Search for electroweak production of charginos and neutralinos using leptonic final states in pp collisions at √s = 7 TeV

Abstract

The 2011 dataset of the CMS experiment, consisting of an integrated luminosity of 4.98 fb−1 of pp collisions at 7 TeV, enables expanded searches for direct electroweak pair production of charginos and neutralinos in supersymmetric models, or for the analog in other models of new physics. Searches sensitive to such processes, followed by decays to leptonic final states, are presented. Final states with three leptons, with a same-sign lepton pair, and with an opposite-sign lepton pair in conjunction with two jets, are examined. No excesses above the standard-model expectations are observed. The results are used in conjunction with previous results on four-lepton final states to exclude a range of chargino and neutralino masses from approximately 200 to 500 GeV, depending on the decay modes of these particles.

This analysis is documented in arXiv:1209.6620, submitted to JHEP.

Approved Tables and Plots from SUS-12-006 ( click on plot to get .pdf )

Tables in the paper Abbreviated Caption
Table1.png Table 1: The observed and total expected background in bins of Missing Transverse Energy (MET) for three-lepton events with HT < 200 GeV, an opposite-sign same-flavor lepton pair, and no Z boson candidate. These results were published JHEP

Table2.png Table 2: Relative systematic uncertainties for the WZ mean background. On-Z refers to events in which the OSSF pair satisfies 81 < Mll < 101 GeV. Off-Z refers to events with ei- ther Mll < 81 GeV or Mll > 101 GeV. The events are further categorized according to whether they have low (< 100 GeV) or high (> 100 GeV) MT. The off-Z, low MT column corresponds to the sum of events in regions I and V in Fig. 3, while the off-Z, high MT region corresponds to the sum of regions II and IV

Table3.png Table 3: Summary of mean expected backgrounds and observations for each region in the three-lepton search based on Mll and Emiss.

Table4.png Table 4: Summary of mean expected backgrounds and observed yields in the Emiss > 200 GeV search region for all six same-sign dilepton channels. The background categories comprise non- prompt and misidentified leptons, charge misassignment, and rare standard model processes. Uncertainties include statistical and systematical contributions.

Table5.png Table 5: Summary of mean expected backgrounds and observed data in each of the Emiss search regions, in final states with two opposite-sign leptons, two jets, and Emiss. The total background is the sum of the Z + jets background evaluated with γ + jets events, the flavor-symmetric back- ground predicted from opposite-flavor events (OF background), and the WZ/ZZ background expected from simulation (WZ/ZZ background). All uncertainties indicate the statistical and systematic components added in quadrature.

Table6.png Table 6: Summary of the results from the broad multi-lepton analysis, used as input to the combined limit on the GMSB model. All categories have four leptons including an OSSF pair consistent with a Z. N(τ) denotes the number of these leptons that are identified as taus.

Table7.png Table 7: The parameters for the efficiency function ε(x) where x represents pT(μ), pT(e), Emiss, MT, and the pT(parton) with different flavours.

Figures in the Paper Abbreviated Caption
Figure1a.pngFigure1b.png Figure 1: Diagrams of chargino-neutralino pair production followed by decays to sleptons leading to a final state with three leptons, two LSPs, and a neutrino. In the case of left-handed sleptons, both diagrams, top and bottom, can be realized in nature and for each of the diagram neutralino can also decay to neutrino-sneutrino pair. Thus only 50% of produced chargino-neutralino pairs result in three leptons. For right handed sleptons on the other hand, neutralino decays to a slepton-neutrino pair 100% of the time.
Figure2a.png Figure2b.png Figure 2: Diagrams of chargino-neutralino (left) and neutralino-neutralino (right) pair production followed by decay to on-shell W or Z bosons and LSPs.
Figure3.png Figure 3:Distribution of the selected 3 lepton events with MET > 50 GeV in transverse mass (MT) and dilepton invariant mass Mll plane. For μμμ and eee channels, the opposite-sign-same-flavour (OSSF) lepton pair that forms an invariant mass closest to Z boson mass is used for Mll calculation and the third lepton and MET for the MT calculation. Two events appear outside the limits of the plot; one is a μμμ event at (Mll, MT) = (240 GeV, 399 GeV) and the other is an eee event at (95 GeV, 376 GeV).
Figure4a.png Figure 4a: One dimentional projection of Figure 3 onto MT for Mll < 81 GeV where observed data overlaid with predicted SM backgrounds. This plot corresponds to events in search regions I and II. Backgrounds due to non-prompt leptons
Figure4b.png Figure 4b: One dimentional projection of Figure 3 onto MT for Mll between 81 and 101 GeV. Observed data overlaid with predicted SM backgrounds.
Figure4c.png Figure 4c: One dimentional projection of Figure 3 onto MT for Mll > 101 GeV. Observed data overlaid with predicted SM backgrounds. This plot corresponds to events in search regions IV and V.
Figure5a.png Figure 5a: Selected exclusive same-sign di-lepton events mapped onto MET and HT plane where events of ee, μμ , eμ , tau-tau, tau-e and tau-μ pairs displayed seperately.
Figure5b.png Figure 5b: Mean expected background yields with their uncertainty and observed number of events in the six channels, for the signal region with MET > 200 GeV.
Figure6.png Figure 6: Selected events in opposite-sign-same-flavour di-leptons and dijet analysis as a function of MET. Observed data is compared with the corresponding SM expectation. The total expected SM curve (red histogram) is the sum of the OF prediction (purple histogram), the diboson background prediction (green histogram), and the prediction from γ + jets. For purposes of illustration, expected signal events from WZ+2LSP SMS model (see Figure 2 left above ), with M(chargino) = M(neutralino) = 200 GeV and M(LSP) = 0 GeV, are added on top of total expected SM background shown in orange dashed curve. The bottom plot shows the ratio of the observed and predicted distributions.
Figure7a.png Figure 7a: The observed 95% CL upper limit on the direct chargino-neutralino production cross section times branching ratio, in a scenario where slepton masses are set to 0.5*[M(chargino/neutralino) + M(LSP)], mapped onto M(chargino/neutralino) and M(LSP) obtained using MET based tri-lepton analysis. The solid (dotted) contours bound the observed (expected) mass region excluded at 95% CL for a branching fraction of 50%, as appropriate for the three-lepton decay products in the flavor-democratic scenario. These results assume democratic branching fractions to leptons.
Figure7b.png Figure 7b: Same as Figure 7a above but in this case chargino decays to stau+LSP 100% of the time and neutralino decays always to a slepton and charged lepton pair.
Figure8a.png Figure 8a: The 95% CL upper limit on chargino-neutralino production cross section times branching fraction in the flavor-democratic scenario, for the combined analysis of the three-lepton search using Mll and MT, and the same- sign dilepton search. The contours bound the mass regions excluded at 95% CL for a branching fraction of 50%, as appropriate for the visible decay products in this scenario. The contours based on the observations are shown for the separate searches and for the combination; in addition, the expected combined bound is shown. The results correspond to slepton mass M(slepton) = 0.25*M(chargino/neutralino) + 0.75*M(LSP).
Figure8b.png Figure 8b: Same as Figure 8a except that slepton mass is set to M(slepton) = 0.5*M(chargino/neutralino) + 0.5*M(LSP).
Figure8c.png Figure 8c: Same as Figure 8a except that slepton mass is set to M(slepton) = 0.75*M(chargino/neutralino) + 0.25*M(LSP).
Figure9a.png Figure 9a: The 95% CL upper limit on chargino-neutralino production cross section times branching fraction in the tau-enriched scenario, for the combined analysis of the three-lepton search using Mll and MT, and the same- sign dilepton search. The contours bound the mass regions excluded at 95% CL for a 100% branching fraction of neutralino to charged leptons and chargino to stau and neutrino pair. The contours based on the observations are shown for the separate searches and for the combination; in addition, the expected combined bound is shown. The results correspond to slepton mass M(slepton) = 0.25*M(chargino/neutralino) + 0.75*M(LSP).
Figure9b.png Figure 9b: Same as Figure 9a except that slepton mass is set to M(slepton) = 0.5*M(chargino/neutralino) + 0.5*M(LSP).
Figure9c.png Figure 9c: Same as Figure 8a except that slepton mass is set to M(slepton) = 0.75*M(chargino/neutralino) + 0.25*M(LSP).
Figure10.png Figure 10: Interpretation of the WZ + MET and three-lepton + MT/Mll analysis results in the context of the WZ+2LSP SMS. The observed and the expected 95% C.L exclusion curves are shown seperately for dilepton+dijet and tri-lepton with MT and Mll searches as well as the statistical combination of these two results. The wino-like cross section with coupling gγ is assumed.
Figure11.png Figure 11: Interpretation of the ZZ + Emiss (2l2j) and multi-lepton results in the context of the GMSB model described in the text. The cross section upper limits are indicated for the ZZ + Emiss observed (dotted blue line), multi-lepton observed (dotted green line), the combined observed (solid black line), and the combined expected (thick black line) results. The theory prediction for the cross section is indicated by the solid thick red line.
Figure13.png Figure 12: Summary of the observed 95% C.L. exclusion curves on M(chargino/neutralino) versus M(LSP) plane for: the three-lepton+MET search (Figures 7a and 7b above), separately for the left and right handed slepton scenarios; the combination of the three-lepton analysis, based on Mll and MT, with the SS dilepton analysis, separately for the left and right handed scenarios; and the combination of the diboson analysis with two leptons and two jets (VZ+MET analysis) with the three-lepton analysis based on Mll and MT, for the WZ + Emiss model. Regions excluded by searches at LEP2 for sleptons and charginos are also indicated. The implied branching fractions are noted in the legend. For models with intermediate sleptons (including the LEP2 slepton limit), the interpretations correspond to slepton masses set to M(slepton)= 0.5[ M(chargino/neutralino)+M(LSP)].

Additional public Figures Abbreviated Caption
Figure1_add.png Figure 1 add: MET distribution of observed events compared to pure simulation normalized to the integrated luminosity in a WZ enriched sample. We select events with tri-lepton where a tightly selected same-sign pair and an additional lepton passing a looser selection that makes an opposite-sign same-flavor Z candidate with one of the two hypothesis leptons are present. With this selection we obtain a control sample where WZ constitutes about 90% of the yield towards the high end tail of the distribution.
Figure2a_add.png Figure2b_add.png Figure 2 add: MET distribution of the events in exclusive same-sign tau-tau, e-tau, μ - tau (left) and ee, μμ eμ (right) events. The expected mean background distributions for each background sources is also shown.
Figure3_add.png Figure 3 add: The distribution of missing ET in thri-lepton events where the OSSF di-lepton pair form an invariant mass away from Z mass (abs(Mll-MZ) > 10 GeV) but the invariant mass of the thri-leptons is close to Z mass (abs(Mll-MZ) < 10 GeV). This control sample is highly enhanced in ZGamma(*). A rayleigh-fit to this spectrum is performed at MET < 50 GeV to predict the background at high MET region.
Figure4a_add.pngFigure4b_add.png Figure4c_add.png Figure4d_add.png Figure 4 add: Data to MC comparison for the response (top two plots) and resolution (bottom two plots) of the transverse energy of the recoil system in selected Z+Jets events. They are presented seperately for the parallel, u1, (left) and perpendicular, u2, (right) components with respect to the direction of the Z boson.
Figure13_Add.png Figure 5 add: The distribution of the di-lepton, ee (left) and μμ (right), invariant mass for events passing the pre-selection for 4.98 fb−1 in VZ+MET analysis. Pre-selection is defined with exactly two opposite-sign-same-flavour leptons with pT > 20 GeV and at least two jets with pT > 30 GeV where events with a b-tagged jet are vetoed. Filled histograms correspond to expected SM backgrounds obtained using simulation.
Figure16_Add.png Figure 6 add: Interpretation in direct neutralino-neutralino pair production that results in ZZ+2LSP final state. The signal selection efficiency times acceptance for OSSF di-lepton + di-jet analysis (VZ+MET analysis) after MET > 60 GeV selection is shown on the left plot. The observed 95% CL cross section upper limits is shown in the right plot.
Figure7a_add.png Figure7b_add.png Figure7c_add.png Figure 7 add: Lepton reconstruction and identification efficiency as a function of lepton pT obtained in the WZ MC sample (left) . Selection efficiency for MET>50 GeV and mT > 100 GeV as a function of true MET and MT obtained in WZ MC (middle). b-jet identification probability for three types of jets; b-quark jets, c-quark jets and light- flavour jets shown as a function of parton pT (right). The first two obtained in ttbar MC and the last one in WZ MC.

Table2_Add.png Table 1: Summary of mean expected backgrounds and observed data in each of the ETmiss search regions. The details are the same as in Table 5 of the paper except that here the results are quoted in inclusive ETmiss regions.
Table1_Add.png Table 2: Summary of mean expected backgrounds and observed data in each of the ETmiss search regions. The details are the same as in Table 5 of the paper except that here the results are quoted in inclusive ETmiss regions, and the dijet mass requirement and veto of events with at least three selected leptons are removed.
Table3_Add.png Table 3: Summary of data and MC expected yields after the preselection requiring a Z boson candidate, at least two jets, no b-tagged jets, dijet mass in the range 70 < M(jj) < 110 GeV, and exactly two selected leptons.

-- DidarDobur - 26-Jul-2012

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