Top Quark Pair Production Cross Section in the Lepton+Jets Final State at DØ

Preliminary and published results obtained by the DØ Collaboration on the top quark pair production cross-section in p ¯ p collisions at a center-of-mass energy of ¡ s ¢ 1£ 96 TeV are presented. The measurements are obtained using the lepton+jets final states with two methods : in the first one, signal is discriminated from background using event kinematics and in the other method signal events are enumerated in a sample purified using b-lifetime tagging. Since its discovery in 1995 the Top quark can only be studied at Fermilab with the Tevatron which produces p ¯ p collisions at s ¡ 1¢ 96 TeV. At the Tevatron Run II, Top pairs are produced mostly by quark annihilation (85%) and gluon fusion (15%). The theoretical prediction for the pair production is 6.7£ 0.8 pb [1] with the mass assumption 1 of 175 GeV. The electroweak production of Top quark (single Top) has not yet been observed. In order to study Top quark properties, Top enriched samples are used, which are controlled with the cross section measurement. Top pair production measurements allow to test the Standard Model (SM) and perturbative QCD predictions. Moreover, since Top pair production is a major background in analysis seeking for more rare decays such as light Higgs production, it has to be measured precisely. The Top quark has a relatively short lifetime (10¤ 25 s) compared to the other quarks and to the typical hadronisation time (10¤ 23 s). Within the SM prediction, the Top decays predominantly (¥ 100%) into a b quark and a W boson, therefore Top pair events contain at least 2 b-jets. This SM assumption is made only in the b-lifetime tagging analysis which uses an algorithm to distinguish heavy (b and c quarks) from light flavor jets (u, d, s quarks and gluon). Charged tracks, primary and secondary verticies are utilized as input of this algorithm. They are reconstructed with the inner tracking system, which consists of a silicon micro-strips tracker and a scintillating fiber tracker both inside a superconducting solenoid creating a longitudinal magnetic field of 2 Tesla. Top pair decay channels are completely determined by the W boson decay modes. They are : all hadronic channel (45%), dilepton channels (2.4% and 1.2 %) and lepton+jets channels (15%). The Lepton+jets channel analysis for electron and muon are reported in this paper. In the lepton+jets channels presented, one of W boson decays …