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Title: A measurement of the top quark's charge

Abstract

The top quark was discovered in 1995 at the Fermilab National Accelerator Laboratory (Fermilab). One way to confirm if the observed top quark is really the top quark posited in the Standard Model (SM) is to measure its electric charge. In the Standard Model the top quark is the isospin partner of the bottom quark and is expected to have a charge of +2/3. However, an alternative 'exotic' model has been proposed with a fourth generation exotic quark that has the same characteristics, such as mass, as our observed top but with a charge of -4/3. This thesis presents the first CDF measurement of the top quark's charge via its decay products, a W boson and a bottom quark, using ~ 1 fb -1 of data. The data were collected by the CDF detector from proton anti-proton (p$$\bar{p}$$) collisions at √s = 1.96 TeV at Fermilab. We classify events depending on the charges of the bottom quark and associated W boson and count the number of events which appear 'SM-like' or 'exotic-like' with a SM-like event decaying as t → W +b and an exotic event as t → W -b. We find the p-value under the Standard Model hypothesis to be 0.35 which is consistent with the Standard Model. We exclude the exotic quark hypothesis at an 81% confidence level, for which we have chosen a priori that the probability of incorrectly rejecting the SM would be 1%. The calculated Bayes Factor (BF) is 2 x Ln(BF)=8.54 which is interpreted as the data strongly favors the Standard Model over the exotic quark hypothesis.

Authors:
 [1]
  1. Michigan State Univ., East Lansing, MI (United States)
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
923065
Report Number(s):
FERMILAB-THESIS-2007-48
TRN: US0801963
DOE Contract Number:
AC02-07CH11359
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; DAUGHTER PRODUCTS; ELECTRIC CHARGES; FERMILAB; FERMILAB ACCELERATOR; FERMILAB COLLIDER DETECTOR; HYPOTHESIS; INTERMEDIATE BOSONS; ISOSPIN; PROBABILITY; PROTONS; QUARKS; STANDARD MODEL; T QUARKS; Experiment-HEP

Citation Formats

Unalan, Zeynep Gunay. A measurement of the top quark's charge. United States: N. p., 2007. Web. doi:10.2172/923065.
Unalan, Zeynep Gunay. A measurement of the top quark's charge. United States. doi:10.2172/923065.
Unalan, Zeynep Gunay. Mon . "A measurement of the top quark's charge". United States. doi:10.2172/923065. https://www.osti.gov/servlets/purl/923065.
@article{osti_923065,
title = {A measurement of the top quark's charge},
author = {Unalan, Zeynep Gunay},
abstractNote = {The top quark was discovered in 1995 at the Fermilab National Accelerator Laboratory (Fermilab). One way to confirm if the observed top quark is really the top quark posited in the Standard Model (SM) is to measure its electric charge. In the Standard Model the top quark is the isospin partner of the bottom quark and is expected to have a charge of +2/3. However, an alternative 'exotic' model has been proposed with a fourth generation exotic quark that has the same characteristics, such as mass, as our observed top but with a charge of -4/3. This thesis presents the first CDF measurement of the top quark's charge via its decay products, a W boson and a bottom quark, using ~ 1 fb-1 of data. The data were collected by the CDF detector from proton anti-proton (p$\bar{p}$) collisions at √s = 1.96 TeV at Fermilab. We classify events depending on the charges of the bottom quark and associated W boson and count the number of events which appear 'SM-like' or 'exotic-like' with a SM-like event decaying as t → W+b and an exotic event as t → W-b. We find the p-value under the Standard Model hypothesis to be 0.35 which is consistent with the Standard Model. We exclude the exotic quark hypothesis at an 81% confidence level, for which we have chosen a priori that the probability of incorrectly rejecting the SM would be 1%. The calculated Bayes Factor (BF) is 2 x Ln(BF)=8.54 which is interpreted as the data strongly favors the Standard Model over the exotic quark hypothesis.},
doi = {10.2172/923065},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}

Thesis/Dissertation:
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  • 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
  • According to the Standard Model, the top quark decays to a W boson and a b quark virtually 100% of the time. The measurements of tmore » $$\bar{t}$$ production cross section depend strongly on that assumption. We test this hypothesis with a measurement of R = B(t → W b)/B(t → Wq), using a combination of event kinematics and b- tagging techniques. The measurement is carried out using a data sample produced in p$$\bar{p}$$ collisions at 1.96 TeV and collected at the Collider Detector at Fermilab between March 2002 and September 2003 with an integrated luminosity of ~ 162 pb -1. The branching ratio R is determined from the relative t$$\bar{t}$$¯ tagging rates making the measurement independent of any assumption on the t$$\bar{t}$$ cross section. Any two tagging rates are sufficient to determine the R but the problem is overconstrained if more than two tagged subsamples are used. The t$$\bar{t}$$ events are classified by the number vii of leptons in the final state. In lepton-plus-jets channel only one of the W bosons decays leptonically, whereas in dilepton channel both W bosons decay leptonically. The measurement of R is performed in both lepton-plus-jets and dilepton samples. In the lepton-plus-jets channel the background is estimated using the artificial neural network (ANN) technique. The ANN approach allows us to measure the signal fraction in samples with any number of tags. By applying this method alone the branching ratio was measured to be R = 1.06$$+0.27\atop{-0.24}$$(stat.) ± 0.16(syst.). Alternatively, the tagged background contamination in lepton-plus-jets channel is determined from a traditional a priori method using data driven and Monte Carlo based techniques. A similar approach is used to determine the t$$\bar{t}$$ content in the dilepton sample. The combination of ANN background measurement in lepton-plus- jets data sample with the a priori lepton-plus-jets and dilepton estimations leads to improved sensitivity in the final value of R = 1.12$$+0.21\atop{-0.19}$$(stat.)$$+0.17\atop{-0.13}$$(syst.). Finally, we construct the confidence level bands based on the method proposed by Feldman and Cousins. From these bands and our final measurement of R we set the lower limit of R > 0.61 at 95% confidence level.« less
  • Discovery of the top quark in 1995 at the Fermilab Tevatron collider concluded a long search following the 1977 discovery of bottom (b) quark [1] and represents another triumph of the Standard Model (SM) of elementary particles. Top quark is one of the fundamental fermions in the Standard Model of electroweak interactions and is the weak-isospin partner of the bottom quark. A precise measurement of top pair production cross-section would be a test of Quantum Chromodynamics (QCD) prediction. Presently, Tevatron is the world's highest energy collider where protons (p) and anti-protons (more » $$\bar{p}$$) collide at a centre of mass energy √s of 1.96 TeV. At Tevatron top (t) and anti-top ($$\bar{t}$$) quarks are predominantly pair produced through strong interactions--quark annihilation (≅ 85%) and gluon fusion (≅ 15%). Due to the large mass of top quark, t or $$\bar{t}$$ decays (~ 10 -25 sec) before hadronization and in SM framework, it decays to a W boson and a b quark with ~ 100% branching ratio (BR). The subsequent decay of W boson determines the major signatures of t$$\bar{t}$$ decay. If both W bosons (coming from t and $$\bar{t}$$ decays) decay into leptons (viz., ev e, μv μ or τc τ) the corresponding t$$\bar{t}$$ decay is called dileptonic decay. Of all dileptonic decay modes of t$$\bar{t}$$, the t$$\bar{t}$$ → WWb$$\bar{b}$$ → ev eμv μb$$\bar{b}$$ (eμ channel) decay mode has the smallest background contamination from Z 0 production or Drell-Yan process; simultaneously, it has the highest BR (~ 3.16%) [2] amongst all dileptonic decay modes of t$$\bar{t}$$. During Run I (1992-1996) of Tevatron, three eμ candidate events were detected by D0 experiment, out of 80 candidate events (inclusive of all decay modes of t$$\bar{t}$$). Due to the rarity of the t$$\bar{t}$$ events, the measured cross-section has large uncertainty in its value (viz., 5.69 ± 1.21(stat) ± 1.04(sys) pb {at} √s = 1.8 TeV measured by D0 [3]). This analysis presents a cross section measurement in eμ channel utilizing ~ 228 pb -1 of data collected by D0 experiment during Tevatron Run II (between June 2002 and April 2004).« less