Observation of $t\bar{t}$ Quark Production Using a Soft Lepton B Tag in $p\bar{p}$ Collisions at $\sqrt{s}$ = 1.8-TeV

Abstract

This thesis presents the results of a search for tl production in 110 pb-1 of pp collisions at .JS=l.8 TeV. The data were collected using the Collider Detector at Fermilab (CDF). The search identifies events consistent with the Standard Model decay (tl ~ w+bw-'b). W bosons are identified by the presence of high momentum electrons or muons associated with missing energy transverse to the beam. At least three jets are required to reduce the background from W bosons produced with associated jets. Finally the presence of ab-quark is inferred by searching for a soft lepton from the semileptonic decay (b~ iv X) or the sequential decay (b~ c ~ lvX). Backgrounds from fake leptons or from real leptons occurring in generic jets are estimated from the data and account for 90% of the total background. The remaining backgrounds from Drell-Yan, z~ r+r-, We, bb and Diboson production are estimated from a combination of Monte Carlo simulated events and data. Forty candidate events are identified containing 44 soft lepton tags with an expected background of 24.6 ± 3.0 tags. The probability of the background fluctuating upward to give the observed excess is 0.18%. The acceptance for top quark events is calculated and the tl cross section measured to be U£i = 8.5 ± 3.4 pb. The results of this analysis are combined with the results of other searches and a kinematic fit for the top mass is performed yielding a top mass of 175.6 ± 5.7 (stat)± 7.1 (syst) Ge V / c 2 • For the measured mass the cross section is in good agreement with predictions from the Standard Model.