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

This paper reports a measurement of the cross section for the pair production of top quarks in p anti-p collisions at s**(1/2) =1.96 TeV at the Fermilab Tevatron. The data was collected from the CDF Run 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 t anti-t 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 tt-bar 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_tt-bar =7.6 +/- 1.2(stat.)+0.6_-0.5(sys.) +/- 0.6(lum.) pb, assuming a branching ratio of BR( W --> l nu) =0.8% and a top mass of m_t =175 GeV/c2.