public: Lepton + Jets ttbar cross section with SLT b-tagging

Measurement of ttbar Production Cross-Section

in l+Jets channel with SLTm b-tagging

Alice Abulencia, Lucio Cerrito+, Ulysses Grundler,

Tony Liss, Anyes Taffard, Xiaojian Zhang

*University of Illinois Urbana-Champaign

+University College London

This page is for the measurement of the t-tbar cross section using a muon tag to identify b-jets.
The analysis was blessed on September 28^th, 2006.
Last updated on Oct 05.

The Analysis:

In this analysis we measure the t-tbar production cross section using lepton+jets events. Lepton plus jets events result from top decays (t->Wb) in t-tbar events when one of the W bosons decays to an electron/muon plus neutrino (taus are not included) and the other decays hadronically, resulting in a topology of one high-Pt electron or muon, missing transverse energy and four jets. We require 3 or more jets to allow for the possibility that one of the jets is not found. The dominant background for this process is QCD production of W+jets. We enhance the signal to background ratio in this analysis by identification of one of the jets in the event as a candidate b-jet. The identification of candidate b-jets ("b-tagging") is done by looking for relatively low momentum muons in or near the jets that result from semileptonic decays of B hadrons. This identification is referred to as "Soft Lepton Tagging" (SLT). In order to further reduce the dominant backgrounds we require H_T>200 GeV, where H_T is the scalar sum of the lepton p_T, jet E_T, and missing E_T.

The dataset consists of 760 pb^-1 of proton-antiproton collisions at root(s)=1.96 TeV. In events with a 3 or more jets, H_T>200 GeV and at least one SLT-tagged jet, we measure the t-tbar production cross section to be:

result

where the first uncertainty is statistical and the second systematic.

NB: To see hatched histogram on eps files you should disable Antialias.

evt sel

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l+ Jets event selection

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SLTm tagger & b-tagging

result_final

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Summary table of background contributions, ttbar expected (@6.7 pb) and observed data event.

Systematic Uncertainties

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SLT_m tagged events per jet multiplicity.

The color stack histograms represent the different background contributions in each jet bin.

The hashed boxes are ±1s error (statistical & systematic) on the background estimate. The dots are the data events.

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Number of tagged events and background summary.

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Comparison of the jet E_T distribution in W+>=3 jets for tagged events and for the expectations from fakes+Wbbar+Wccbar,

non-W and ttbar events (scaled to the measure cross section of 7.8 pb).

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Comparison of the H_T distribution in W+>=3 jets for tagged events and for the expectations from fakes+Wbbar+Wccbar, non-W and ttbar events (scaled to the measure cross section of 7.8 pb).

H_T is defined as the sum of the lepton E_T or p_T, jet E_T and missing E_T.

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Comparison of the SLTm p_T distribution in W+>=3 jets for tagged events and for the expectations from fakes+Wbbar+Wccbar, non-W and ttbar events (scaled to the measure cross section of 7.8 pb).

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Comparison of the SLTm d0 significance distribution in W+>=3 jets for tagged events and for the expectations from fakes+Wbbar+Wccbar, non-W and ttbar events (scaled to the measure cross section of 7.8 pb).

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Comparison of the SLTm p_TRel (transverse momentum with respect to the jet axis) distribution in W+>=3 jets for tagged events and for the expectations from fakes+Wbbar+Wccbar, non-W and ttbar events (scaled to the measure cross section of 7.8 pb).

central_slteff

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SLTm efficiency for the central muon detector (CMU & CMP) as a function of p_T measure from JPsi & Z⁰

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SLTm efficiency for the extended muon detector (CMX) as a function of p_T measure from JPsi & Z⁰

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Last Updated : Monday, October 10, 2006 12:47 by Xiaojian