Wt analysis

Tasks of the priority:

  • Check why ttbar dilepton events has only one jet reconstructed: Proposal new cuts to reduce ttbar
  • Subtract method for fake lepton contribution estimation
  • Optimization of Wt dilepton selection

Introduction

Single top Wt channel is not accessible at Tevatron. While its cross section is enhanced at LHC. The main backgrounds for Wt dilepton channel are ttbar, Drell-Yan, dibosons. The single top group working on its own D3PDs. the D3PD files available here for data and here for MC. My work based on MSU SingleTopRootAnalysis package. The svn position of STRA is here.

The cross sections for MC are here. The cross section for single top at 7 TeV are 58723 fb (t channel) 13102 fb(Wt channel) and 3937 fb (s channel). More information on single top are in SingleTop working group .

Event Selection

Object definition

The event Definition is based on top common objects definitions.

Wt dilepton channel Selections

exactly 2 tight highly isolated leptons with Pt>20 GeV, |eta| < 2.47(2.5 for muons), and exactly one jets with Pt > 20 GeV, |eta| < 5; details are here.

Background estimation strategy

Drell-Yan

Using ABCD method to estimate DY from data. The two uncorrelated variables are M_{ll} and MET. The MET-M_{ll} plane is divided into A,B,C/D,E,F region,

DY_A = N_f * (B_data - k * B_ttbar)/ (E_data - k * E_ttbar) * (D_data - k * D_ttbar)

The N_f is here to correct the facts that MET-Mll correlation, k scales MC ttbar to take into account contamination from non DY. Fit N_f and k are from really data using a likelihood based on a poisson assumption of the event in region A and the increased events when lower MET cuts.

Backgrounds from lepton fakes from jets

Besides the well known methods(matrix method and templete fit method) used in single top group. Here proposed another method. The basical ideal is chose an isolation variable, assume harder isolation cut will remove more fake leptons than isolated lepton. so, the survived events over isolation efficiency will follow a constant term (true isolated lepton) + a asymptotic term(the fake lepton from jets). A fit performed on changing of isolation cut to get these 2 terms.

ttbar

The control region is number jets >=2, the 1 jet bin over >=2 jet bin ratio obtained from MC, then the 1 jet bin ttbar can be predicted from control region.

Multi Variable Analysis (BDT)

Scale factors, Systematic uncertainties

Lepton energy scale factors

Obtained from Z peak fitting, should applied on real Data only

Lepton trigger and ID efficiency scale factor

Obtained from comparison of data and MC, should applied on MC only

Btagging scale factor

Obtained from comparison of data and MC, should applied on MC only

Systematic uncertainties

TopSysUncertainies: including Luminosity, PDF, JES...

Statistical combinations and Limits/cross sections

HistFactory RootStats TopHistFactory

MC samples:

https://twiki.cern.ch/twiki/bin/view/AtlasProtected/TopMC10For2011Data

-- Huaqiao2Zhang - 16-Feb-2011

Topic attachments
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JPEGjpg TAB_XSECTION_VS_ECM.jpg r1 manage 73.3 K 2011-02-16 - 14:09 Huaqiao2Zhang  
JPEGjpg XSECTION_SGTOP_VS_ECM.jpg r1 manage 23.5 K 2011-02-16 - 14:10 Huaqiao2Zhang  
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Topic revision: r9 - 2011-06-26 - Huaqiao2Zhang
 
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