CDF Logo Search for the Supersymmetric Partner of the Top Quark in Dilepton Events,
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We present a search for the lightest supersymmetric partner of the top quark (stop) in proton-antiproton collisions. This search was conducted within the framework of the Rp conserved Minimal Supersymmetric extension of the Standard Model, assuming the decay Stop into lepton, sneutrino and b quark is dominant. We searched a total of 1 fb-1 of data collected by the CDF experiment looking for two leptons (ee, eμ, μμ) of opposite electric charge, jets and missing transverse energy. No significant evidence of the stop signal has been found. New limits at 95% confidence level in the stop versus sneutrino mass plane have been set.


Description of the analysis

In R-parity conserved MSSM stop quarks are produced in pairs. Feynman diagrams for dominant stop production processes at the Tevatron are shown below.


PS or GIF


PS or GIF


PS or GIF


PS or GIF

In some scenarios decay of the lightest stop quark into lepton, sneutrino (via virtual chargino) and b quark is dominant. In this analysis we assume that the branching ratio of such process is 100%. Sneutrino may be either LSP, or decay into neutrino and neutralino(LSP). We used PYTHIA as a signal MC generator. Next-to-leading order cross-section for stop masses from 50 to 200 GeV/c2 was calculated using PROSPINO with CTEQ6M1 Parton Distribution Function (PDF).


PS or GIF

In this analysis we look for two isolated leptons with pT ≥10 and ≥5 GeV/c and Invariant mass ≥ 15 GeV/c2, at least one jet with ET ≥ 15 GeV and missing ET. To crosscheck our background estimations we looked at several control regions : Z peak (for ee, μμ), zero jets, low MET. Reasonable agreement in all control regions was found. All the above control regions were removed from the signal region, which is defined as following : no same flavor leptons with invariant mass in Z peak (76,106), at least one jet, MET ≥ 15 GeV, any object (leptons, jet) should be greater than 20° away in φ from MET.


EPS or GIF

Analysis cuts depend dominantly on Δm (stop - sneutrino mass difference). Taking this into account, we divided our signal region into 4 cut groups (a,b,c,d). For each group we tuned cuts to get best expected exclusion limit using all three dilepton channels (ee, eμ, μμ). We use statistical program CORLIM (Bayesian) for 95% CL exclusion limit calculations. This program takes into account systematic error correlations between different dilepton channels. Systematic errors are (per channel) : 6% - luminosity, 2% - efficiency of the dilepton triggers, 2% - lepton ID efficiency, 2% - PDF, 3% - isr/fsr. Uncertainties on theoretical calculations of the cross sections are the following : DY - 2%, tt - 8%, WW - 6%, WZ - 8%, ZZ - 10%, Wγ - 7%. Stop cross-section systematic uncertainty (20%) was not used in this analysis (central value was used for limit calculations). Jet energy scale systematic : tt - 0.3%, dibosons - 11%, DY - 35%.

Blessed Plots and Tables:

Results (CDF only):
95% CL Bayesian exclusion limits

EPS or GIF
Cross-section upper limits

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Definitions of Control Regions used in the analysis
(y axis - Δφ between leptons) :
0 jets

EPS or GIF
≥1 jet

EPS or GIF

Z peak (ee, μμ with InvM∈(76,106), plots) is removed from the rest.
Heavy flavor contribution (bb,cc) is estimated from data.

MET plots for Control Regions with at least one jet are here.
MET Pre-signal plots are here. Corlim tuning curves are here.

Tables:
Pre-Signal definition/# of events

EPS or GIF
Cuts for different Groups

EPS or GIF

Pre-Signal clean up cut :
any object (both leptons and jet) should be 20° away in φ from MET.

Expected (observed) plots (blue dots are excluded,
red squares are not excluded, green stars used for tuning cuts) :
Groups in stop/snu mass plane

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Expected (with group lines)

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Tables:
Number of events after all cuts for different cut groups.
Groups a & d

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Groups b & c

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(N-1) Met Plots for four cut groups (ee+eμ+μμ) :
Group a

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Group b

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Group c

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Group d

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Here are separate (ee, eμ, μμ) plots for cut groups a, b, c and d,
as well as for the pre-signal region.



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