Date of Award

Fall 2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Engineering Physics

First Advisor

Zeno Greenwood

Abstract

The top quark is the heaviest of the known elementary particles in the Standard Model. Top quark decay can result into various final states; therefore, careful study of its production rate and other properties is very important for particle physics. With the shutdown of the Tevatron, The Large Hadron Collider (LHC) is the only facility currently capable of studying top quark properties. The data obtained by proton-proton collisions in the LHC is recorded by two general purpose detectors, ATLAS and CMS. The results in the dissertation are from the ATLAS detector. A new measurement is reported of &sgr;(pp [special characters omitted] tt¯) at [special characters omitted]= 7 TeV using 4.7 fb -1 of data collected during 2011. In this analysis, the final state of the top quark decay is a hadronically decaying tau lepton and a pair of light quark jets. Only those events in which the τ lepton subsequently decays to one or three charged hadrons, zero or more neutral hadrons and a tau neutrino, are selected. Boosted Decision Trees are used for hadronic tau identification. The signature thus consists of one hadronically decaying tau lepton and four or more jets, of which at least one is initiated by a b quark accompanying the W in the top quark decays, and a large net missing momentum in the transverse plane due to the energetic neutrino-antineutrino pair. This momentum is not detected by the ATLAS detector. For multi-jet background estimation, a template fitting method is used. The template is fitted to the data to obtain the fractions for the signal and it's various backgrounds. The measured cross section along with the uncertainties on the statistics, systematics and luminosity is:

&sgr;tt¯ = 170.6 [special characters omitted]12 (stat.) [special characters omitted] (syst.) [special characters omitted]3 (lumi.) pb.

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