MEASUREMENT OF TOP QUARK PAIR PRODUCTION CROSS SECTION IN PROTON ANTIPROTON COLLISIONS AT √s = 1.96 TeV IN THE τ + jets FINAL STATE USING 1 fb^-1 OF DATA

Abstract

This dissertation presents a new measurement of p_p → t_t X production at √s = 1.96 TeV using 974.2 pb^-1 of data collected with the DØ detector between 2002 and 2006. We focus on the final state where the W boson from one of the top quarks decays into a τ lepton and its associated neutrino, while the other W boson decays into a quark-antiquark pair. We aim to select those events in which the τ lepton subsequently decays hadronically, meaning to one or three charged hadrons, zero or more neutral hadrons and a tau neutrino (the charge conjugate processes are implied in all of the above). The observable signature thus consists of a narrow calorimeter shower with associated track(s) characteristic of a hadronic tau decay, four or more jets, of which two are initiated by b quarks accompanying the W's in the top quark decays, and a large net missing momentum in the transverse plane due to the energetic neutrino-antineutrino pair that leave no trace in the detector media. The preliminary result for the measured cross section is:\n \n \n σ(t_t) = 6.9 ^+1.2-1.2 (stat)^+0.8 -0.7 (syst) ± 0.4 (lumi) pb This indicates that our finding is consistent with the Standard Model prediction.