The SUSY discovery potential of CMS and ATLAS detectors at the LHC is evaluated in different regions of the mSUGRA parameter space. In inclusive searches the parameters can be constrained by considering the event topology. The observation of kinematic end points at large statistics makes possible to infer the SUSY mass spectrum. The LHC SUSY discovery region extents up to the mass scale of 2 TeV at L_{int}> 10fb^{-1} and is limited by uncertainties in the Standard Model backgrounds

Zhukov, Valery

on behalf of CMS The SUSY discovery potential of CMS and ATLAS detectors at the LHC is evaluated in different regions of the mSUGRA parameter space. In inclusive searches the parameters can be constrained by considering the event topology. The observation of kinematic end points at large statistics makes possible to infer the SUSY mass spectrum. The LHC SUSY discovery region extents up to the mass scale of 2 TeV at L  	  10fb  and is limited by uncertainties in the Standard Model backgrounds

V. Zhukov

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LHC SUSY discovery potential

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Available on CMS information server CMS CR 2006/049CMS Conference Report30 August 2006LHC SUSY discovery potentialV. Zhukov IEKP, University Karlsruhe, Germanyon behalf of CMSAbstractThe SUSY discovery potential of CMS and ATLAS detectors at the LHC is evaluated in differentregions of the mSUGRA parameter space. In inclusive searches the parameters can be constrainedby considering the event topology. The observation of kinematic end points at large statistics makespossible to infer the SUSY mass spectrum. The LHC SUSY discovery region extents up to the massscale of 2 TeV at L 	 10fb  and is limited by uncertainties in the Standard Model backgrounds.Presented at ICHEP06, July 28, 2006, MoscowOn leave from SINP MSU1 IntroductionThe potential of the Large Hadron Collider (LHC) to discover supersymmetry(SUSY) has been studied in detail inthe past decade. Each new step in this study involves more detailed detector simulation and advanced analysis ofsignal signatures. Any deviation from the Standard Model (SM) predictions will be tested against SUSY hypothesisand the SUSY discovery would mean observation of at least one or several new sparticles predicted by the model:squarks(  ) and sleptons (   ), gauginos( ﬀ ﬁﬂ ﬃ ,    ), gluino ( ! ) or heavy Higgses(H,H,A). In the minimalSupergravity (mSUGRA) model the mass spectrum and couplings are dened by only ve free parameters: theuniversal gaugino m #" and scalars masses m, the Higgs-sfermions trilinear coupling A, the ratio of Higgsvacuum expectation values tan $ and sgn( % ). The lightest SUSY particle (LSP) in R-parity conserved scenario is theneutralino (  ) being an attractive candidate for the relic Dark Matter(DM). The mSUGRA m-m #" parametersplane is constrained at large mby the ElectroWeak Symmetry Breaking (EWSB) requirements and at low mthe LSP is a charged stau disfavored by the relic DM interpretation. The experimental limits from LEP on thelight Higgs boson (m &('*)+)ﬂ, GeV) and the -/.10ﬀ2 ([3.43 3 0.36] 10 4 ﬃ ) branching ratio from b-factories alreadyexclude low mass region (m' 1000GeV and m #"' 300 GeV )[1]. The LEP mass limit on the chargino massexcludes region with m 5"' 140 GeV for all m. Further constraints can be obtained from the relic DM densitymeasurement by WMAP and indirect DM search with gamma rays [2]. At the LHC a typical search strategyhave two major steps. First, all possible mSUGRA nal states can be detected with an inclusive search which isa ’counting like’ experiment. All uncertainties in the background calculations are very important in this searchwhich can hardly be used to identify the underlying model. In the second step the sparticles decay chain can beanalyzed. The invariant mass distributions of different particles combinations have the particular kinematic endpoints related to the mass differences of sparticles participating in the cascade [4]. With a sufcient statistics andlarge signal to noise ratio the mass spectrum, and therefore the SUSY Lagrangian can be reconstructed.In this report the expected mSUGRA discovery potential is summarized for the CMS and ATLAS detectors,ready for operation in 2008 at the LHC. The results are based on the full(CMS:ORCA) or fast(CMS:FAMOS,ATLAS:ATLFAST) detector simulations and are presented in the CMS Physics TDR [3] or recent ATLAS publi-cations [5, 6].2 SUSY signal and background signatures at LHCThe mSUGRA production cross section at LHC depends on the SUSY mass scale 687:9<;=6?>@6A>B ) and drops fast atlarger m #" ( 6A>BDCFEHGJI 6 #" ) and m. The regions in Fig.1 correspond to the dominating squarks   (1) or gluino!! (3) production and in the region (2) both  ! can be produced. For larger m(K2000 GeV the direct gauginosproduction (     , ﬀ  ) dominates.The signal topology is dened by the cascade decays of produced sparticles: the general signal signatures canbe written as N LNMPOﬀQ=R5SUT +NVWMQ=T +Missing Transverse Energy(MET). The number of hard central jets N VWMQ=T (E V XKY+ZU[]\^, _ `_a' EHG , ), MET, highest lepton P L X and highest jet E X will depend on the mSUGRA parameters mand m #" , see Fig.2. The MET and jets E X are decreasing toward the EWSB limit and can be quite small at lowm 5" making the background suppression with the MET and N VWMQ=T cuts very difcult in this region. The largestjets multiplicity is achieved in the region (2) where m >@ and m >B are almost degenerate in mass and the cascadesare long. The leptons PXis increasing with m #" where the gauginos, main source of hard leptons, are heavier(m bdce C Z G f 6 5" ).Some important SM backgrounds with large MET, high jets multiplicity and hard leptons are listed in Table 1together with some CMS test points (m #" ,m,tan $ , A=0). The largest average MET is expected for the gihg events.The ZW, DY, W+jets, Z+jets channels are producing relatively hard leptons. The QCD, W+jets have large crosssections and can be important for the multijet nal states. There are more backgrounds important for the specicexclusive studies.3 mSUGRA discovery reachFor most of inclusive searches the SUSY events are selected with large MET and jets multiplicity. One hard leptonor two same sign (SS) and opposite sign (OS) same avor (SF) leptons can be added to the selection. A summary ofconsidered nal states and some important selection requirements used in the CMS studies with the full simulationis presented in Table 2. The background suppression is optimized for a particular mSUGRA test point, usuallyat low mass, and then extrapolated to the whole parameter space [3]. A typical L1 trigger for SUSY events isjet+MET (in CMS E V XK88GeV, METK45GeV) or dileptons (P jXKYU[]\^	kl, P MXK)I[]\^mkl ). Trigger efciency2mh<114 GeVχm  <103 GeVFigure 1: mSUGRA regions in the m n -m o#pWq mass plane and contours for the inclusive gluino rtsPuﬂvxw and squarksrts:uzydw production.Table 1: SM backgrounds and mSUGRA test points(m o5pWq ,m n ,tan { , A n =0) cross sections [pb], reconstructed aver-age MET [GeV] and jets multiplicity (E |~} 30 GeV)channel  a 830 ŁU 69 2.8W(  )+jets( : GeV) 42 10  ŁU 51 1.8Z  +jets( 	P GeV) 14 10  ŁU 25 1.6DY(  ) 53 10  ŁU 22 0.7QCD ( 	: GeV) 2.4 10  Ł+ 18 1.8ZW(  ) 52 ŁU 41 0.6SUSY(60,250,10) 55 ŁU 240 2.8SUSY(1450,175,50) 40 ŁU 105 3.1is U for the low mass region and is decreasing to  ¡¢ for large m nA} 2000 GeV. The SM backgroundrejection up to  10 £ is obtained with MET }¥¤§¦d¡ GeV. Jets multiplicity N ¨W©ª=« , hard leptons N ¬N©P­ﬀª=®5¯U« and cutson some angular variables ( °±s² o ² q w , °±s² oﬂ³F´]µ w , ¶ ) increase the rejection further to  10 ·ﬀ¸º¹ . The 5 r discoveryreaches in mSUGRA mass plane is shown in Fig.3 for »½¼ ¯ª =10 fb ¸ o . The signal signicance is calculated bytaking into account theoretical and reconstruction systematic uncertainties which can contribute up to ¾U¡ to thebackground uncertainties.4 Determination of model parameters.The reconstruction of SUSY mass spectrum from kinematic end points is discussed in [4, 5, 6]. This techniqueworks well at m o5pWqÀ¿ 500 GeV m nÁ¿ 300 GeV, where two body decays dominate: ÂvFÃ y o ÂyÁÃÄy o y qﬂÅ nqÃyoyqzÆPÇÂÆ:ÈÃÉyoyqzÆ:ÇÊÆPÈÊÅno. The kinematic end point in invariant masses of OSSF leptons M ¬J¬ and different com-binations of jets ( y o , y q ) and leptons M ¬J¬Ë , M ¬NË are related to the mass differences m Ì -m Ì  Í , m ÎÏ -m ÎË , m ÎË -m Ì ,etc. System of equations involving all end points is overestimated and all mass differences can be reconstructed inprinciple. However, the sparticles are produced in pairs and one needs a correct association to the cascade chain.This can be done with ÐU¡ efciency by grouping particles around two axes a oÒÑ q and for ex. maximizing thescalar product ÓÔ ÓÕ [3]. The mass spectrum of sparticles can be reconstructed from the end points with an accuracybelow 10  [5] for low mass region, see Fig. 4. The uncertainties can cause an another ghost solution for the3m00 1000 2000 3000 40001/2m50010000246810<Nj>m00 1000 2000 3000 40001/2m5001000050100150200250300350400450<MET>m00 1000 2000 3000 40001/2m5001000020406080100120>µ<PTm00 1000 2000 3000 40001/2m500050100150200250300350400>j1<ETFigure 2: Average jets multiplicity N ÖW×Ø=Ù (E ÖW×Ø=ÙÚ ÛÝÜ+Þ , ß àáßmâäãxå æ ), MET [GeV], highest lepton P çÚ [GeV/c] andhighest jet E Ú [GeV] for mSUGRA(tan è =50) at generator level.Figure 3: CMS 5 é mSUGRA discovery reaches for different channels at L êNëdØ =10 fb ìáí .4Table 2: Summary of the CMS mSUGRA inclusive studies with full simulations. The selection cuts on MET, jetsand leptons are presented together with number of signal and background events at the optimization point.signature MET Nî5ï:ðñ N ò ïóÒðô#õzñ Nsig, 10fb öø÷ Nbkg, 10fb öù÷[GeV] (E úûýü GeV) ( P û ü GeV/c) ( þ ß  þ÷	 )jet+MET ü 200  3 (180,110,30) - 6 10  (60,250,10) 2.5 10  (qcd,   ) +jets+MET ü 130  3 (440,440,50)  1  (30) 311 (60,250,10) 2.5 (Wjets,    )SS 2  +jets+MET ü 200  3 (175,130,55) SS  (10) 341 (60,250,10) 1.5 (   )OS 2  +jets+MET ü 150  2 (150,150) OS  1140 (185,350,35) 427 (   )OS 2l+jets+MET ü 200  2 (100,60) OSSF (10) 8.5 10  (60,250,10) 2 10  (   , Wjets)top+jet+MET ü 150 4 (30)  e,  (5) 380 (60,250,10) 220 (    )Z ß +MET ü 255 - OSSF M ò ò =m  1289 (210,285,10) 440 (   )h ß +jets+MET ü 200  4 (200,150,50,30) - 1.4 10(60,250,10) 200 (   )sleptons (OSSF+MET) ü 130 0 veto OSSF (20) 60 (60,250,10) 45 (    , WZ)trilept (OSSF+lept.) - 0 veto OSSF (  :10,e:17)+l(10) 53 (1450,175,50) 157 (DY,Zjets,    )masses (blue or gray curves) which can be rejected considering in addition the cross section information [6]. ThemSUGRA parameters can be reconstructed from the masses with an accuracy of ﬀﬂﬁﬃ! , #"%$'&(ﬁ*),+ -! ,.0/!1324ﬁ65! , 789ﬁ;:<-! in the low mass region [5].Figure 4: Reconstructed mass spectrum for mSUGRA(m  =100 m "$'& =250,tan 2 =10) at L=300 fb = " using ATLASfast simulations.The kinematic end point method may not be so effective for the focus point region at large m  . In this casethe different signal topologies can be used to constrain mSUGRA regions already in inclusive search, withoutreconstruction of the whole cascade chain.5 ConclusionThe discovery of SUSY at LHC will be possible at L >@?BACﬁ 1 fb = " in low mass region, after 10 fb = " the mSUGRAregion m "%$'&ED 1000 GeV and m ED 2000 GeV will be exploited in details with the mass reconstruction method.The higher m "$'& are limited by the SUSY production cross section and backgrounds uncertainties. The focus pointregion at large m  is difcult for the LHC and will require a good understanding of all backgrounds.References[1] LEPHIGGSWG CERN-EP/2003-011 (2003), ALEPH, Phys.Lett. B583 (2004).[2] W. de Boer et al., Phys.Lett. B636 13 (2005).5[3] CMS PTDR Vol.2., CERN-LHCC-2006/021 (2006).[4] ATLAS TDR Vol.2. CERN-LHCC-1999/015 (1999).[5] B.K. Gjelsten et al., J.High Energy Phys. 015 (2005).[6] C. Lester et al., ATL-PHYS-PUB-2005-013, hep-ph/0508143 (2005).6

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