Title: Measurement of the W boson helicity in top quark decays

Aus was besteht die Welt? Die Frage nach den fundamentalen Bausteinen der Materie beschaftigte Wissenschaftler und Gelehrte zu allen Zeiten. Ausgehend von abstrakten philosophischen Uberlegungen wurde das Konzept von kleinsten, nicht weiter zerteilbaren Grundbausteinen der Materie bereits einige Jahrhunderte vor Christus von indischen und griechischen Philosophieschulen entwickelt. Etwa 450 v. Chr. pragte Demokrit den Begriff ´atomos, das “Unzerschneidbare”, fur die diskreten Grundbausteine der Materie. Doch erst in der jungeren Vergangenheit konnte dieses philosophische Konzept auch experimentell uberpruft werden.

The thesis presented here includes two parts. The first part discusses the production of endcap modules for the ATLAS SemiConductor Tracker at the University of Geneva. The ATLAS experiment is one of the two multi-purpose experiments being built at the LHC at CERN. The University of Geneva invested extensive efforts to create an excellent and efficient module production site, in which 655 endcap outer modules were constructed. The complexity and extreme requirements for 10 years of LHC operation with a high resolution, high efficiency, low noise tracking system resulted in an extremely careful, time consuming production and quality assurance of every single module. At design luminosity about 1000 particles will pass through the tracking system each 25 ns. In addition to requiring fast tracking techniques, the high particle flux causes significant radiation damage. Therefore, modules have to be constructed within tight and accurate mechanical and electrical specification. A description of the ATLAS experiment and the ATLAS Semiconductor tracker is presented, followed by a detailed overview of the module production at the University of Geneva. My personal contribution to the endcap module production at the University of Geneva was taking part, together with other physicists, in selecting components to be assembled to a module, including hybrid reception tests, measuring the I-V curve of the sensors and the modules at different stages of the production, thermal cycling the modules and performing electrical readout tests as an initial quality assurance of the modules before they were shipped to CERN. An elaborated description of all of these activities is given in this thesis. At the beginning of the production period the author developed a statistics package which enabled us to monitor the rate and quality of the module production. This package was then used widely by the ATLAS SCT institutes that built endcap modules of any type, and kept being improved and updated. The production monitoring and summary using this package is shown in this thesis. The second part of the thesis reports a measurement of the fraction of longitudinal and right-handed helicity states of W bosons in top quark decays. This measurement was done using 955 pb ^-1 of data collected with the CDF detector at the TEvatron, where protons and anti-protons are collided with a center-of-mass energy of 1.96 TeV. the helicity fraction measurements take advantage of the fact that the angular distribution of the W boson decay products depends on the helicity state of the W which they originate from. They analyze tmore » $$\bar{t}$$ events in the 'lepton+jets' channel and look at the leptonic side of decay. They construct templates for the distribution of cosθ*, the angle between the charged lepton and the W flight direction in the rest frame of the top quark. Using Monte Carlo techniques, they construct probability distributions ('templates') for cosθ* in the case of left-handed, longitudinal and right-handed Ws and a template for the background model. They extract the W helicity fractions using an unbinned likelihood fitter based on the information of these templates. The Standard Model predicts the W helicity fractions to be about 70% longitudinal and 30% left-handed, while the fraction of right-handed W bosons in top decays is highly suppressed and vanishes when neglecting the mass of the b quark.« less

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 asymmetrymore » 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.« less

We have measured the fraction of longitudinally polarized W bosons produced top-quark decays. The result of the analysis in single-lepton tmore » $$\bar{t}$$ events is fully consistent with the Standard Model expectation. The 2σ discrepancy in the dilepton analysis is suggestive of new phenomena. However, given the significance of the discrepancy, any claim of new physics based on this analysis is highly speculative. It is worth noting that another analysis of the kinematic properties of these data finds a similar discrepancy [63]. It will be interesting to see if this discrepancy persists as more data are collected during run II of the Tevatron. In order to make a stronger statement about the nature of the tWb coupling with this method, larger statistics are required. However alternative methods, particularly the matrix-element method developed at D0 [36, 35], should be especially powerful, even with limited statistics.« less

This thesis describes a measurement o f the decay properties of the top quark. The six quarks are fundamental building blocks matter in the universe. The most common quarks, named up and down, combine to form the protons and neutrons which exist at the nucleus of all atoms.