Search for supersymmetry in hadronic final states using MT2 in pp collisions at sqrt(s) = 7 TeV

Abstract

A search for supersymmetry or other new physics resulting in similar final states is presented using a data sample of 4.73 fb-1 of pp collisions collected at $\sqrt{s} $ = 7 TeV with the CMS detector at the LHC. Fully hadronic final states are selected based on the variable MT2, an extension of the transverse mass in events with two invisible particles. Two complementary studies are performed. The first targets the region of parameter space with medium to high squark and gluino masses, in which the signal can be separated from the standard model backgrounds by a tight requirement on MT2. The second is optimized to be sensitive to events with a light gluino and heavy squarks. In this case, the MT2 requirement is relaxed, but a higher jet multiplicity and at least one b-tagged jet are required. No significant excess of events over the standard model expectations is observed. Exclusion limits are derived for the parameter space of the constrained minimal supersymmetric extension of the standard model, as well as on a variety of simplified model spectra.

This analysis is documented in arXiv:1207.1798, submitted to JHEP.

Further documentation is given in CMS-PAS-SUS-12-002

Approved Plots from SUS-12-002 ( click on plot to get .pdf )

MT2-Analysis

MT2-distributions
Figure Abbreviated Caption
MT2Analysis_MT2plot_4.73fb-1.png
MT2Analysis_MT2plot_ratio_4.73fb-1.png
MT2 distribution for the MT2-Analysis with HT > 750 GeV. The last bin contains the overflow. The different MC backgrounds are stacked on top of each other and normalized to 4.73 fb−1. The LM6 signal distribution is normalized to the same integrated luminosity and overlayed on the Standard Model backgrounds. The lower plot also includes a data/MC ratio comparison. Only statistical uncertainties are shown in the ratio plot, both for data and MC.
MT2Analysis_MT2plot_variableBinSize_4.73fb-1.png
MT2Analysis_MT2plot_variableBinSize_ratio_4.73fb-1.png
MT2 distribution for the MT2-Analysis with HT > 750 GeV. The bin size is variable chosen such that the last five bins coincide with the definition of the signal regions. The last bin contains the overflow. The different MC backgrounds are stacked on top of each other and normalized to 4.73 fb−1. The LM6 signal distribution is normalized to the same integrated luminosity and overlayed on the Standard Model backgrounds. The lower plot also includes a data/MC ratio comparison. Only statistical uncertainties are shown in the ratio plot, both for data and MC.

QCD factorization method
Figure Abbreviated Caption
MT2Analysis_QCD_minDPhiRatio_4.73fb-1.png Figure illustrating the QCD estimation method in data. The data shows the ratio $N(\Delta \varphi_{\mathrm{min}} \ge 0.3)/N(\Delta \varphi_{\mathrm{min}} \le 0.2)$ as a function of MT2. The solid points correspond to raw data after all selection cuts, while the dashed points correspond to data after subtracting processes other than QCD (i.e., electroweak and tt̄). An exponential fit (green curve) is performed in the region 50 < MT2 < 80 GeV where the non-QCD contribution is minimal. The constant term of the final functional model (blue curve) is taken as the value of the exponential at MT2 = 250 GeV.

Z(nunu) from W(mu nu)
Figure Abbreviated Caption
MT2Analysis_MuonPt_ZnunuFromWtomunu_BVeto_4.73fb-1.png Muon transverse momentum for the W enriched selection used for the Z(nunu) background estimate from W(mu nu) events. All selection cuts are applied and a b-tag veto is imposed. Only statistical uncertainties are shown in the ratio plot, both for data and MC.
MT2Analysis_MT2_ZnunuFromWtomunu_BVeto_4.73fb-1.png MT2 distribution for the events with a reconstructed muon in the W enriched selection. The muon momentum is added to the MET-vector to mimic the Z(nunu) kinematics. Only statistical uncertainties are shown in the ratio plot, both for data and MC.

Z(nunu) from Gamma+jets
Figure Abbreviated Caption
MT2Analysis_PhotonMT2spectrum_4.73fb-1.png
MT2Analysis_PhotonMT2spectrum_ratio_4.73fb-1.png
Photon MT2 spectrum, where the photon was added to the MET-vector to mimic the Z(nunu) kinematics. The lower plot also includes a data/MC ratio comparison.
MT2Analysis_Vboson_PtSpectra_4.97fb-1.png
MT2Analysis_Vboson_PtSpectra_ratio_4.97fb-1.png
Top: Vector boson transverse momenta for $Z(ee, \mu\mu)$ and $\gamma+jets$ events for data and MC in 4.97 fb-1. Bottom: $Z(ee, \mu\mu)$ to $\gamma+jets$ ratio as a function of vector boson transverse momenta both for data and MC. Only statistical errors are shown.

Lost Lepton background estimate
Figure Abbreviated Caption
MT2Analysis_MuonPt_4.73fb-1.png
MT2Analysis_MuonPt_ratio_4.73fb-1.png
Muon transverse momentum for the lepton selection used in the Lost Lepton method. The lower plot also includes a data/MC ratio comparison. Only statistical uncertainties are shown in the ratio plot, both for data and MC.
MT2Analysis_MuonMT2plot_4.73fb-1.png
MT2Analysis_MuonMT2plot_ratio_4.73fb-1.png
MT2 distribution for the events with a reconstructed muon. All selection cuts are applied (but the lepton veto is inverted). The lower plot also includes a data/MC ratio comparison. Only statistical uncertainties are shown in the ratio plot, both for data and MC.

Other distributions
Figure Abbreviated Caption
MT2Analysis_MT2_METgt300_4.73fb-1.png MT2 with MET > 300 GeV and all selection cuts applied.

MT2b-Analysis

Figure Abbreviated Caption
MT2bAnalysis_MT2Plot_4.73fb-1.png
MT2bAnalysis_MT2Plot_ratio_4.73fb-1.png
MT2 distribution for the MT2b-Analysis with HT > 750 GeV. The last bin contains the overflow. The different MC backgrounds are stacked on top of each other and normalized to 4.73 fb−1. The LM9 signal distribution is normalized to the same integrated luminosity and overlayed on the Standard Model backgrounds. The lower plot also includes a data/MC ratio comparison. Only statistical uncertainties are shown in the ratio plot, both for data and MC.
MT2bAnalysis_MT2Plot_variableBinSize_4.73fb-1.png
MT2bAnalysis_MT2Plot_ratio_variableBinSize_4.73fb-1.png

MT2 distribution for the MT2b-Analysis with HT > 750 GeV. The bin size is variable chosen such that the last four bins coincide with the definition of the signal regions. The last bin contains the overflow. The different MC backgrounds are stacked on top of each other and normalized to 4.73 fb−1. The LM9 signal distribution is normalized to the same integrated luminosity and overlayed on the Standard Model backgrounds. The lower plot also includes a data/MC ratio comparison. Only statistical uncertainties are shown in the ratio plot, both for data and MC.

Background prediction

Figure Abbreviated Caption
MT2_highHT_4p7fb-1.png Total background prediction in the five MT2 signal regions for the high-HT bin of the MT2 Analysis. The gray shaded area corresponds to the total (statistical and systematic) uncertainty on the background prediction for each bin.
MT2_lowHT_4p7fb-1.png Total background prediction in the five MT2 signal regions for the low-HT bin of the MT2 Analysis. The gray shaded area corresponds to the total (statistical and systematic) uncertainty on the background prediction for each bin.
MT2b_highHT_4p7fb-1.png Total background prediction in the four MT2 signal regions for the high-HT bin of the MT2b Analysis. The gray shaded area corresponds to the total (statistical and systematic) uncertainty on the background prediction for each bin.
MT2b_lowHT_4p7fb-1.png Total background prediction in the four MT2 signal regions for the low-HT bin of the MT2b Analysis. The gray shaded area corresponds to the total (statistical and systematic) uncertainty on the background prediction for each bin.

Interpretation

mSUGRA/CMSSM limits
Figure Abbreviated Caption
MT2_and_MT2b_mSUGRA.png Exclusion limit at 95% CL in the mSUGRA m0, m1/2 plane for the MT2 and MT2b analyses.
BestOf_MT2_and_MT2b_mSUGRA.png The results from both the MT2 and MT2b selections are combined into one single limit exclusion curve based on the best expected limit in each point of the plane.

SMS limits
Simplified Model Topology Name Limit
SMS_T1_MT2Analysis.png Limits in the T1 model using the MT2 analysis.
SMS_T1bbbb_MT2bAnalysis.png Limits in the T1bbbb model using the MT2b analysis.
SMS_T5zz_MT2Analysis.png Limits in the T5zzInc model using the MT2 analysis.
SMS_T5zz_MT2bAnalysis.png Limits in the T5zzInc model using the MT2b analysis.
SMS_T1tttt_MT2Analysis.png Limits in the T1tttt model using the MT2b analysis.
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