The CERN Large Hadron Collider (LHC) is scheduled to commence operation in
2008 and inclusive searches for supersymmetry (SUSY) will be one of our primary
tasks in the first days of LHC operation. It is certain that the final state of
multijets + missing transverse energy will provide a superior performance in
SUSY searches. Strategies to understand the instrumental background and to
understand the Standard Model (SM) background are still under development and
are urgent issues for the coming data. We describe the strategy for early SUSY
searches at the ATLAS experiment using the fist data corresponding to an
integrated luminosity up to 1fb^-1, which comprises much progress in the
data-driven technique for the SM background estimations.Comment: Submitted for the SUSY07 proceedings, 4 pages, LaTeX, 8 eps figure

Yamamoto, Shimpei

English

arXiv

The CERN Large Hadron Collider (LHC) is scheduled to commence operation in 2008 and inclusive searches for supersymmetry (SUSY) will be one of our primary tasks in the first days of LHC operation. It is certain that the final state of multijets plus missing transverse energy will provide a superior performance in SUSY searches. As yet, well-considered strategies for the understanding of instrumental effects of detectors and the realistic estimations of the Standard Model (SM) backgrounds would not be clear: they are urgent issues for the coming data. We describe the strategy for early SUSY searches at the ATLAS experiment using the fist data corresponding to the integrated luminosity up to 1fb^-1, which comprises many progresses in the data-driven technique for the SM background estimations.The CERN Large Hadron Collider (LHC) is scheduled to commence operation in 2008 and inclusive searches for supersymmetry (SUSY) will be one of our primary tasks in the first days of LHC operation. It is certain that the final state of multijets + missing transverse energy will provide a superior performance in SUSY searches. Strategies to understand the instrumental background and to understand the Standard Model (SM) background are still under development and are urgent issues for the coming data. We describe the strategy for early SUSY searches at the ATLAS experiment using the fist data corresponding to an integrated luminosity up to 1fb^-1, which comprises much progress in the data-driven technique for the SM background estimations

CERN Document Server

Strategy for early SUSY searches at ATLAS

The CERN Large Hadron Collider (LHC) is scheduled to commence operation in 2008 and inclusive searches for supersymmetry (SUSY) will be one of our primary tasks in the first days of LHC operation. It is certain that the final state of multijets plus missing transverse energy will provide a superior performance in SUSY searches. As yet, well-considered strategies for the understanding of instrumental effects of detectors and the realistic estimations of the Standard Model (SM) backgrounds would not be clear: they are urgent issues for the coming data. We describe the strategy for early SUSY searches at the ATLAS experiment using the fist data corresponding to the integrated luminosity up to 1fb^-1, which includes many progresses in the data-driven technique for the SM background estimations

Yamamoto, S

Strategy for early SUSY searches at ATLASJuly 26, 2007Shimpei Yamamoto (ICEPP, University of Tokyo)on behalf of the ATLAS collaboration“SUSY07” The 15th International Conference on Supersymmetry and the Unification of Fundamental InteractionsKarlsruhe, Germany2S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)Outline? Introduction? Strategy for early SUSY searches? Commissioning for missing transverse energy? Data-driven estimation for SUSY backgrounds? Summary3S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)Introduction? Large Hadron Collider (LHC) is getting ready for 2008 spring startup with hardware fully commissioned up to Ebeam=7TeV.? ATLAS, one of two general purpose experiments/detectors at LHC, is also approaching completion toward the first p-p collision at sqrt(s)=14TeV in 2008 summer.Current SUSY studies at ATLAS:─ Usual SUSY breaking scenarios: mSUGRA, GMSB, AMSB─ Full detector simulation basis for detector commissioning and understanding systematics.─ Techniques to estimate background using real data.─ (Post-discovery measurements of mass scale etc. and unusual signatures of long-lived stau and R-hadron.)This talk focuses on the early SUSY searches (first data of 1031-32cm-2s-1,  up to ~1fb-1)1. Strategy for early SUSY searches5S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)Strategy for early SUSY searches? We use the golden discovery channels “multi-Jets + n-leptons + ETmiss”─ Fair and less model-dependent features of SUSY :? gluinos/squarks are produced via strong interactions.? gluinos/squarks are heaviest.? their decays give rise to high-pT jets? neutralinos/charginos decay via emission of leptons? (assuming R-parity conservation) LSP is stable and neutral, escaping from the detector (large ETmiss) ? Early SUSY studies will proceed :─ Commissioning for missing transverse energy (ETmiss)  ? part 2.─ Data-driven background estimation  ? part 3.lqqlg~ q~l~χ02~ χ01~p pNeed to establish a robust background estimation6S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)Event topologies and baseline selectionBaseline selection (to be optimized)? Jet multiplicity ≥ 4,  pT1st > 100GeV,  pTothers > 50GeV? ETmiss > max(100GeV, 0.2xMeff)? Transverse sphericity > 0.2? (Additional cuts depending on signature)─ Transverse mass > 100GeV ,  pTlepton > 20GeV ( for one-lepton mode)Jet multiplicity Additional signature SUSY scenario BackgroundsNo leptonmSUGRA, AMSB, split SUSY, heavy squarkQCD, ttbar, W/ZOne lepton (e,μ) mSUGRA, AMSB, split SUSY, heavy squarkttbar, Wdi-lepton mSUGRA, AMSB, GMSB ttbardi-tau GMSB, large tanβ ttbar, Wγγ GMSB free~2 light squark Z≥ 4Early searches try to cover a broad range of experimental signatures, but they are classified based on the event topology:Large ETmiss +7S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)SUSY searches with each event topology? Effective mass (Meff = Σi pTi + ETmiss) discriminates SUSY and SM backgrounds.? Moreover, Meff is strongly correlated to the mass of sparticles produced via p-p collisions.? We hopefully find excess over the background estimation in the signal region of large Meff.ttbar (→lνlν)+jetsttbar (→lνqq)+jetsW+jetsAll backgroundsSUSY signal (SU3)ttbar+jetsZ (→νν)+jetsW+jetsQCD jetsAll backgroundsSUSY signal (SU3)Meff (GeV) Meff (GeV)No-lepton One-lepton(full detector simulation, 1fb-1)Di-leptonBenchmark point SU3:  m0=100GeV, m1/2=300GeV, A0=-300, tanβ=6, sign(μ)=+ ATLAS PreliminaryMeff (GeV)ATLAS PreliminaryATLAS Preliminary8S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)Discovery potential with L~1fb-1? 5σ-discovery potential on m1/2-m0(mgluino-msquark) space is shown.─ Require S>10 and S/sqrt(B)>5─ Factor of 2 generator-level uncertainty included (hatched)? ATLAS has the potential for discovery of ~1TeV scale SUSY with first data for each event topology.─ Also shows a stable potential against tanβ.tanβ=10ATLAS PreliminaryATLAS Preliminary2. Commissioning for missing transverse energyusing early data10S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)Transverse missing energy? ETmiss is a discriminating variable for SUSY discovery─ Our searches rely on the excess in the ETmiss(Meff) distribution.? However, controlling its energy scale and resolution is very difficult experimentally.─ Fake muons─ Dead material and crack─ Industrial effects in the detector(hot, dead and noisy calorimeter cells)? Large tail in ETmiss due to the fake is serious for SUSY searches.─ Especially for QCD-jet background (almost no truth ETmiss, but large x-section)In-situ measurements for ETmiss scale/resolution determination and understanding of fake ETmiss sources are our priorities straight.D0 experience:(V. Shary@CALOR04)11S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)In-situ measurements for ETmiss scale/resolution determinationZ→ττ (l-h channel) sampleW→ lν sampleMττ vs. ETmiss scale (signal only)10% in ETmiss scale ? 3% shift in Z massZ→ττQCD jetsW →μνW→eνttbarmean = 90.7 GeVσ = 14.2 GeVInvariant mass (100pb-1)σ(Mττ) ~ ETmiss/|sinφττ|Shape of transverse mass distribution depends on ETmiss scale and resolution.-10% scale biasno scale biasσ(ETmiss) = 7GeVσ(ETmiss) = 8GeVUse the collinear approximation for mass reconstruction.ATLAS PreliminaryATLAS PreliminaryATLAS Preliminary12S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)Fake and large ETmiss tails? The industrial effect in the detector is crucial.? Its suppression is under study.─ Online/offline monitoring tools ─ Event-by-event basis corrections------- Perfect detector- - - - mis-calibration in Lar calo.-∙ -∙ - mis-calibration in Lar and tile calo.Detector failure (example):0.1% LAr EM HV lines2 LAr Frond-end crate (barrel and endcap)2 tile drawersETmiss (GeV)Z(→ee)+multi-jetsATLAS Preliminary3. Data-driven background estimation for early SUSY searches14S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)Data-driven background estimation for no-lepton signatureEven if we get early indications,  they cannot justify the discovery of “beyond SM”:─ Generator-level uncertainties affect the normalization and shapes of backgrounds.We use the real data for BG estimations (data-driven)Example of Z(→νν) background in no-lepton mode:? determine the BG MC normalization factor by comparing Z→ ll data with MC? apply it to normalize the MC distribution of Z→ νν.? it can be also applied to W→ lν (almost the same production mechanism)--- data (pseudo) --- estimated--- data (pseudo) --- estimatedZ→ νν W→ lνThis scaling method works fine!ETmiss (GeV) ETmiss (GeV)ATLAS Preliminary ATLAS Preliminary15S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)Including SUSY signal (SU3)•Satisfying performances with the MT discrimination technique.•However, taking account of SUSY signal contamination in the control sample, this estimate appears to be over the mark.  (By a factor of 2.5 for SU3)Meff (GeV)Data-driven background estimation for one-lepton signature#BG (Meff>800GeV):24.8±1.6 (real)60.8±2.5 (estimated)Events/100GeV/1fb-1signaleffective mass (@1fb-1)Meff (GeV)#BG (Meff>800GeV):24.8±1.6 (real)22.0±0.9 (estimated)Events/100GeV/1fb-1ATLAS PreliminaryATLAS PreliminaryMT (GeV)control region signal region ttbar (→lνlν)+jetsttbar (→lνqq)+jetsW+jetsAll backgroundsSUSY signal (SU3)ATLAS Preliminary1. Isolate the background and signal processes based on transverse mass (control sample with <100GeV, signal sample with >100GeV)2. Estimate the ETmiss/Meff shapes of background processes using control sample3. Determine the normalization of backgrounds with low ETmiss regions of control and signal samples.Method of estimate :step 1: Measure jet smearing function from data─ Select events: ETmiss > 60 GeV, Δφ(ETmiss, jet) < 0.1─ Estimate pT of jet closest to ETmiss as  pTestimate = pTjet + ETmissstep 2: Smear QCD events with low ETmiss by the smearing function.ETmissjetsfluctuating jetReproduces the ETmiss tail well!QCD background estimation? QCD background is hard to estimate based on MC:─ Mainly originated from the industrial fake ETmiss─ Full understanding of detector response needs time… (difficult at the early stage)ATLAS PreliminaryATLAS Preliminary17S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)Summary? The countdown to the startup of LHC and experiments has begun, and studies of early SUSY searches and detector commissioning have been performed actively.? The discovery potential using early data (1fb-1) reaches ~1TeV scale SUSY with each event topology.? However, establishing robust background estimations is crucial for the discovery at the early stage.─ Origins of fake ETmiss and tails are going to be made known with the full detector simulation.─ New techniques for ETmiss calibration have been also performed.─ Variety of data-driven SM background estimations have been studied.Large efforts of many working groups toward the early discovery.News of “beyond SM” hopefully in 2009 summer!18S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)SUSY search with gamma signatureLeading Photon Pt (GeV)G1A (Λ=90TeV)BG TotalL=1fb-12nd Leading Photon Pt (GeV)G1A (Λ=90TeV)BG TotalATLAS Preliminary ATLAS PreliminaryNon-pointing photon is a distinctive signature of GMSB:• NLSP decays only via gravitational coupling• NLSP could have a long lifetimeATLAS could cover GMSB requiring “photons + multi-jets + large ETmiss”.19S.Yamamoto    SUSY07 (Karlsruhe, July 06, 2007)1. Top mass is largely uncorrelated with ETmiss─ used as a calibration variable2. Select semi-leptonic top candidates─ mass window: 140-200 GeV3. Contributions of combinatorial BG to top mass are estimated from the side-band events (200GeV<mtop<260GeV) 4. Normalize the ETmiss distribution in low ETmiss region where SUSY signal contamination is small.5. Extrapolate it to high ETMISS region and  estimate the background with SUSY signal selection.ttbar sidebandttbar signalETmiss (GeV)ttbar background estimationEstimateSUSY selectionETmiss (GeV)top mass (GeV)ATLAS PreliminaryATLAS Preliminary

The CERN Large Hadron Collider (LHC) is scheduled to commence operation in 2008 and inclusive searches for supersymmetry (SUSY) will be one of our primary tasks in the first days of LHC operation. It is certain that the final state of âﾜmultijets + missing transverse energyâ will provide a superior performance in SUSY searches. As yet, well-considered strategies for the understanding of instrumental effects of detectors and the realistic estimations of the Standard Model (SM) backgrounds would not be clear: they are urgent issues for the coming data. We describe the strategy for early SUSY searches at the ATLAS experiment using the fist data corresponding to the integrated luminosity up to 1fbâ1, which comprises many progresses in the data-driven technique for the SM background estimations

Strategy for early SUSY searches at ATLASShimpei Yamamotoa (for the ATLAS collaboration)International Center for Elementary Particle Physics, the University of Tokyo7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, JapanAbstract. The CERN Large Hadron Collider (LHC) is scheduled to commence operation in 2008and inclusive searches for supersymmetry (SUSY) will be one of our primary tasks in the firstdays of LHC operation. It is certain that the final state of “multijets + missing transverse energy”will provide a superior performance in SUSY searches. Strategies to understand the instrumentalbackground and to understand the Standard Model (SM) background are still under developmentand are urgent issues for the coming data. We describe the strategy for early SUSY searches atthe ATLAS experiment using the fist data corresponding to an integrated luminosity up to 1fb−1,which comprises much progress in the data-driven technique for the SM background estimations.PACS. 11.30.Pb Supersymmetry1 IntroductionSupersymmetry (SUSY) imposes a new symmetry be-tween the fermions and bosons [1–5]. The supersym-metric extension of the Standard Model (SM) makesimprovements to phenomenological problems in thephysics of elementary particles: it provides a naturalsolution for the gauge hierarchy problem, if sparticlesexist at the TeV scale. Moreover, the extrapolation ofLEP data within the framework of supersymmetric ex-tension yields a precise unification of gauge couplingsat a scale of ∼ 1016 GeV [6,7]. Due to these proper-ties, SUSY is one of the most attractive alternativesbeyond the SM and has been the subject of many stud-ies in particle physics. However, up to now, no directevidence for SUSY has been found. It is essential toexamine the properties of any new states of matter atenergy scales close to the threshold for the new phe-nomena, or in high energy collisions at the TeV energyscale.The CERN Large Hadron Collider (LHC), a p-pcollider, is scheduled to operate at the center-of-massenergy of 14 TeV in 2008. The ATLAS experiment isone of two general purpose experiments being nearlyconstructed for the CERN LHC. The ATLAS detec-tor is designed to have a good sensitivity to the fullrange of high-pT physics in p− p collisions. Details re-garding the detector and its performance are describedin Ref. [8]. Currently we focus on preparation for theearly data in 2008, aiming for early SUSY discoveries.We describe the strategy for early SUSY searches atthe ATLAS experiment using the data from the firstyear of data taking corresponding an integrated lumi-nosity of 1 fb−1. The results in this paper are obtainedusing the full detector simulation to understand recon-a Email: shimpei.yamamoto@cern.chstruction performances, trigger efficiencies and system-atic effects of the detector.2 Inclusive searchesSparticle production of gluinos (g˜) and squarks (q˜) oc-curs dominantly via strong interactions and its ratemay be expected to be considerably large at the LHC.Gluino production leads to a large rate for events withmultijets via series of cascade decay and the neutrallightest supersymmetric particle (LSP) in the final statewhich remains stable and undetectable, if R-parity isconserved. LSP’s carry off apparently large missingtransverse energy (EmissT ). We note that there are nothird generation partons in the initial state, gluino andsquark production rates are fixed by QCD in termsof the gluino and squark masses (mg˜ and mq˜). Thus,inclusive SUSY searches with the early data rely onexcesses of events in the channel of “multijets + largeEmissT ” [9] which is a model-independent feature.In our searches, the events are classified based onthe topology, the number of identified isolated lep-tons, and we try to cover a broad range of experi-mental signatures. The experimental signatures, cor-responding SUSY scenarios and their background pro-cesses are summarized in Table 1. They cover real-istic supersymmetric models of minimal supergravity(mSUGRA) [10–13], anomaly-mediated SUSY break-ing (AMSB) [14,15] and gauge-mediated SUSY break-ing (GMSB) [16,17].ATL-PHYS-CONF-2008-00108 January 2008Colliders - SUSY Phenomenology Contributed TalkTable 1. Summary of experimental signatures with EmissT and corresponding SUSY scenarios and SM backgroundprocesses.jet multiplicity additional signature covered scenario background≥ 4 no lepton mSUGRA, AMSB, split SUSY, heavy squark QCD, tt¯, W/Zsingle lepton (e,µ) mSUGRA, AMSB, split SUSY, heavy squark tt¯, Wdilepton (e,µ) mSUGRA, AMSB, GMSB tt¯ditau GMSB, large tan β tt¯, Wγγ GMSB —∼ 2 — light squark Z2.1 Event selectionConsidering the features of sparticle production andits decay, the signal candidate events are selected byrequiring:– Njet ≥ 4,– pJ1T > 100 GeV/c & pJ4T > 50 GeV/c,– ST > 0.2,– EmissT > 100 GeV & EmissT > 0.2×Meff ,where Njet, pJ1(4)T , ST andMeff are the number of jets ,the transverse momentum of first (fourth) leading jet,the transverse sphericity and the effective mass, re-spectively. The effective mass is formulated as Meff =∑i≤4i=0 piT + EmissT , where piT is the transverse momen-tum of i-th leading jet. In addition, for the single-lepton signature, the events are selected by requiringone isolated lepton with pT larger than 20 GeV and thetransverse mass (MT ) should be larger than 100 GeV.The selection cuts described here are based on the def-inition given in Ref. [8], but will be optimized.Finally, we look for SUSY events with large Meffat which the signal exceeds the SM backgrounds. Thevalue of Meff also provides a first estimate of the spar-ticle masses. Fig. 1 shows the Meff distributions forno and single-lepton signatures. For the SUSY signal,we set a bench mark point in the bulk region, wherem0 = 100 GeV, m1/2 = 300 GeV, A0 = 300 GeV,tanβ = 6 and µ > 0, referred to SU3. If SUSY existswe expect there is an excess over the SM backgroundexpectation in the distribution for each event topologywith an integrated luminosity up to 1fb1.2.2 Discovery reachFigure 2 shows 5 − σ discovery reach in the mg˜-mq˜space for each event topology in the mSUGRA model.We see that even for sparticle mass as heavy as ∼1 TeV, discoveries are expected for an integrated lu-minosity of 1fb−1. The discovery reach also shows goodstability against the values of tanβ.500 1000 1500 2000 2500110210310 All backgroundsWSUSY (SU3)ZQCDttbarATLAS PreliminaryMeff (GeV)500 1000 1500 2000 2500−110110210All backgroundsWSUSY (SU3)ttbarttbar(−>lnln)(−>lnqq)ATLAS PreliminaryMeff (GeV)Fig. 1. The Meff distributions for no-lepton (upper) andsingle-lepton (bottom) signatures. The histograms are nor-malized to the integrated luminosity of 1fb−1.3 In-situ measurements for the EmissT scaleand resolutionAs stated above, the EmissT is the discriminating sig-nature for the SUSY searches, but it is also a com-plex object: apart from undetected neutral particles,it will comprise contributions from beam halo, cosmic-ray muons and instrumental effects such as noise, hotor dead channels or cracks of the detector. The mis-measurements or inefficiencies for jets and muons alsocontribute to the EmissT ; these make up the fake EmissT .A precise understanding of these contributions andtheir reduction are crucial, especially in the tails ofthe distribution.Shimpei Yamamoto Strategy for early SUSY searches at ATLAS0 200 400 600 800 1000 1200 1400 1600 1800 20000200400600800100012001400160018002000) (GeV)q~m() (GeV)g~m(0 lepton mode1 lepton mode2 lepton modeATLAS PreliminaryFig. 2. 5σ-discovery reaches in the mg˜−mq space for eachevent topology. Generator-level uncertainties of 100% areshown by the hatched regions. The results are obtained onthe assumption of tanβ = 10, A0 = 0 and µ > 0.For the calibration and commissioning of EmissT , weuse the transverse mass distributions of W → lν andthe invariant mass of Z → τ τ¯ where τ ’s decay into alepton and hadrons. For the Z → τ τ¯ , as an example,the invariant mass is reconstructed using the collinearapproximation; it is sensitive to the EmissT scale andresolution. Fig. 3 shows the Z → τ τ¯ invariant masspeak as a function of the EmissT scale. A variation of10% in the EmissT scale results in a 3% shift on the Zmass scale. The Z mass can be reconstructed with anerror of 1% using an accumulated data of 100 pb−1.Considering the uncertainty of 1% on the absolute en-ergy scale of the calorimeters, we can evaluate theEmissT scale with an accuracy of ∼ 4% accordingly.The fake EmissT originating from the instrumentaleffects of the detector have also been studied. All theresults for the commissioning are given in Ref. [18].ETmiss scale0.6 0.8 1 1.2 1.4Z Mass (GeV) 707580859095100105110Z Mass vs ETmiss scaleATLAS PreliminaryFig. 3. The Z → τ τ¯ invariant mass as a function of EmissTscale.4 Data-driven approaches for backgroundestimationsEven if we get early indications of SUSY, we shouldjustify “beyond SM” signatures with full understand-ings of generator-level uncertainties and instrumentaleffects of the detector because they affect the normal-ization and shapes of predicted backgrounds. However,these uncertainties are hard to estimate in the earlystage of the experiment, and they are also expected tobe large. Thus, in the early SUSY searches, we takedata-driven approaches for the SM background esti-mation.4.1 Z/W boson production backgroundThe process of Z → νν¯ in association with multijetswill give rise to final states with large EmissT and couldbe a dominant background for the no-lepton signature.For this background contamination, the expectation isderived from the MC distribution of Z → νν¯ with thenormalization determined by the Z → ll¯ data, wherel is e or µ. We can better measure the Z → ll¯ yieldthanks to small backgrounds in the final state. Thisnormalization factor can be also applied to theW → lνbackground. Differences due to different masses andproduction mechanisms can be taken from theory.4.2 QCD multijet backgroundThe QCD multijet production could be one of the mostdominant SM background sources due to its large crosssection. In QCD events, neutrinos via leptonic decaysand the mismeasurements of jets contribute to the tailsof EmissT distribution. For the QCD background con-tamination, the estimation is derived from the multijetdata with a function representing the fluctuations ofmeasured jet energies. The fluctuation of jet energy isevaluated using events of– EmissT > 60 GeV,– ∆φ(EmissT , jet) < 0.1,where ∆φ(EmissT , jet) is the φ-angle between the miss-ing transverse energy and an isolated jet in radian. Wesuppose that EmissT is originated from the fluctuatingjet close to the EmissT direction, and the pT of initial jetis estimated to be the vectorial sum of pjetT and EmissT .Then we can obtain the fluctuating function of jet en-ergies (R ≡ 1 − pjetT · (pjetT + EmissT )/|pjetT + EmissT |2).The jets in QCD multijet events with small EmissT aresmeared according to the jet fluctuating function, whichresult in fluctuations of EmissT . Fig. 4 shows the jet fluc-tuating function and the EmissT distribution with thesuperimposed QCD background estimation. We canobtain a fair description of the QCD background withthis method, especially in the tails of the distribution.4.3 MT discrimination methodAnother advanced method to estimate the SM back-grounds, especially for single-lepton signature, is touse MT which shows a discriminating power betweenSUSY and SM backgrounds but are less dependent onEmissT . At first, we select events of MT < 100 GeV toColliders - SUSY Phenomenology Contributed TalkclosestJet_measResEntries  812Mean   0.7287RMS    0.1465R0 0.2 0.4 0.6 0.8 1 1.20102030405060ATLAS PreliminaryETmiss (GeV)0 100 200 300 400 500 600 700 800 900 1000110210310410ATLAS PreliminaryAll BG’s + SignalEstimated QCDSignal (SU3)QCDZFig. 4. The jet fluctuating function (left) and the EmissTdistribution (right) for the no-lepton signature. The his-tograms are normalized to an integrated luminosity of22 pb−1.enhance the backgrounds (background control sample)and suppose the EmissT distribution of these events rep-resents that of all the SM backgrounds. Then, the over-all normalization of SM backgrounds is determined bycomparing the number of events with MT < 100 GeVand that with MT ≥ 100 GeV in the low EmissT regionbelow 200 GeV.The MT distribution and the resultant Meff areshown in Fig. 5. The estimated number of SM back-grounds with large Meff (> 800 GeV) and the MCprediction are 22.0 ± 0.9 and 24.8 ± 1.6, respectively.They show a good agreement, however, there couldbe a contamination of signal events in the backgroundcontrol sample, which results in the overestimation ofbackgrounds by a factor of ∼ 2. We will make furtherinvestigations to improve this method and to reducethe signal contamination in the normalization region.Results will be reported in Ref. [18].0 50 100 150 200 250 300 350−110110210M  (GeV)TAll backgroundsWSUSY (SU3)ttbarttbar(−>lnln)(−>lnqq)ATLAS Preliminary100 200 300 400 500 600 700 800 900−110110210MC predictiondata−driven estimationETmiss (GeV)ATLAS PreliminaryFig. 5. The MT distribution of single-lepton candidateevents (left) and the Meff distribution of SM backgrounds(right). The histograms are normalized to an integratedluminosity of 1fb−1.5 ConclusionsWe described the strategies for early SUSY searchesat the ATLAS experiment. Using the inclusive sig-nature of multijets plus large EmissT , we can make anearly discovery of SUSY. Our searches are less model-dependent but sensitive to models with squark andgluino masses up to 1 TeV with an integrated lumi-nosity of 1fb−1. However, the precise understandingof the SM backgrounds is crucial for the early discov-ery. To settle this issue, we have been making muchprogress in the in-situ calibration of EmissT and thedata-driven technique for the SM background estima-tions. The opening of a new era of physics beyond theSM is coming with the LHC.AcknowledgmentsThe author wishes to thank SUSY, Jet/Etmiss andmany other working groups of the ATLAS collabora-tion. The work described here relied on their joint ef-forts.References1. J. Wess and B. Zumino, Nucl Phys. B70, (1974) 39.2. P. Fayet and S. Ferrara, Phys. Rep. 32, (1977) 249.3. H. P. Nilles, Phys. Rep. 110, (1984) 1.4. H, Haber and G. Kane, Phys. Rep. 117, (1985) 75.5. R, Barbieri, Riv. Nuovo Cimento. 11, (1988) 1.6. W. de Boer and C. Sander, Phys. Lett. B585 (2004)276.7. J. R. Ellis, S. Heinemeyer, K. Olive and G. Weiglein,JHEP 0502 (2005) 13.8. The ATLAS collaboration, ATLAS Detector andPhysics Performance Technical Design Report, (1999)CERN/LHCC/99-15.9. H. Baer, C. H. Chen, F. Paige and X. Tera, Phys.Rev. D52, (1995) 2746.10. A. Chamseddine, R. Arnowitt and P. Nath, Phys.Rev. Lett. 49 (1982) 970.11. R. Barbieri, S. Ferrara and C. Savoy, Phys. Lett.B119 (1982) 343.12. N. Ohta, Prog. Theor. Phys. 70 (1983) 542.13. L. Hall, J. Lykken and S. Weinberg, Phys. Rev. D27(1983) 2359.14. L. Randall, and R. Sundrum, Nucl. Phys. B557(1999) 79.15. G. F. Giudice, M. A. Luty. H. Murayama and R. Rat-tazzi, JHEP 9812 (1998) 027.16. M. Dine and A. Nelson, Phys. Rev. D48 (1993) 1277.17. M. Dine , A. Nelson, Y. Nir and Y. Shirman, Phys.Rev. D53 (1996) 2658.18. The ATLAS collaboration, Computing System Com-missioning (CSC) notes (in preparation).

Strategy for early SUSY searches at ATLAS

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