Title: Measurement of the W-boson helicity fractions in top-quark decays at CDF

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

The authors present a measurement of the fractions F{sub 0} and F{sub +} of longitudinally polarized and right-handed W bosons in top quark decays using data collected with the CDF II detector. The data set used in the analysis corresponds to an integrated luminosity of approximately 318 pb{sup -1}. They select t{bar t} candidate events with one lepton, at least four jets, and missing transverse energy. The helicity measurement uses the decay angle {theta}*, which is defined as the angle between the momentum of the charged lepton in the W boson rest frame and the W momentum in the topmore » quark rest frame. The cos {theta}* distribution in the data is determined by full kinematic reconstruction of the t{bar t} candidates. They find F{sub 0} = 0.85{sub -0.22}{sup +0.15}(stat){+-}0.06(syst) and F{sub +} = 0.05{sub -0.05}{sup +0.11}(stat) {+-} 0.03(syst), which is consistent with the standard model prediction. They set an upper limit on the fraction of right-handed W bosons of F{sub +} < 0.26 at the 95% confidence level.« less

We present a measurement of the fractions F{sub 0} and F{sub +} of longitudinally polarized and right-handed W bosons in top-quark decays using data collected with the CDF II detector. The data set used in the analysis corresponds to an integrated luminosity of approximately 318 pb{sup -1}. We select tt candidate events with one lepton, at least four jets, and missing transverse energy. Our helicity measurement uses the decay angle {theta}*, which is defined as the angle between the momentum of the charged lepton in the W boson rest frame and the W momentum in the top-quark rest frame. Themore » cos{theta}* distribution in the data is determined by full kinematic reconstruction of the tt candidates. We find F{sub 0}=0.85{sub -0.22}{sup +0.15}(stat){+-}0.06(syst) and F{sub +}=0.05{sub -0.05}{sup +0.11}(stat){+-}0.03(syst), which is consistent with the standard model prediction. We set an upper limit on the fraction of right-handed W bosons of F{sub +}<0.26 at the 95% confidence level.« less

The properties of the W boson can be probed in top quark decays t {yields} bW. The CDF and DZero collaborations have released updated measurements of the W boson helicity fractions f{sub 0}, f{sub +} and f{sub -}. The analysis techniques and updated measurements are discussed. This paper summarizes the talk given at the DIS2008 conference.