We measure the top pair production cross section where top decays to a W boson and a b quark ( t → Wb ), and both W bosons in turn decay leptonically. The candidate sample is events with two opposite-sign leptons, two or more jets, and significant missing transverse energy (MET). We require that at least one of the jets be identified as having come from the decay of a b hadron. This requirement suppresses the backgrounds, increasing the purity of the candidate sample. We fully reconstruct the first lepton but only require the second lepton to be identified as an isolated track. This increases the top acceptance by including leptons that may not be fiducial to calorimetry or muon detectors, and most decay channels (including "1-prong" hadronic) of tau leptons.
This is the first cross section measurement requiring b-tagging using the lepton+isolated track selection. A similar analysis using two identified leptons was done with 750 pb-1 of CDF data, this analysis is done with 1.0 fb-1.The primary backgrounds in the candidate sample are Z+bb, mistags due to resolution and material tags, and top events in the lepton+jets channel that fake the dilepton signal. The Z+bb and mistag background are estimated using a data driven technique. A five dimensional matrix of tagging rates for generic jets is created, and used to predict the tagging rates of the jets in the pretag candidate events. The fake background due to top decays in the lepton+jets channel is estimated using both data and Monte Carlo. We use Monte Carlo to estimate the acceptance to top lepton+jets events in our selection, and find lepton+jet candidate tag rates in data.
The top acceptance for lepton plus isolated track events is 0.843 ± 0.026% without requiring a tagged jet. The event tagging efficiency is 0.669 ± 0.035. In 1.0 fb-1 of data collected during Run 2 at the Fermilab Tevatron at a center of mass energy of √ s = 1.96 TeV, we observe 69 candidate events, with a predicted background of 11.9 ± 4.1. events. We measure a cross-section of
σ = 10.1 ± 1.8(stat) ± 1.1(sys) ± 0.6(lumi) pb.
assuming BR(W → l ν) = 10.8 % and m_top = 175 GeV / c ^ 2.
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