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

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 themore » 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

Authors:
 [1]
  1. Univ. of Karlsruhe (Germany)
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
897585
Report Number(s):
FERMILAB-THESIS-2005-80
TRN: US0701470
DOE Contract Number:
AC02-07CH11359
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ASYMMETRY; B QUARKS; BOUND STATE; ELEMENTARY PARTICLES; FERMILAB COLLIDER DETECTOR; FERMILAB TEVATRON; HELICITY; INTERMEDIATE BOSONS; PAIR PRODUCTION; QUANTUM CHROMODYNAMICS; QUARKS; RADIATIVE CORRECTIONS; STANDARD MODEL; SYMMETRY BREAKING; T QUARKS; TRANSVERSE ENERGY; WEAK INTERACTIONS; Experiment-HEP

Citation Formats

Hirschbuehl, Dominic. Measurement of the charge asymmetry and the W boson helicity in top-antitop quark events with the CDF II experiment. United States: N. p., 2005. Web. doi:10.2172/897585.
Hirschbuehl, Dominic. Measurement of the charge asymmetry and the W boson helicity in top-antitop quark events with the CDF II experiment. United States. doi:10.2172/897585.
Hirschbuehl, Dominic. Fri . "Measurement of the charge asymmetry and the W boson helicity in top-antitop quark events with the CDF II experiment". United States. doi:10.2172/897585. https://www.osti.gov/servlets/purl/897585.
@article{osti_897585,
title = {Measurement of the charge asymmetry and the W boson helicity in top-antitop quark events with the CDF II experiment},
author = {Hirschbuehl, Dominic},
abstractNote = {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.},
doi = {10.2172/897585},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Fri Dec 23 00:00:00 EST 2005},
month = {Fri Dec 23 00:00:00 EST 2005}
}

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  • 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.
  • In 1995 the top quark was discovered at the Tevatron proton-antiproton collider at Fermilab by the CDF and D0 collaborations [1, 2]. It is the most massive known elementary particle and its mass is currently measured with a precision of about 1.3% [3, 4]. However, the measurements of several other top quark properties are still statistically limited, so the question remains whether the Standard Model of elementary particle physics successfully predicts these properties. This thesis addresses one interesting aspect of top quark decay, the helicity of the produced W boson. Until the start of the Large Hadron Collider (LHC) atmore » CERN, the Tevatron with a center-of-mass energy of {radical}s = 1.96 TeV is the only collider, where top quarks can be produced. In the Standard Model the top quark decays predominantly into a W boson and a b quark, with a branching ratio close to 100%. The V-A structure of the weak interaction of the Standard Model predicts that the W{sup +} bosons from the top quark decay t {yields} W{sup +}b are dominantly either longitudinally polarized or left handed, while right handed W bosons are heavily suppressed and even forbidden in the limit of a massless b quark. Under the assumption of a massless b quark, for a top quark mass of 173 GeV/c{sup 2} the Standard Model predicts the fraction F0 of longitudinally polarized W bosons to be 0.7 and 0.3 for the fraction F{_} of left handed W bosons, while the fraction F{sub +} of right handed W bosons is predicted to be zero. Since next-to-leading order corrections change these fractions only slightly, a significant deviation from the predicted value for F{sub 0} or a nonzero value for F{sub +} could indicate new physics. Left-right symmetric models [5], for example, lead to a significant right handed fraction of W bosons in top decays. Such a right handed component (V+A coupling) would lead to a smaller left handed fraction, while F{sub 0} would remain unchanged. Since the decay rate to longitudinal W bosons depends on the Yukawa coupling of the top quarks, the measurement of F{sub 0} is sensitive to the mechanism of electroweak symmetry breaking. Alternative models can lead to an altered F{sub 0} fraction. In this analysis the W helicity fractions are measured in a selected sample rich in t{bar B} events where one lepton, at least four jets, and missing transverse energy are required. All kinematic quantities describing the t{bar t} decay are determined. As a sensitive observable, we use the cosine of 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 boson momentum in the top quark rest frame. The data used in this analysis were taken with the Collider Detector at Fermilab (CDF II) in the years 2002-2006 and correspond to an integrated luminosity of about 955 pb{sup -1}. Previous CDF measurements of the W boson helicity fractions in top quark decays used either the square of the invariant mass of the charged lepton and the b quark jet, M{sub {ell}b}{sup 2}, or the lepton p{sub T} distribution as a discriminant. The D0 collaboration used a matrix-element method to extract a value of F{sub 0}; in a second analysis the reconstructed distribution of cos {theta}* was utilized to measure F{sub +}. CDF gives the latest value of F{sub 0} = 0.74{sub -0.34}{sup +0.22}, while D measured F{sub 0} = 0.56 {+-} 0.31. The CDF collaboration also gives the current upper limit of F{sub +} < 0.09.« less
  • 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
  • We report a measurement of the top quark mass with the upgraded collider detector at Fermilab (CDF-II). The top quarks are produced in pairs (tt) in proton-antiproton collisions with a center-of-mass energy of 1.96 TeV.
  • We present a measurement of forward-backward asymmetries in top-antitop quark pairs produced in proton-antiproton collisions decaying via the lepton+jets channel. Using data recorded by the D0 experiment at the Fermilab Tevatron collider and corresponding to an integrated luminosity of 5.4 fb -1, we measure the forward-backward asymmetry in top-antitop quark events to bemore » $$\left(9.2 \pm 3.7\right)\%$$, after background processes have been subtracted. After correcting for the effects of acceptance and detector reconstruction, we measure an asymmetry of $$\left(19.6 \pm 6.5\right)\%$$. In addition, we measure an acceptance-corrected asymmetry based on the lepton from top-antitop quark decay of $$\left(15.2 \pm 4.0\right)\%$$. We compare these results to predictions from the MC@NLO next-to-leading-order QCD simulation.« less