Title: Measurement of the Top Quark Pair Production Cross Section in Lepton+jets Final States at D0 using a Topological Event Selection and Lifetime b Tagging.

Abstract:

Measurement of the top quark pair (ttbar) production cross section at hadron colliders can be used to test perturbative QCD predictions. Within the Standard Model (SM), the top quark almost always decays to a W boson and a b quark. I present two alternative measurements of the ttbar production cross section at sqrt{s}= 1.96 TeV in proton-antiproton collisions using 230 pb-1 for the topological analysis, respectively 165 pb-1 for the analysis using lifetime b tagging of data collected by the D0 experiment during Run II at the Fermilab Tevatron collider. I consider the lepton+jets final state, with emphasis on the muon+jets final state, which is characterized by four or more jets, a high transverse momentum isolated electron or muon and high missing transverse energy, resulting from the decay of one W boson into a charged lepton and a neutrino and the other W boson into a quark-antiquark pair. In the first part I discuss the measurement of the top quark pair production cross section based purely on a topological event selection. Within the SM, the top quark almost always decays to a W boson and a b quark, thus leading to the presence of two b jets in the event. By using its new silicon tracker, the D0 experiment is capable of identifying b jets based on lifetime tagging. The second analysis presented here uses the expected b jet content of ttbar events to discriminate between the top signal and the SM backgrounds.