The cascade decay of heavy supersymmetric particles is expected to produce energetic jets, missing transverse momentum and leptons. A selection of the search strategies employed in CMS are described and the results obtained with the 35 pb−1 of data recorded in 2010 are presented. The data did not show any significant deviation with respect to the expectation from Standard Model processes. The limits on the existence of supersymmetric particles set by TeVatron and LEP experiments are abundantly extended

Pioppi, M.

English

The cascade decay of heavy supersymmetric particles is expected to produce 
energetic jets, missing transverse momentum and leptons. A selection of the 
search strategies employed in CMS are described and the results obtained 
with the 35 pb$^{-1}$ of data recorded in 2010 are presented. 
The data did not show any significant deviation with respect the expectation from Standard Model processes. 
The limits on the existence of supersymmetric particles set by TeVatron and LEP experiments are abundantly extended

Pioppi, Michele

CERN Document Server

SUSY searches at LHC with the CMS detector

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-11097-2Colloquia: IFAE 2011IL NUOVO CIMENTO Vol. 34 C, N. 6 Novembre-Dicembre 2011SUSY searches at LHC with the CMS detectorM. Pioppi on behalf of the CMS CollaborationImperial College London - London, UK(ricevuto il 29 Luglio 2011; pubblicato online il 21 Dicembre 2011)Summary. — The cascade decay of heavy supersymmetric particles is expectedto produce energetic jets, missing transverse momentum and leptons. A selectionof the search strategies employed in CMS are described and the results obtainedwith the 35 pb−1 of data recorded in 2010 are presented. The data did not show anysignificant deviation with respect to the expectation from Standard Model processes.The limits on the existence of supersymmetric particles set by TeVatron and LEPexperiments are abundantly extended.PACS 12.60.Jv – Supersymmetric models.1. – IntroductionThe Standard Model (SM) of particle physics has been enormously successful in de-scribing all phenomena at the highest attainable energies thus far. Yet, it is widely be-lieved to be only an effective description of a more complete theory which is valid at thehighest energy scales. Of particular theoretical interest is SuperSymmetry (SUSY) [1,2]which solves the hierarchy problem of the SM by compensating for each of the fermionicand bosonic degrees of freedom in the SM with a supersymmetric bosonic and fermionicdegree of freedom, respectively. The resulting superfields have the same quantum num-bers as their SM counterparts, except for spin. Since no SUSY particle has been observedso far, they must have higher masses than their SM partners, implying that SUSY is abroken symmetry. At the Large Hadron Collider (LHC) at CERN, supersymmetric par-ticles, if they exist, are predicted to be produced dominantly via strong interaction,through the fusion of two gluons into a pair of gluinos, a pair of squarks, or a gluino anda squark.The LHC, with a proton-proton√s = 7TeV, is a copious source of high-energypartons which allows to probe squark and gluino masses beyond the limits previouslyset at LEP and at the TeVatron. Squarks and gluinos initiate a decay cascade in whichquarks, leptons and photons are produced, until the lightest supersymmetric particle(LSP) is created. The dynamics of the expected cascade depends on the SUSY modelunder consideration, and in particular on the masses of the supersymmetric particles.c© CERN on behalf of the CMS Collaboration under CC BY-NC 3.0 326SUSY SEARCHES AT LHC WITH THE CMS DETECTOR 327In these proceedings, the emphasis is put on the search for SUSY in final statesfully hadronic, with two leptons with the same sign and with two photons. The resultspresented in these proceedings are based on the data collected by the CMS experiment [3]in 2010 (∼ 35 pb−1).2. – Search for SuperSymmetry in All-Hadronic final stateIn the decay of gluinos and squarks there is an abundant production of hadronic jetswhile the presence of leptons and photons is not guaranteed, thus the production of allhadronic final states is the most frequent final state deriving from SUSY particles. InCMS, three complementary approaches are followed. The jets + Missing Energy analy-sis, described elsewhere [4], consists of looking for an excess of multi-jet events at highMissing Energy. This approach is the most efficient of the three, but requires the QCDbackground to be accurately controlled. The razor analysis [5] relies on the kinemati-cal properties of the production of two heavy particles. A third method based on thekinematical properties of the QCD background [6] has been developed. With respect theother two approaches, this analysis is very robust against jet energy mis-measurementwhich is the most important source of fake missing transverse energy. The data sampleused in this analysis is recorded using a trigger based on the scalar sum (HT) of thetransverse energy (ET) of jets reconstructed at the trigger level. The pseudo-rapidity ofthe leading jet is required to be within |η| < 2.5 and the transverse energies of each of thetwo leading jets must exceed 100GeV. Events with isolated photon with PT > 25GeVor isolated leptons (electron or muon) with transverse PT > 10GeV are vetoed. Jet mis-measurements are potentially the dominant source of (fake) missing transverse energyin the events from the QCD background. To control this background and to separateit from a genuine missing energy signal, a variable which is robust against energy mis-measurements, αT = Ej2T /MT [7], is used. For events with two jets, Ej2T is the transverseenergy of the jet with the lower ET in the event and MT is the transverse mass of thedi-jet system. For a perfectly-measured di-jet event, with Ej1T = Ej2T and jets back-to-back in φ, and in the limit where the jet momenta are large compared to their masses,the value of αT is nearly 0.5 (fig. 1 left). In case of an imbalance of the measured trans-verse energies, αT takes on values < 0.5. For larger jet multiplicities, the n-jet system isreduced to a di-jet system by combining the jets in the event into two pseudo-jets. Thechosen combination is the one which minimizes the ET difference of the two pseudo-jets.To protect against multiple jets failing the > 50GeV threshold, the jet-based estimate ofthe missing energy, MHT = |−∑jets PTjets|, is compared to the calorimeter tower-basedestimate of the missing energy, EmissT , and events with MHT/EmissT > 1.25 are rejected.In the signal region (HT > 350GeV) 13 events pass the requirements. The total back-ground is estimated with the ratio of the numbers of events which pass and fail the αTselection requirement (RαT ). The total number of background events with αT > 0.55from SM processes in the signal region is therefore the product of the extrapolated RαTin the signal region with the number of events with αT < 0.55 and HT > 350GeV.This estimate (9.4+4.8−4.0 (stat) ± 1.0 (syst)) is found to be compatible with the observednumber of events in data after the full cut-flow. An upper limit on the number of non-SM events compatible with the measurements is derived in this section using the ProfileLikelihood method. To interpret the consistency of the observed number of events withthe background expectation in the context of a model, and also to facilitate compar-ison with previous experimental results, an exclusion limit in the CMSSM [8] is set.328 M. PIOPPI on behalf of the CMS COLLABORATION (GeV)0m0 100 200 300 400 500 (GeV)1/2m100150200250300350400450(500)GeVq~(500)GeVg~(650)GeVq~(650)GeVg~(800)GeVq~(800)GeVg~-1,   2 fbq~,g~CDF-1,   2.1 fbq~,g~D0±1χ∼LEP2±l~LEP2ν∼D0 = LSPτ∼NLO Expected LimitNLO Observed LimitLO Observed Limit = 7 TeVs,-1 = 35 pbintCMS preliminary 2010, L)>0μ = 0, sign(0 = 3, AβtanTα0.2 0.4 0.6 0.8 1 1.2 1.4Events/0.025-110110210310410510CMS = 7 TeVs,-1 L dt = 35 pb∫DataStandard ModelQCD, W, Z + JetsttLM0LM1Fig. 1. – Left: αT distribution in data and simulation for events with only 2 reconstructedjets. Right: 95% CL exclusion contour at NLO in the CMSSM (m0,m1/2)-plane (tanβ = 3,A0 = 0GeV, sign(μ) > 0).The 95% CL limit for tanβ = 3 and A0 = 0 is shown in fig. 1 (right) and the excludedregion significantly extends the previous limits from TeVatron and LEP experiments.3. – Search for SuperSymmetry in same-sign di-lepton final stateSame-sign isolated lepton-pairs are very rare in SM but appear very naturally in manysupersymmetric scenarios [9-11]. In order to maximize the sensitivity to the presence ofnew physics from SUSY, four different search regions are defined [12]: 1) HT > 200GeV,EmissT > 30GeV, 1 > 20GeV, 2 > 10GeV with 1,2 = e, μ, 2) HT > 60GeV, EmissT >80GeV, 1 > 20GeV, 2 > 10GeV with 1,2 = e, μ, 3) HT > 300GeV, EmissT > 30GeV,e > 10GeV, μ > 5GeV, 4) HT > 350GeV, EmissT > 50GeV, e > 10GeV, μ > 5GeV,τ > 15GeV. The first two search regions are selected with single electron/muon triggerswhile the last two regions are selected with a pure HT trigger that allows to explore thephase space of low-PT electrons and muons, as well as final states with hadronic τ decays.In all the regions both the leptons must be isolated. Backgrounds in all of our searches aredominated by one or two jets mimicking the lepton signature. Such lepton candidates canbe real leptons from heavy flavor decay, electrons from unidentified photon conversions,muons from meson decays-in-flight, hadrons reconstructed as leptons, or jet fluctuationsleading to hadronic τ signatures. Backgrounds from jets mimicking leptons are estimatedusing the so-called “Tight-Loose” (TL) method [13]. In this method a probability, fora lepton passing loose cuts to also pass the tight analysis cuts is measured in QCDmulti-jet events. To predict the background from real lepton + jets in a signal region,the TL probability is applied to a sample of di-lepton events satisfying all requirementsbut where one of the leptons fails the tight cuts and passes the loose cuts. A secondpotentially important source of background consists of opposite-sign di-lepton eventswith at least one electron where the sign of the charge of a reconstructed electron ismis-measured because of hard bremsstrahlung in the tracker volume. The probabilityof mis-measuring the charge of the electron is obtained in data by selecting a sampleof Z → ee events. The background due to charge mis-identification is estimated byscaling the opposite-sign yields by the above probability. Figure 2(left) summarizes thesignal region yields and background composition in all four search regions. Estimates forbackgrounds due to events with one or two fake leptons and from electron charge mis-SUSY SEARCHES AT LHC WITH THE CMS DETECTOR 329 (GeV)0m0 100 200 300 400 500 (GeV)1/2m150200250300350400450(500)GeVq~(500)GeVg~(650)GeVq~(650)GeVg~(800)GeVq~(800)GeVg~-1=5, 2 fbβ, tanq~,g~CDF-1=3, 2.1 fbβ, tanq~,g~D0±1χ∼LEP2±l~LEP202χ∼,±1χ∼D0 = LSPτ∼NLO Observed LimitNLO limit (efficiency model) = 7 TeVs,-1 = 35 pbintCMS, L)>0μ = 0, sign(0 = 3, Aβtan0123HT Trig  tausLept TrigMET>80 Events observed bkgd with two fake leptons bkgd with one fake lepton opposite-sign leptons and charge mis-identification same-sign leptonsCMS     sqrt(s)=7 TeV   Ldt=35 pb-1Lept TrigHT>200HT TrigleptonsFig. 2. – Left: Number of expected and observed events in the 4 search regions defined to searchSUSY in the same sign di-lepton final state. Right: Exclusion contour in the (m0,m1/2)-planefor CMSSM.measurement were obtained directly from data in appropriately chosen control regions.The remaining irreducible background from two real same-sign leptons (WZ, ZZ, tt¯W ,etc.) amounts to at most 10% of the total and is estimated based on theoretical crosssection predictions and Monte Carlo simulation. No evidence of an event yield in excessof the background prediction is observed and a 95% CL upper limits are set on thenumber of observed events using a Bayesian method with a flat prior on signal strengthand lognormal for efficiency and background uncertainties. The absence of new physicsis interpreted in the CMSSM model (fig. 2 right).4. – Search for SuperSymmetry in Di-Photon + Missing Energy final stateOne of the well-established scenarios for physics beyond SM is General Gauge-Mediation (GGM) SUSY [14,15], with the gravitino as the lightest SUSY particle (LSP)and the lightest neutralino as the next-to-lightest (NLSP). The latter promptly decaysinto a photon and a gravitino, which escapes detection, leading to apparent EmissT . IfR-parity is conserved, strongly-charged SUSY particles are pair-produced, resulting inthe presence of two neutralinos and multiple jets per event. The topology of interest)  [GeV]q~M(500 1000 1500 2000)  [GeV]g~M(500100015002000) = 50 GeV10χ∼M() = 150 GeV10χ∼M() = 500 GeV10χ∼M(tcMET (GeV)0 20 40 60 80 100 120 140Number of Events-210-110110210310410510 Candidate SampleNorm. Uncertainty of BGTotal Background UncertaintyElectro Weak BackgroundGGM - -1CMS Preliminary, 35.5 pb1+ Jet RequirementFig. 3. – Left: Observed and expected from SM background EmissT distribution. Right: Exclusioncontour in the (mg,mq˜)-plane for various choices of the neutralino mass.330 M. PIOPPI on behalf of the CMS COLLABORATIONfor this search [16] is, therefore, two isolated high-ET photons, at least one hadronic jet,and EmissT . Photon candidates are required to be isolated and to have ET ≥ 30GeV and|η| ≤ 1.4. At least one jet with ET > 30GeV and EmissT > 50GeV are required.The SUSY signal can be mimicked in several ways. The dominant contribution comesfrom mis-measurement of EmissT in QCD processes such as direct di-photon, photon plusjets, and multi-jet production, with jets mimicking photons in the latter two cases (QCDbackground). The strategy for determining this background is to use control sampleswhich are kinematically similar to the candidate sample while having no real EmissT . Thecontrol sample contains events with two electrons with invariant mass between 70 and110GeV, and is dominated by the Z → ee decays.The second background comes from events with real EmissT . It is dominated by theevents with a real or fake photon and a W that decays into a neutrino and an electron,with the latter mis-identified as a photon (EWK background). Since all componentsof this background involve electron-photon mis-identification, in order to estimate itscontribution to the signal sample, we weight the eγ sample with fe→γ/(1− fe→γ) wherefe→γ is the probability to mis-identify an electron as a photon. Figure 3 (left) shows theEmissT distribution for the selected events and the expected background. Only 1 event isobserved in the region with EmissT ≥ 50 and the predicted background is 1.2±0.8. Basedon the probability to observe zero or one event at the predicted signal cross section, the95% CL exclusion regions shown in fig. 3 (right) for GGM SUSY is determined in thesquark-gluino mass plane for various choices of the χ˜01 neutralino mass.∗ ∗ ∗I would like to thank the members of the CMS Collaboration as well as the membersof the CERN accelerator department.REFERENCES[1] Wess J. and Zumino B., Nucl. Phys. B, 70 (1974) 39.[2] Nilles H. P., Phys. Rep., 110 (1984) 1.[3] CMS Collaboration, JINST, 0803 (2008) S08004.[4] CMS Collaboration, CMS-PAS-SUS-10-005 (2011).[5] CMS Collaboration, CMS-PAS-SUS-10-009 (2011).[6] CMS Collaboration, Phys. Lett. B, 698 (2011) 196.[7] Randall L. and Tucker-Smith D., Phys. Rev. Lett., 101 (2008) 221803.[8] Kane G. L. et al., Phys. Rev. D, 49 (1994) 6173.[9] Barnett R. M. et al., Phys. Lett. B, 315 (1993) 349.[10] Guchait M. and Roy D. P., Phys. Rev. D, 52 (1995) 133.[11] Baer H. et al., Phys. Rev. D, 53 (1996) 6241.[12] CMS Collaboration, JHEP, 1106 (2011) 077.[13] CMS Collaboration, CMS PAS-SUS-10-001 (2010).[14] Meade P. et al., hep-ph (arXiv:0801.3278v3).[15] Buican M. et al., hep-ph (arXiv:0812.3668v4).[16] CMS Collaboration, Phys. Rev. Lett., 106 (2011) 211802.

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SUSY searches at LHC with the CMS detector

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