CMS Conference Report

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1 Available on information server CR 6/37 Conference Report July 6 SUSY searches with pposite Sign Dileptons at M. Chiorboli, M. Galanti, A. Tricomi Università and IF, Catania Abstract A full simulation study with the detector is presented. The Leptons Jets Missing Energy (l = e, ) final state for SUSY events is investigated at msugra benchmark point. The end point in the dilepton pair invariant mass distribution is reconstructed and a scan of the plane is performed in order to determine the observability reach. Presented at Physics at LHC 6, Cracow, July 38, 6

2 z z Table 1: Cross section at L, selection efficiencies and number of events surviving cuts for signal and background processes. 1 Introduction Process (pb) Ev. analysed in 1 fb SUSY () 78k k jets k 1. 6 jets 66k! DY$# k %& 1 ' %( DY)#* k '.,,,1,3 /...76 *98 P:;=<?>A@CB,, GeV/D 3.3 k jets 11 37k..7 jets 1.E 176k B9GF!H k 8. 'E.83 The SUSY production cross section at LHC is dominated by gluinos and squarks which decay mainly through a chain to the lightest neutralinos. For low and moderate I?JLKM values, many decay chains end up with the decays.qp3.r and.ts3..p3., with.u6, in which the two final state leptons provide a natural trigger. Leptons (electrons and muons) from the decay exhibit a peculiar.vpw. invariant mass distribution with a sharp edge. In this report the method to reconstruct the dilepton end point is described [1]. We proved the feasibility of the method at benchmark point (X, X `#a[z\1]b^d, c gih K 8j YB[Z\1]_^D Y, IdJKMef, ) [], and the observability of the signal in the msugra (, ) plane for I?JLKMk. Signal The msugra benchmark point has been chosen as the working point (total L cross section lma# pb, calculated with PRSPI [3]). The branching ratio of the decay. S..QPW. is 11.%. The events analysed in this note have been produced using PYTHIA 6. [] interfaced with ISAJET 7.69 []. A full detector simulation has been employed. Low luminosity pileup has also been taken into account. The Level and High Level Trigger paths require a single isolated electron or muon to select the event. 3 Background The Standard Model backgrounds considered for this analysis are: I I, WWjets, DY n#l., Zbb o.q., Wjets, Zjets, QCD, ZZjets and I Iip p. The full simulation of the detector has been used also for the backgrounds. Table 1 summarizes the number of expected events in 1 fb for SUSY events and the Standard Model backgrounds considered, after the following selections are applied: q at least two sameflavour oppositesign (SFS) isolated leptons, with rs:(@tsz\1]b^d and u vwuw%x# for both electrons and muons; q at least two jets with ry: and and u vwu{%( 9 ; The total number of SUSY events after all selection cuts is 83 in 1 fb over the whole SUSY sample. Results at, corresponding to an efficiency of 1.6% The invariant mass distribution of the same flavour opposite sign lepton pairs after all selection cuts and for an integrated luminosity of 1 fb is shown, superimposed over the I I background, in Figure 1. In SUSY events, the presence of two SFS leptons can also be due to processes different from. S._.P3. decay. If the two leptons are independent of each other, one would expect equal amounts of SFS leptons,,

3 and different flavour opposite sign (DFS) leptons. Their distributions should also be identical. The background SFS contribution can hence be removed by subtracting the DFS events. Figure shows the same SFS distribution for SUSY events together with the distribution of the DFS lepton pairs. The flavour subtracted SFS dilepton pair distributions for both SUSY and I I background are shown in Figure 3 for an integrated luminosity of 1 fb. The I I background contribution after the flavour subtraction is close to zero since the lepton pairs are always uncorrelated. The numbers of SUSY and background dilepton pairs surviving cuts are respectively 913 and, giving a signal to background ratio of.1. A statistical significance of sigma, calculated using [7], can be obtained with 1 pb of integrated luminosity ttbar Entries 913 Mean 6.3 RMS 8.81 verflow 19 Entries 1 Mean 18.6 RMS 8. verflow SFS DFS Entries 913 Mean 6.3 RMS 8.81 verflow 19 Entries 87 Mean 6. RMS verflow Figure 1: Same flavour opposite sign lepton pair distributions of SUSY and I I events for 1 fb. Figure : SFS and DFS distributions of events for 1 fb. The value of the end point can be extracted by fitting the flavour subtracted distribution with a convolution of a triangle and a Gaussian function (Fig. ). The value obtained from the fit is < #~ jz\]^d where the error quoted is only statistical. The theoretical end point value is 81. Z\]b^LD. The analysis has been repeated using also a sample corresponding to an integrated luminosity of 9. fb measured end point is <. 9 ab~=f Z\1]^LD (1) : the Systematics.1 Misalignment The effect of tracker and muon chambers misalignment expected in the first months of data taking has been evaluated. Efficiencies are lowered by l % for muons and by lt % for electrons. The endpoint in the invariant mass distributions is however still visible (Fig., 6). The final selection efficiencies for SUSY events at point are 1.% for first data misalignment, compared to an efficiency of 1.6% expected without misalignment. The shift of the endpoint due to the misalignment is of about 1 GeV/c.. Jet and electron energy scale The error due to the absolute electron energy scale (ElecES) and jet energy scale (JES) uncertainties has also been evaluated. An uncertainty of.% at all integrated luminosities has been considered for the ElecES [9], while 7% for 1 fb and of % for fb are the values used as JES uncertainty. With 1 fb, an increase of % 3

4 3 1 ttbar Entries 66 Mean 8.17 RMS. verflow 11 Entries 3 Mean 91 RMS verflow χ / ndf 7.6 / p 3. ± 11. p1 8. ±.8 p 1.17 ± ) M(l l ) (GeV/c Figure 3: Flavour subtracted distributions of SUSY and I I events for 1 fb. Figure : Flavour subtracted distributions of SUSY and I I events for 1 fb. The fit function is shown superimposed. number of muon pairs Ideal_alignment FirstData Entries 71 Mean 61. RMS 31.8 verflow 6 Entries 38 Mean 9.19 RMS 31.3 verflow 6 number of electron pairs Ideal_alignment FirstData Entries 3 Mean 6.66 RMS 31.6 verflow 9 Entries 36 Mean 6.69 RMS 3.9 verflow M(µ µ ) (GeV/c ) M(e e ) (GeV/c ) Figure : Muon invariant mass distribution for ideal alignment and in first data scenario. Figure 6: Electron invariant mass distribution for ideal alignment 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 the JES. With 1 fb, these increases are of % and % respectively. The integrated luminosity needed to reach sigma significance at, when a % systematic error is taken into account for the background, is of 17 pb. The position of the endpoint is shifted by about a GeV^D by a mismeasurement of ElecES, while the JES uncertainty gives a negligible shift. 6 Scan In order to check the observability of SUSY events in the leptons jets missing transverse energy final state, a scan of the msugra plane 8 has been performed using the software for the fast simulation of the detector, keeping Y, g h K 8A, and I?JLK M 8jf. Background used is the same as in the analysis at

5 point. The same selection cuts for background and signal have been used. Results of the scan are shown for 1, 1, and 3 p of integrated luminosity in Fig. 7. With systematic uncertainties the range is slightly reduced, but no dramatic changes are expected (Fig. 8) τ LSP 1 HM1 HM HM τ LSP 1 HM1 HM HM3 7 7 (GeV) m 1/ 6 3 ~ 1 l LM LM6 Teva tron LM LM LM8 LM3 m χ = 13 GeV 3 fb 1 fb = 11 GeV m h 1 fb (GeV) m 1/ 6 3 ~ 1 l LM LM6 Teva tron LM LM LM8 LM3 m χ = 13 GeV 3 fb 1 fb = 11 GeV m h 1 fb EWSB m (GeV) EWSB m (GeV) Figure 7: Discovery reach at tanmt for an integrated luminosity of 1, 1, and 3 fb, when no systematic uncertainties are taken into account. Figure 8: Discovery reach at tanmy for an integrated luminosity of 1, 1, and 3 fb, when systematic uncertainties are taken into account. 7 Conclusions The observability of the. S._. P. decay produced in SUSY chains through the two same flavour opposite sign lepton pairs Jets Missing Transverse Energy final state has been studied at msugra benchmark point, with a full simulation of the detector. The dilepton end point can be measured with a statistical error of lugasz\]y^ld at 1 fb. The main systematic uncertainties have also been evaluated. The integrated luminosity needed to reach sigma significance is 1 pb without systematics, and 17 pb with systematic uncertainties taken into account. The observability of the SUSY events in the plane has also been evaluated. References [1] M. Chiorboli et al., Leptons Jets Missing Energy Analysis at, TE6/133 [] M. Battaglia et al. [arxiv:hepph/3619] [3] W.Beenakker et al. [arxiv:hepph/96113] [] T. Sjöstrand et al. Computer Phys. Commun. 13 (1) 38 [] isajet/ [6] V. Konoplianikov et al., Jet Calibration using gammajet Events in the Detector, TE 6/ [7] S. I. Bityukov et al. IM A : 18, [8] I. Belotelov et al., Simulation of Misalignment Scenarios for Tracking Devices, TE 6/8 [9] V. Büge et al., Prospects for the Precision Measurement of the W Mass with the Detector at the LHC, TE6/61