi-jet background is suppressed by a cut on the azimuthal angulardifference between the EmissT vector and the leading jets; control regions are obtainedreversing this cut. The Monte Carlo prediction normalized to the rate observed in thesecontrol regions is consistent with a data-driven estimate obtained smearing a multi-jet control sample with low EmissT with the measured jet resolution. The Monte Carlodescription of the Z(νν)+jets and W(τν)+jets backgrounds was verified using controlselections of Z candidates decaying into two leptons and leptonic W decays.The number of observed data events in the four signal regions was found to be con-sistent with the expected Standard Model background. Limits have been placed on botha simplified model containing only squarks of the first two generations, a gluino octetand a massless neutralino, as well as on minimal supergravity (also called constrainedMSSM) models (mSUGRA/CMSSM) [4]. In the simplified model, gluino masses below500GeV are excluded at the 95% confidence level with the limit increasing to 870GeVfor equal mass squarks and gluinos. In the mSUGRA models equal mass squark andgluinos below 775GeV are excluded. Both exclusion plots are shown in fig. 1(1).3. – Searches with jets, a lepton and missing transverse energyThe signal region of this analysis [6] requires the presence of one isolated electron ormuon with pT > 20GeV, three jets with pT > 30GeV, of which at least one possessespT > 60GeV, EmissT > 125GeV, EmissT > 0.25meff and meff > 500GeV, where theeffective mass meff is defined as the scalar sum of the pT of the three leading jets, thepT of the lepton and EmissT . The lepton is the basis of the trigger selection and greatly(1) More stringent preliminary limits have been recently placed by ATLAS in this search channelusing 2011 data [5].SEARCHES FOR SUPERSYMMETRY WITH THE ATLAS DETECTOR 305gluino mass [GeV]0 250 500 750 1000 1250 1500 1750 2000squark mass [GeV]025050075010001250150017502000 = 10 pbSUSYσ = 1 pbSUSYσ = 0.1 pbSUSYσ)10χ∼Squark-gluino-neutralino model (massless =7 TeVs, -1 = 35 pbintL0 lepton combined exclusionATLASq~LEP 2 FNAL MSUGRA/CMSSM, Run ID0 MSUGRA/CMSSM, Run IICDF MSUGRA/CMSSM, Run IIObserved 95% CL limitMedian expected limitσ1±Expected limit  [GeV]0m200 400 600 800 1000 [GeV]1/2m150200250300350400 (400)g~ (600)g~ (800)g~ (400)q~ (600)q~ (800)q~>0.μ= 0, 0 = 3, AβMSUGRA/CMSSM: tan=7 TeVs, -1 = 35 pbintL0 lepton combined exclusionATLASReference point±  l~LEP 2 1± χ∼LEP 2 20χ ~,1± χ∼D0 -1<0, 2.1 fbμ, q~, g~D0 -1<0, 2 fbμ=5, β, tanq~,g~CDF Observed 95% CL limitMedian expected limitσ1±Expected limit -1, 35pbTαCMS Fig. 1. – Left plot: 95% exclusion limits in the (gluino, squark) mass plane for a simplifiedMSSM model with a massless neutralino. Comparison with existing limits is illustrative onlyas some are derived in the context of mSUGRA/CMSSM and might not assume a masslessneutralino. Right plot: 95% CL exclusion limits in the slice of mSUGRA/CMSSM definedby tan(β) = 3, A0 = 0 and μ = 0 [4], together with existing limits with the different modelassumptions given in the legend.reduces the QCD multi-jet background. Further reduction is achieved by a cut on thesmallest azimuthal separation between the EmissT vector and the three leading jets. Themain backgrounds which survive these cuts are W+jets and top pair production, whichare suppressed by requiring the transverse mass MT between the lepton and the missingtransverse momentum to be larger than 100GeV.In order to determine the background prediction for the signal region, several controlselections have been defined. The W and top control selections are defined by keepingthe same jet and lepton selection of the signal region but requiring 30 < EmissT < 80GeVand 40 < MT < 80GeV. The requirement of no or at least one b-tag jet creates twocontrol selections dominated by W and top pair production, respectively. The transferof the normalization factors measured in the control region to the signal region is doneby Monte Carlo and the uncertainty is carefully studied.The left plot of fig. 2 shows the distribution of effective mass after all the other cuts.In the electron selection 1.81 ± 0.75 background events are expected and one observed.In the muon channel 2.25±0.94 are expected and one is observed. The resulting limit onthe effective cross section of new processes in the signal region, including the effects ofexperimental acceptance and efficiency, is 0.065 pb and 0.073 pb for the electron and muonchannels, respectively. Limits are also set on the parameters of the mSUGRA/CMSSMmodel, shown in the right plot of fig. 2. For equal squark and gluino masses, gluinomasses below 700GeV are excluded at 95% confidence level. Preliminary results fromthe analysis of 2011 data are reported in [7].4. – Searches with two leptons and missing transverse energyEvents with two isolated leptons are a potentially clean signal for SUSY particles. Asearch was performed [8] for events with opposite sign or same sign lepton pairs. Thesignal region was defined by EmissT > 150GeV for opposite sign pairs and EmissT > 100GeV306 T. LARI for the ATLAS COLLABORATIONFig. 2. – Left plot: effective mass of the candidates selected by the 1-lepton analysis after all selec-tion criteria except the cut on effective mass itself. The hatched band indicates the uncertaintyon the Monte Carlo prediction. Right plot: 95% CL exclusion limits in the mSUGRA/MSSMslice defined by tan(β) = 3, A0 = 0 and μ = 0 [4], together with existing limits with the differentmodel assumptions given in the legend.for same sign pairs. The main backgrounds are top pair production for the opposite signchannel and events with at least one fake or non-isolated lepton for the same sign channel.The fake background is estimated from the number of observed events with a looser leptonselection and measurements in a control sample of the probability of isolated and fakelepton to pass the tighter selection. The top background is estimated from a dedicatedcontrol selection. Again no significant excess is observed in either the opposite or samesign selection.Under the hypothesis of a scalar lepton lighter than the second lightest neutralino(which enhances the fraction of signal events with leptons) limits are placed on thesquark and gluino masses, as shown in fig. 3.Dilepton events were also studied with the “flavour subtraction” method [9]. Thissearch is sensitive to the correlated production of leptons of the same flavour, for ex-ample in χ02 → ll˜ → l+l−χ01 and subtracts the observed number of eμ events from theobserved same flavour pairs. Background in which the production of the two leptons isuncorrelated, like top pair events, cancel in the subtraction. Again, no significant excesswas observed. [GeV]g~m300 350 400 450 500 550 600 650 700 750 800 [GeV]q~m300350400450500550600650700750800 [GeV]q~m [GeV]q~mMSSM PhenoGrid2=7 TeVs,  -1 = 35 pbintL2-lepton OS Observed limit 95% CL (Comp. spectrum)Median expected limit (Comp. spectrum)Observed limit 95% CL (Light neutralino)Median expected limit (Light neutralino)ATLAS [GeV]g~m300 350 400 450 500 550 600 650 700 750 800 [GeV]q~m300350400450500550600650700750800 [GeV]q~m [GeV]q~mObserved limit 95% CL (Comp. spectrum)Median expected limit (Comp. spectrum)Observed limit 95% CL (Light neutralino)Median expected limit (Light neutralino)MSSM PhenoGrid2=7 TeVs,  -1 = 35 pbintL2-lepton SSATLASFig. 3. – Limits on the squark and gluino masses placed by the searches for opposite sign (left)and same sign (right) lepton pairs and large missing transverse momentum.SEARCHES FOR SUPERSYMMETRY WITH THE ATLAS DETECTOR 3075. – Searches with b-jets and missing transverse momentumEvents with jets, no or one lepton and missing transverse momentum are also studiedwith a b-tag requirement, in order to increase the sensitivity to SUSY decay chainsinvolving third generation squarks [10]. The dominant background source is top pairproduction in both the zero and one lepton selection. Since no significant excess is foundover Standard Model expectations, limits have been placed on several scenarios. Underthe hypothesis that the gluino decays in sbottom-bottom pairs with 100% branchingratio, gluino masses below 590GeV are excluded.6. – Searches for new slow-moving massive particlesATLAS also pursued a search for slow-moving particles (SMPs) with color charge [11].Such particles are expected if small couplings or decays proceeding via highly virtualparticles makes the lifetime of some squark or gluinos much longer than the time of flightthrough the detector. These particles would hadronize, forming so called R hadrons.In this search, the ionization measured by the pixel detector (dE/dx) and the time offlight measured by the tile calorimeter were used to measure the speed of charged parti-cles. Events are triggered by EmissT and an Inner Detector track requirement. For eachevent, the mass is determined by dividing the momentum by βγ. Since the two velocitymeasurements and the momentum measurements are not correlated, the background isestimated by random combinations of measured momentum and velocity values.No event with both measurements of the mass larger than 100GeV was found. De-pending on the model assumed for the interactions of R-hadrons with matter, a 95%CL limit on stable gluinos between 562 and 586GeV is derived, while the lower masslimits for scalar sbottom and stops are found to be 294GeV and 309GeV. Each of theseconstraints is the most stringent to date.7. – ConclusionsDue to the large cross section for squark and gluinos the sensitivity of 2010 LHCdata to these particles exceeds by far that of all previous collider experiments. ATLASperformed a search for a supersymmetric signal in a variety of final states. No signal hasbeen observed yet, and the most stringent limits have been placed in many scenarios.REFERENCES[1] For a review, see Martin S. P., arXiv/9709356.[2] ATLAS Collaboration, JINST, 3 (2008) S08003.[3] ATLAS Collaboration, Phys. Lett. B, 701 (2011) 186.[4] Kane G. L. et al., Phys. Rev. D, 49 (1994) 6173 and references therein.[5] ATLAS Collaboration, ATL-CONF-2011-086.[6] ATLAS Collaboration, Phys. Rev. Lett., 106 (2011) 131802, arXiv:1102.2357.[7] ATLAS Collaboration, ATL-CONF-2011-090.[8] ATLAS Collaboration, Eur. Phys. J. C, 71 (2011) 1682, arXiv:1103.6214.[9] ATLAS Collaboration, Eur. Phys. J. C, 71 (2011) 1647, arXiv:1103.6208.[10] ATLAS Collaboration, Phys. Lett. B, 701 (2011) 398, arXiv:1103.4344.[11] ATLAS Collaboration, Phys. Lett. B, 701 (2011) 1, arXiv:1103.1984.

Searches for supersymmetry with the ATLAS detector

First ATLAS searches for signals of Supersymmetry in proton-proton collisions at the LHC are presented. These searches are performed with the full data sample recorded in 2010, corresponding to an integrated luminosity of 35 pb-1. Results for various channels with large transverse missing energy and different jet and lepton multiplicity are reported. A search for long lived strongly interacting particles is also presented. Good consistency with the Standard Model predictions has been found in all channels; limits on squark and gluino masses are derived, which considerably improve previous results

Lari, T

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Searches for supersymmetry with the ATLAS detector

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