Search for supersymmetry in hadronic final states using MT2 in 7 TeV pp collisions at the LHC

Abstract

A search for supersymmetry or similar new physics was carried out using a $\sqrt{s} = 7$ TeV $pp$ collisions data sample corresponding to 1.1 of integrated luminosity collected by the CMS experiment at the LHC. Fully hadronic final states were selected based on the stransverse mass variable $M_{T2}$. Two complementary analyses were performed. A first one 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 strong cut on $M_{T2}$. A second analysis has been optimized to look for events with a light gluino but heavy squarks. In this case, the $M_{T2}$ cut was relaxed, but a higher jet multiplicity and an additional $b$-tag were required. The dominant backgrounds in both analyses are different. They were estimated using both simulation and data-driven methods. As no excess of events over the expected background was observed, exclusion limits were derived.

Link to Physics Analysis Summary (PAS)

Public PAS

Approved Plots from SUS-11-005 ( click on plot to get .pdf )

Table Abbreviated Caption
HighMT2cutflow.png

Data and MC event yields for the High MT2 analysis after applying all selection cuts and for various cuts on MT2.

The event selection consists of basic data quality cuts and noise rejection, HT>600 GeV, at least three good jets were required, the two leading jets with pT > 100 GeV, a maximum difference between the MET and MHT vectors of 70 GeV, a veto on events with jets with pT ≥ 50 GeV failing the jet identification criteria; a minimum difference in phi between any selected jet and the MET, a veto on isolated electrons and muons. The signal region is defined for MT2 > 400 GeV

LowMT2cutflow.png

Data and MC event yields for the Low MT2 analysis after applying all selection cuts and for various cuts on MT2.

The event selection is the one from the High MT2 Analysis, but with HT >650, the request of at least 4 selected jets, pt >150 GeV for the hardest jet, at least one b-tagged jet. Signal region is defined for MT2 > 150 GeV

Figure Abbreviated Caption
HighMT2_1fb-1_uniform.png MT2 distribution after having applied all selection cuts for the High MT2 Analysis, with constant binning. The last bin contains the overflow. The different MC backgrounds are stacked on top of each other and normalized to 1.1 fb−1. The LM6 signal distribution is normalized to the same integrated luminosity and overlayed on the Standard Model backgrounds.
HighMT2_1fb-1_variable.png MT2 distribution after having applied all selection cuts for the High MT2 Analysis, with variable binning. The last bin contains the overflow. The different MC backgrounds are stacked on top of each other and normalized to 1.1 fb−1. The LM6 signal distribution is normalized to the same integrated luminosity and overlayed on the Standard Model backgrounds.
LowMT2_1fb-1_uniform.png MT2 distribution after having applied all selection cuts for the Low MT2 Analysis, with constant binning. The last bin contains the overflow. The different MC backgrounds are stacked on top of each other and normalized to 1.1 fb−1. The LM9 signal distribution is normalized to the same integrated luminosity and overlayed on the Standard Model backgrounds.
LowMT2_1fb-1_variable.pdf MT2 distribution after having applied all selection cuts for the Low MT2 Analysis, with variable binning. The last bin contains the overflow. The different MC backgrounds are stacked on top of each other and normalized to 1.1 fb−1. The LM9 signal distribution is normalized to the same integrated luminosity and overlayed on the Standard Model backgrounds.
ExclusionLimit_tanb10_HighMT2.pdf

Exclusion limit in the mSUGRA/CMSSM (m0,m1/2) plane for the High MT2 Analysis with $\tan \beta$ = 10.

ExclusionLimit_tanb10_LowMT2.pdf

Exclusion limit in the mSUGRA/CMSSM (m0,m1/2) plane for the Low MT2 Analysis with $\tan \beta$ = 10.

ExclusionLimit_tanb10_BestOf.pdf

Exclusion limit in the mSUGRA/CMSSM (m0,m1/2) plane for the High and Low MT2 Analyses with $\tan \beta$ = 10. Here, the High and Low MT2 Analyses are combined by showing the limit of the analysis with the better performance for each point on the mSUGRA plane.

Re-interpretation in terms of simplified models

The language of simplified models has been introduced recently (see e.g. here and here) to establish an efficient communication channel between the LHC experiments and the theory communities who have proposed a general set of simplified models with which LHC data can be interpreted. A similar approach has been taken before in the hadronic channel with 2010 data -- see here.

The above results have been re-interpreted in terms of the simplified models given in the following table. The signal efficiencies as a function of the mass parameters of the simplified model are also given below, together with upper limit plots on the production cross section. The data for these plots are also available as a ROOT file T1bbbb_MT2b.root. The regions with small 
ΔΜ(gluino,LSP)
 and
 low gluino 
mass
 are 
excluded since they are
 dominated by 
statistical 
errors 
and
 signal 
modeling uncertainty. Signal contamination in control regions has not been taken into account when computing the upper limits on the production cross sections.

Simplified model topology name Simplified model topology Efficiency Limit Caption
"T1bbbb" T1bbbb.png h_eff_MT2b_T1bbbb.png h_limit_MT2b_T1bbbb.png Upper limit on production cross section, for T1bbbb, assuming a branching ratio of 1. Superimposed are 95% CL exclusion contours, assuming a "reference SUSY cross section", for decoupled squarks: m(~q)>>m(~g).

-- PascalNef - 22-Aug-2011

Topic attachments
I Attachment History Action Size Date Who Comment
C source code filec BestOfMT2_allLimits.C r1 manage 16.4 K 2011-08-24 - 14:53 UnknownUser ROOT macro with TGraphs describing the best MT2 limits (High and Low MT2). Observed, expected and 1-sigma band limits are included
PDFpdf ExclusionLimit_tanb10_BestOf.pdf r1 manage 23.7 K 2011-08-22 - 19:47 PascalNef  
PNGpng ExclusionLimit_tanb10_BestOf.png r1 manage 34.4 K 2011-08-22 - 19:30 PascalNef  
PDFpdf ExclusionLimit_tanb10_HighMT2.pdf r1 manage 36.1 K 2011-08-22 - 19:48 PascalNef  
PNGpng ExclusionLimit_tanb10_HighMT2.png r1 manage 34.8 K 2011-08-22 - 19:30 PascalNef  
PDFpdf ExclusionLimit_tanb10_LowMT2.pdf r1 manage 23.9 K 2011-08-22 - 19:48 PascalNef  
PNGpng ExclusionLimit_tanb10_LowMT2.png r1 manage 34.5 K 2011-08-22 - 19:30 PascalNef  
PDFpdf HighMT2_1fb-1_uniform.pdf r1 manage 13.0 K 2011-08-22 - 19:48 PascalNef  
PNGpng HighMT2_1fb-1_uniform.png r1 manage 68.2 K 2011-08-22 - 19:30 PascalNef  
PDFpdf HighMT2_1fb-1_variable.pdf r1 manage 11.4 K 2011-08-22 - 19:49 PascalNef  
PNGpng HighMT2_1fb-1_variable.png r1 manage 55.5 K 2011-08-22 - 19:30 PascalNef  
PDFpdf HighMT2cutflow.pdf r1 manage 46.6 K 2011-08-22 - 19:49 PascalNef  
PNGpng HighMT2cutflow.png r1 manage 106.1 K 2011-08-22 - 19:31 PascalNef  
PDFpdf LowMT2_1fb-1_uniform.pdf r1 manage 9.8 K 2011-08-22 - 19:49 PascalNef  
PNGpng LowMT2_1fb-1_uniform.png r1 manage 64.6 K 2011-08-22 - 19:31 PascalNef  
PDFpdf LowMT2_1fb-1_variable.pdf r1 manage 11.3 K 2011-08-22 - 19:50 PascalNef  
PNGpng LowMT2_1fb-1_variable.png r1 manage 56.8 K 2011-08-22 - 19:31 PascalNef  
PDFpdf LowMT2cutflow.pdf r1 manage 39.0 K 2011-08-22 - 19:50 PascalNef  
PNGpng LowMT2cutflow.png r1 manage 106.9 K 2011-08-22 - 19:31 PascalNef  
PDFpdf T1bbbb.pdf r1 manage 8.0 K 2011-11-18 - 20:02 MariarosariaDalfonso  
PNGpng T1bbbb.png r1 manage 19.9 K 2011-11-18 - 20:01 MariarosariaDalfonso  
PDFpdf h_eff_MT2b_T1bbbb.pdf r1 manage 8.2 K 2011-11-18 - 20:01 MariarosariaDalfonso  
PNGpng h_eff_MT2b_T1bbbb.png r1 manage 108.3 K 2011-11-18 - 20:00 MariarosariaDalfonso  
PDFpdf h_limit_MT2b_T1bbbb.pdf r1 manage 8.8 K 2011-11-18 - 20:00 MariarosariaDalfonso  
PNGpng h_limit_MT2b_T1bbbb.png r1 manage 133.3 K 2011-11-18 - 20:00 MariarosariaDalfonso  
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