Measurement of the charge asymmetry and the W boson helicity in top-antitop quark events with the CDF II experiment

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Abstract

In 1995 the heaviest elementary particle, top quark, was discovered at the Tevatron collider in top-antitop quark pair production. Since the top quark mass is of the same order as the electroweak symmetry breaking scale, measurements of the properties of the top quark like mass, charge, spin or the production mechanism, offer a good opportunity to test the Standard Model at such high energies. Top quarks at the Tevatron are predominantly pair-produced through light quark-antiquark annihilation. Higher order perturbative QCD calculations predict a sizeable asymmetry between the number of top quarks and antitop quarks produced in forward direction. This asymmetry is induced through radiative corrections. A measurement of the asymmetry can check the perturbative QCD predictions. Due to the high mass of the top quark, nearly the mass of a gold nucleus, the life time of the top quark is much shorter than the hadronization time-scale. This means that the top quark decays before it has a chance to form a bound state. The Standard Model predicts that the top quark decays in nearly 100% of the cases into a W boson and a b quark via a charge-current weak interaction. The measurement of the W boson helicity probes the V-A structure of the weak interaction and differences to the expectation would give evidence for new physics. Until the start of the Large Hadron Collider at CERN, the Tevatron is the only experiment where top quarks can be directly produced and their properties be measured. The Tevatron reaches a center-of-mass energy of 1.96 TeV in proton antiproton collisions. The data used in this analysis were taken in Run II of the Tevatron with the Collider Detector at Fermilab (CDF) in the years 2001-2004 and represent an integrated luminosity of 319 pb{sup -1}. The thesis is organized in the following way: In the first chapter a short overview of the Standard Model is given. The theoretical aspects of the top quark decay are described with particular emphasis on the different helicities of the W boson. The second focus lies on the production process and the higher order QCD effect causing the charge asymmetry. In the following three chapters the experimental techniques of the CDF detector, hardware and the used software are introduced as well as. In this thesis t{bar t} candidates are selected in the decay mode t {yields} bl{nu}, {bar t} {yields} bjj and the charge conjugated state. An important ingredient for this measurement is the complete reconstruction of the top-antitop partonic process. The reconstruction of the partonic process requires the assignment of reconstructed objects, such as jets, the charged lepton and the missing transverse energy to parton level objects. This assignment implies a certain number of possible permutations and ambiguities. To achieve the optimal reconstruction of the event all combinations have to be considered and evaluated. To measure a t{bar t}-quantity one hypothesis has to be chosen. In chapter five we present a novel technique to fully reconstruct t{bar t} events. The technique is investigated in great detail by comparing to the Monte Carlo truth information. In the sixth chapter the background estimation is given. The identification and selection procedure on data is checked with Monte Carlo samples. Chapter seven describes the measurement of the W boson helicity in the top quark decay. The helicity of the W boson is measured via the angle between the W boson momentum in the top quark rest frame and the lepton momentum in the W boson rest frame. After correcting for acceptance and reconstruction effects the different helicity fractions are extracted by fitting the theoretical expected distribution. The systematic error is determined using the technique of pseudo experiments. In chapter eight the measurement of the charge asymmetry in top-pair production is presented. The measurement of the asymmetry is performed by using the difference of the top quark rapidities times the charge of the lepton, to distinguish between top and anti-top quarks. The results and an outlook are given in the last chapter.

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