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<sup>-1</sup> 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:</dcvalue>\n <dcvalue element="description" qualifier="abstract"/>\n <dcvalue element="description" qualifier="abstract"/>\n <dcvalue element="description" qualifier="abstract">&amp;sigma;(t_t) = 6.9 &lt;sup&gt;+1.2&lt;/sup&gt;-1.2 (stat)&lt;sup&gt;+0.8&lt;/sup&gt; -0.7 (syst) &amp;plusmn;

0.4 (lumi) pb

This indicates that our finding is consistent with the Standard Model

prediction.