A Measurement of the t-tbar Cross Section in p-pbar Collisions at sqrt(s) = 1.96 TeV using Dilepton Events with a Lepton plus Track Selection

TL;DR

This paper measures the top-antitop quark pair production cross section at Fermilab Tevatron using dilepton events with a novel lepton plus track selection, enhancing acceptance especially for tau decays.

Contribution

It introduces a new method combining lepton and track identification to improve top quark pair detection in dilepton channels, including tau decays, with the first b-jet requirement in CDF dilepton analysis.

Findings

Measured cross section: 9.6 pb with uncertainties.

Used lepton plus track selection to increase acceptance.

First CDF result with b-jet requirement in dilepton channel.

Abstract

This paper reports a measurement of the cross section for the pair production of top quarks in ppbar collisions at sqrt(s) = 1.96 TeV at the Fermilab Tevatron. The data was collected from the CDF II detector in a set of runs with a total integrated luminosity of 1.1 fb^{-1}. The cross section is measured in the dilepton channel, the subset of ttbar events in which both top quarks decay through t -> Wb -> l nu b where l = e, mu, or tau. The lepton pair is reconstructed as one identified electron or muon and one isolated track. The use of an isolated track to identify the second lepton increases the ttbar acceptance, particularly for the case in which one W decays as W -> tau nu. The purity of the sample may be further improved at the cost of a reduction in the number of signal events, by requiring an identified b-jet. We present the results of measurements performed with and without the request of an identified b-jet. The former is the first published CDF result for which a b-jet requirement is added to the dilepton selection. In the CDF data there are 129 pretag lepton + track candidate events, of which 69 are tagged. With the tagging information, the sample is divided into tagged and untagged sub-samples, and a combined cross section is calculated by maximizing a likelihood. The result is sigma_{ttbar} = 9.6 +/- 1.2 (stat.) -0.5 +0.6 (sys.) +/- 0.6 (lum.) pb, assuming a branching ratio of BR(W -> ell nu) = 10.8% and a top mass of m_t = 175 GeV/c^2.