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Title: A Measurement of the Top Quark Mass in the Dilepton Decay Channel at CDF II

Abstract

The top quark, the most recently discovered quark, is the most massive known fundamental fermion. Precision measurements of its mass, a free parameter in the Standard Model of particle physics, can be used to constrain the mass of the Higgs Boson. In addition, deviations in the mass as measured in different channels can provide possible evidence for new physics. We describe a measurement of the top quark mass in the decay channel with two charged leptons, known as the dilepton channel, using data collected by the CDF II detector from p$$\bar{p}$$ collisions with √s = 1.96 TeV at the Fermilab Tevatron. The likelihood in top mass is calculated for each event by convolving the leading order matrix element describing q$$\bar{q}$$ → t$$\bar{t}$$ → bℓv $$\bar{b}$$ℓ'v ℓ' with detector resolution functions. The presence of background events in the data sample is modeled using similar calculations involving the matrix elements for major background processes. In a data sample with integrated luminosity of 1.0 fb -1, we observe 78 candidate events and measure M t = 164.5 ± 3.9(stat.) ± 3.9(syst.) GeV/c 2, the most precise measurement of the top quark mass in this channel to date.

Authors:
 [1]
  1. Univ. of Michigan, Ann Arbor, MI (United States)
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
892485
Report Number(s):
FERMILAB-THESIS-2006-17
TRN: US0701012
DOE Contract Number:
AC02-76CH03000
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ACCURACY; DECAY; FERMILAB COLLIDER DETECTOR; FERMILAB TEVATRON; HIGGS BOSONS; LEPTONS; LUMINOSITY; MATRIX ELEMENTS; PHYSICS; RESOLUTION; STANDARD MODEL; T QUARKS; Experiment-HEP

Citation Formats

Jayatilaka, Bodhitha A. A Measurement of the Top Quark Mass in the Dilepton Decay Channel at CDF II. United States: N. p., 2006. Web. doi:10.2172/892485.
Jayatilaka, Bodhitha A. A Measurement of the Top Quark Mass in the Dilepton Decay Channel at CDF II. United States. doi:10.2172/892485.
Jayatilaka, Bodhitha A. Sun . "A Measurement of the Top Quark Mass in the Dilepton Decay Channel at CDF II". United States. doi:10.2172/892485. https://www.osti.gov/servlets/purl/892485.
@article{osti_892485,
title = {A Measurement of the Top Quark Mass in the Dilepton Decay Channel at CDF II},
author = {Jayatilaka, Bodhitha A.},
abstractNote = {The top quark, the most recently discovered quark, is the most massive known fundamental fermion. Precision measurements of its mass, a free parameter in the Standard Model of particle physics, can be used to constrain the mass of the Higgs Boson. In addition, deviations in the mass as measured in different channels can provide possible evidence for new physics. We describe a measurement of the top quark mass in the decay channel with two charged leptons, known as the dilepton channel, using data collected by the CDF II detector from p$\bar{p}$ collisions with √s = 1.96 TeV at the Fermilab Tevatron. The likelihood in top mass is calculated for each event by convolving the leading order matrix element describing q$\bar{q}$ → t$\bar{t}$ → bℓvℓ$\bar{b}$ℓ'vℓ' with detector resolution functions. The presence of background events in the data sample is modeled using similar calculations involving the matrix elements for major background processes. In a data sample with integrated luminosity of 1.0 fb-1, we observe 78 candidate events and measure Mt = 164.5 ± 3.9(stat.) ± 3.9(syst.) GeV/c2, the most precise measurement of the top quark mass in this channel to date.},
doi = {10.2172/892485},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}

Thesis/Dissertation:
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  • We measure the top quark mass using approximately 359 pb-1 of data from pmore » $$\bar{p}$$ collisions at √s =1.96 GeV at CDF Run II. We select t$$\bar{t}$$ candidates that are consistent with two W bosons decaying to a charged lepton and a neutrino following t$$\bar{t}$$ → W +W -b$$\bar{b}$$ → l +l -v$$\bar{v}$$b$$\bar{b}$$.« less
  • We report a measurement of the top quark mass in the all-hadronic channel with the upgraded Collider Detector at Fermilab using an integrated luminosity of 1.02more » $$fb^{-1}$$ . Top quarks are produced mostly in pairs at the Tevatron Collider; they subsequently decay almost 100% of the time in a W boson and a b quark each. Final states are classified according to the decays of the W bosons. Here we study only those events in which both W's decay into quark pairs. This channel has the advantage of a large branching ratio and of being fully reconstructed. On the other hand the signal is overwhelmed by a background which surpasses it by three orders of magnitude even after the requirement of a specific trigger. A neural network is thus used to discriminate signal from background events in order to achieve a good signal over background ratio. We look for a variable which is strongly correlated with the top quark mass, and compare the corresponding distribution in the data to signal and background templates in order to measure the top quark mass. Here is an outline of the following of this work: Chapter 1 presents an introduction to the path that lead to the formalization of the Standard Model of fundamental interactions. We discuss in Chapter 2 the need for a 6th quark in the Standard Model, the phenomenology of the top quark and a summary of the current experimental knowledge of its properties, and motivate the need for a precise measurement of its mass. In Chapter 3 we describe the experimental apparatus needed to produce the top quarks, i.e. the Tevatron Collider, and the detector which collects the data analyzed in this work, CDF II. Chapter 4 describes how CDF II interprets the output from the various subdetectors and translate them into the physics objects which are needed for the measurement. In Chapter 5 we present how we overcome the problem of the huge background which overwhelms the top production by using for the first time in this channel a neural network approach to select the candidate events. Finally, in Chapter 6 we present and discuss the technique used to extract the top quark mass measurement, how we control this technique, evaluate the systematics uncertainties, and finally apply the method to the data to obtain the top quark mass measurement.« less
  • The top quark is the heaviest standard model quark. Discovered in 1995 by the two Tevatron experiments it has atypical properties. In particular its time life is so short that it decays before hadronizing, so the top quark mass could be measured with a high precision. Data collected by the DØ experiment between 2002 and 2009, which represent an integrated luminosity of 5.4 fb⁻¹, are used to measure the top quark mass by using the matrix element method in the three dilepton channels: dielectron, electron--muon and dimuon. The measured mass, 174.0 ± 1.8 (stat.) ± 2.4 (syst.) GeV, is inmore » a good agreement with other measurements and limited by the systematic uncertainties for the first time in these channels. In this thesis different approaches have been studied to improve the accuracy of this measurement: the use of b-quark jet identification in order to optimize the selection of top--anti-top events and a better determination of the main systematic uncertainties. A special attention has been paid to the Monte-Carlo simulation of muons in D0: the improved smearing procedure for the simulated muons, discussed in this thesis, will be used to increase the accuracy of the top properties measurements as well as the precision of many other D0 measurements.« less
  • In this thesis, we present the first exclusive observation of the t -t → hadronic τ + jets decay channel. Using these events, we measure the t -t pair production cross section and the top quark mass in 2.2 fb -1 of data collected with the Collider Detector at Fermilab (CDF). The Tevatron accelerator at Fermilab provides collisions of protons and anti-protons at a center-of-mass energy of √s = 1.96 TeV and is one of only two accelerators in the world with enough energy to produce top quarks. With a branching fraction of nearly 10%, the hadronic τ + jetsmore » decay channel is the third largest t -t decay mode, and it has only been minimally explored. This the first measurement of the t -t pair production cross section in this decay channel at CDF and the first measurement of the top quark mass in this decay channel in the world. The analysis introduces a new method to recover the total momentum of the ν produced in the τ decay and an artificial neural network to reduce the contribution from the largest background source, QCD multijet background. The t -t pair production cross section is extracted by minimizing a negative log likelihood function which compares the number of observed events to the number of expected events for a given t -t cross section. The top quark mass is extracted by minimizing a negative log likelihood function built from signal and ii background probabilities which are based on the matrix elements for t -t production and decay and W + 4 parton production, respectively. Using events selected with exactly 1 hadronically decaying τ, exactly 4 jets with at least 1 identified as having originated from a b quark, and large missing transverse energy, we measure the t -t pair production cross section to be 8.8 ± 3.3 (stat.) ± 2.2 (syst.) pb and the top quark mass to be 172.7±9.3 (stat.) ±3.7 (syst.) GeV. We find both values to be in good agreement with previous measurements in other t -t decay channels, and the cross section to be consistent with next-to-leading order theoretical predictions.« less