A full simulation study with the detector CMS is presented. The Leptons + Jets + Missing Energy (l = e,$\mu$) final state for SUSY events is investigated at mSUGRA benchmark point LM1. The end point in the dilepton pair invariant mass distribution is reconstructed and a scan of the $\left(m_{0},\, m_{1/2}\right)$ plane is performed in order to determine the observability reach

Chiorboli, M

Galanti, M

Tricomi, A

CERN Document Server

Available on CMS information server CMS CR 2006/037CMS Conference Report24 July 2006SUSY searches with Opposite Sign Dileptons atCMSM. Chiorboli, M. Galanti, A. TricomiUniversita` and INFN, CataniaAbstractA full simulation study with the detector CMS is presented. The Leptons + Jets + Missing Energy (l= e,   ) final state for SUSY events is investigated at mSUGRA benchmark point LM1. The end pointin the dilepton pair invariant mass distribution is reconstructed and a scan of the	plane isperformed in order to determine the observability reach.Presented at Physics at LHC 2006, Cracow, July 3-8, 2006Table 1: Cross section at NLO, selection efficiencies and number of events surviving cuts for signal and backgroundprocesses.Process   (pb) Ev. analysed  N  in 1 fb 	SUSY (LM1) 52 478k 0.016 853 830 913k 1.9 	 10 155 jets 188 197k 1.4 	  26 jets 5 	 606k ﬁﬀ ﬂﬃ	! 24DY "$#   3.97 	 10  916k %&ﬀ	 10 ' %(DY ")#+* 3.97 	 10  514k 1.1 	 10 ' 4.5-,,"/.0.,1,32.5476 *98P : ;=<?>A@CB GeV/ D 57.4 621k 8.4 	'E 4.83 , ,3.3 50k 9.8 	3.2- jets 11 37k 2.4 	  2.7 jets 1.5 	E 1765k B9ﬀGF-	!H 11 IntroductionThe SUSY production cross section at LHC is dominated by gluinos and squarks which decay mainly through achain to the lightest neutralinos. For low and moderate I?JLKM values, many decay chains end up with the decaysNO"NO	.QP3.R and NO"N.TS3.-"NO	.0P3. , with .4U6 , in which the two final state leptons provide a naturaltrigger. Leptons (electrons and muons) from the NOdecay exhibit a peculiar .VPW. invariant mass distribution witha sharp edge. In this report the method to reconstruct the dilepton end point is described [1]. We proved thefeasibility of the method at benchmark point LM1 ( X  4YB[Z\1]_^+D, X	  4`#a[Z\1]b^+D, c4Y , IdJKMe4f ,gihK2 8j4 ) [2], and the observability of the signal in the mSUGRA (   ,  	  ) plane for I?JLKMk4 .2 SignalThe LM1 mSUGRA benchmark point has been chosen as the working point (total NLO cross section lma# pb,calculated with PROSPINO [3]). The branching ratio of the decay NO"N.S.ﬃ"NO	.QPW. is 11.2%. The eventsanalysed in this note have been produced using PYTHIA 6.225 [4] interfaced with ISAJET 7.69 [5]. A full detectorsimulation has been employed. Low luminosity pile-up has also been taken into account. The Level-1 and HighLevel Trigger paths require a single isolated electron or muon to select the event.3 BackgroundThe Standard Model backgrounds considered for this analysis are: I I , WW+jets, DY "n#L. , Zbb "o.Q. ,, , W+jets,Z+jets, QCD, ZZ+jets and I Iip p . The full simulation of the detector has been used also for the backgrounds. Table 1summarizes the number of expected events in 1 fb 	for SUSY events and the Standard Model backgroundsconsidered, after the following selections are applied:q at least two same-flavour opposite-sign (SFOS) isolated leptons, with rs:(@tsZ\1]b^+D and u vwuW%x#ﬀ  forboth electrons and muons;q at least two jets with ry:z 	 @&-Z\1]b^+D and ry:z  @CBZ\1]_^+D and u vwu{%(|9ﬀ  ;q~}:@#L-Z\1] .The total number of SUSY events after all selection cuts is 853 in 1 fb 	, corresponding to an efficiency of 1.6%over the whole SUSY sample.4 Results at LM1The invariant mass distribution of the same flavour opposite sign lepton pairs after all selection cuts and for anintegrated luminosity of 1 fb 	is shown, superimposed over the I I background, in Figure 1.In SUSY events, the presence of two SFOS leptons can also be due to processes different from NO"N.S._"NO	.0P3. decay. If the two leptons are independent of each other, one would expect equal amounts of SFOS leptons2and different flavour opposite sign (DFOS) leptons. Their distributions should also be identical. The backgroundSFOS contribution can hence be removed by subtracting the DFOS events. Figure 2 shows the same SFOS dis-tribution for SUSY events together with the distribution of the DFOS lepton pairs. The flavour subtracted SFOSdilepton pair distributions for both SUSY and I I background are shown in Figure 3 for an integrated luminosityof 1 fb 	. The I I background contribution after the flavour subtraction is close to zero since the lepton pairs arealways uncorrelated. The numbers of SUSY and background dilepton pairs surviving cuts are respectively 913 and224, giving a signal to background ratio of 4.1. A statistical significance of 5 sigma, calculated using   [7], canbe obtained with 14 pb 	of integrated luminosity.Entries  913Mean    60.32RMS     28.81Underflow       0Overflow       19)2) (GeV/c-l+M(l0 20 40 60 80 100 120 140 160 180 200number of lepton pairs051015202530354045Entries  155Mean    108.6RMS     48.55Underflow       0Overflow       42CMSLM1ttbarEntries  913Mean    60.32RMS     28.81Underflow       0Overflow       19)2) (GeV/c-l+M(l0 20 40 60 80 100 120 140 160 180 200number of lepton pairs051015202530354045Entries  287Mean    65.05RMS     36.18Underflow       0Overflow        8CMSSFOSDFOSFigure 1: Same flavour opposite sign lepton pair dis-tributions of SUSY and I I events for 1 fb 	.Figure 2: SFOS and DFOS distributions of events for1 fb 	.The value of the end point can be extracted by fitting the flavour subtracted distribution with a convolution of atriangle and a Gaussian function (Fig. 4). The value obtained from the fit is<	4 ﬂﬀ  #~ﬀ ﬂjZ\]ﬃ^+D(1)where the error quoted is only statistical. The theoretical end point value is 81.04 Z\]b^LD.The analysis has been repeated using also a sample corresponding to an integrated luminosity of 9.2 fb 	: themeasured end point is<4 ﬂ9ﬀ aB~ﬀ=F Z\1]ﬃ^LD.5 Systematics5.1 MisalignmentThe effect of tracker and muon chambers misalignment expected in the first months of data taking has been eval-uated. Efficiencies are lowered by l | % for muons and by lt % for electrons. The endpoint in the invariantmass distributions is however still visible (Fig. 5, 6).The final selection efficiencies for SUSY events at LM1 point are 1.2% for first data misalignment, compared to anefficiency of 1.6% expected without misalignment. The shift of the endpoint due to the misalignment is of about1 GeV/c.5.2 Jet and electron energy scaleThe error due to the absolute electron energy scale (ElecES) and jet energy scale (JES) uncertainties has also beenevaluated. An uncertainty of 0.25% at all integrated luminosities has been considered for the ElecES [9], while7% for 1 fb 	and of 2% for   fb 	are the values used as JES uncertainty. With 1 fb 	, an increase of 20%3Entries  626Mean    58.17RMS     24.44Underflow       0Overflow       11)2) (GeV/c-l+M(l0 20 40 60 80 100 120 140 160 180 200number of lepton pairs-10010203040Entries  23Mean     91RMS      55Underflow     0Overflow      3CMSLM1ttbar / ndf 2χ  27.62 / 20p0        115.2± 203.4 p1        0.48± 80.42 p2        0.671± 1.174 )2) (GeV/c-l+M(l0 20 40 60 80 100 120 140 160 180 200number of lepton pairs-1001020304050CMSFigure 3: Flavour subtracted distributions of SUSYand I I events for 1 fb 	.Figure 4: Flavour subtracted distributions of SUSYand I I events for 1 fb 	. The fit function is shownsuperimposed.Entries  471Mean    61.45RMS     31.08Underflow       0Overflow        6)2) (GeV/c-µ+µM(0 20 40 60 80 100 120 140 160 180 200number of muon pairs0246810121416182022Entries  328Mean    59.19RMS     31.43Underflow       0Overflow        6CMSIdeal_alignmentFirstDataEntries  403Mean    62.66RMS     31.64Underflow       0Overflow        9)2) (GeV/c-e+M(e0 20 40 60 80 100 120 140 160 180 200number of electron pairs02468101214161820Entries  362Mean    62.69RMS     32.59Underflow       0Overflow        8CMSIdeal_alignmentFirstDataFigure 5: Muon invariant mass distribution for idealalignment and in first data scenario.Figure 6: Electron invariant mass distribution for idealalignment and in first data scenario.in the number of background events and of 8% in the number of signal events is estimated, mainly due to theJES. With 10 fb 	, these increases are of 5% and 2% respectively. The integrated luminosity needed to reach 5sigma significance at LM1, when a 20% systematic error is taken into account for the background, is of 17 pb 	.The position of the endpoint is shifted by about ﬀa GeV ^+Dby a mismeasurement of ElecES, while the JESuncertainty gives a negligible shift.6 ScanIn order to check the observability of SUSY events in the leptons + jets + missing transverse energy final state, ascan of the mSUGRA plane 2  	  8 has been performed using the software for the fast simulation of the CMSdetector, keeping  4Y ,ghK2 8A4, and I?JLK 2 M 8j4f . Background used is the same as in the analysis at LM14point. The same selection cuts for background and signal have been used. Results of the scan are shown for 1, 10,and 30   p 	of integrated luminosity in Fig. 7. With systematic uncertainties the range is slightly reduced, but nodramatic changes are expected (Fig. 8). (GeV)0m0 100 200 300 400 500 600 700 800 900 1000 (GeV)1/2m01002003004005006007008009001000CMS-130 fb-110 fb-11 fbTevatron = 114 GeVhm = 103 GeV χml~LM1LM2LM3LM4LM5LM6LM8HM1HM2 HM3 LSP1τ∼NO EWSB  (GeV)0m0 100 200 300 400 500 600 700 800 900 1000 (GeV)1/2m01002003004005006007008009001000CMS-130 fb-110 fb-11 fbTevatron = 114 GeVhm = 103 GeV χml~LM1LM2LM3LM4LM5LM6LM8HM1HM2 HM3 LSP1τ∼NO EWSB Figure 7: Discovery reach at tan Mt4  for an in-tegrated luminosity of 1, 10, and 30 fb 	, when nosystematic uncertainties are taken into account.Figure 8: Discovery reach at tan MY4  for an inte-grated luminosity of 1, 10, and 30 fb 	, when system-atic uncertainties are taken into account.7 ConclusionsThe observability of the NO"N.S._"NO	.P. decay produced in SUSY chains through the two same flavouropposite sign lepton pairs + Jets + Missing Transverse Energy final state has been studied at mSUGRA benchmarkpoint LM1, with a full simulation of the CMS detector. The dilepton end point can be measured with a statisticalerror of lUﬀGasZ\]y^LDat 1 fb 	. The main systematic uncertainties have also been evaluated. The integratedluminosity needed to reach 5 sigma significance is 14 pb 	without systematics, and 17 pb 	with systematicuncertainties taken into account. The observability of the SUSY events in the 	plane has also beenevaluated.References[1] M. Chiorboli et al., “Leptons + Jets + Missing Energy Analysis at LM1”, CMS NOTE-2006/133[2] M. Battaglia et al. [arXiv:hep-ph/0306219][3] W.Beenakker et al. [arXiv:hep-ph/9611232][4] T. Sjo¨strand et al. Computer Phys. Commun. 135 (2001) 238[5] http://www.phy.bnl.gov/ l isajet/[6] V. Konoplianikov et al., “Jet Calibration using gamma+jet Events in the CMS Detector”, CMS NOTE-2006/042[7] S. I. Bityukov et al. NIM A 452: 518-524, 2000[8] I. Belotelov et al., “Simulation of Misalignment Scenarios for CMS Tracking Devices”, CMS NOTE-2006/008[9] V. Bu¨ge et al., “Prospects for the Precision Measurement of the W Mass with the CMS Detector at the LHC”,CMS NOTE-2006/0615

SUSY searches with Opposite Sign Dileptons at CMS

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