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Title: Top quark pair production in proton anti-proton collisions

Abstract

This thesis presents a measurement of the t$$\bar{t}$$ cross section in the all-jets channel, measured in p$$\bar{p}$$ collisions at a center of mass energy of 1.96 TeV, using data collected with the D0 detector. The dataset used for this analysis has an integrated luminosity equivalent to L = 162.5 ± 10.6 pb -1. A t$$\bar{t}$$ cross section measurement is a test of the Standard Model predictions for heavy quark production, and the first step towards measurements of the mass and other properties of the top quark. The presented measurement of the cross section for the process p$$\bar{p}$$ → t$$\bar{t}$$ uses the decay channel where both top quarks decay to quarks. The top quark first decays to a b quark and a W boson, and then, for this particular channel, the W boson decays hadronically. Hence, events with six energetic quarks are expected, which ideally leads to events with six jets. These so called all-jets events have a significantly larger branching fraction than other t$$\bar{t}$$ decay channels. The large branching fraction in the all-jets channel means that a significant sample of t$$\bar{t}$$ candidates can be extracted, which can subsequently be used for studies of top quark properties, like the top mass. The background, multijet production through Quantum Chromo Dynamics (QCD) has a cross section three to four orders of magnitude larger than expected for t$$\bar{t}$$ production. The analysis presented in this thesis uses the decay vertices of long-lived b-flavored mesons to identify the b jets. With the silicon detector installed at the start of Run 2 of the Tevatron, the D0 experiment is now able to use this method for b identification. The presence of b quarks in the event makes it possible to reduce the background to a few percent of the original sample, while only rejecting around 45% of the t$$\bar{t}$$ content in the sample.

Authors:
 [1]
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  1. Univ. of Amsterdam (Netherlands)
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
15017276
Report Number(s):
FERMILAB-THESIS-2005-02
TRN: US200606%%467
DOE Contract Number:
AC02-76CH03000
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 43 PARTICLE ACCELERATORS; 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; A CENTERS; B QUARKS; CROSS SECTIONS; DECAY; FERMILAB TEVATRON; INTERMEDIATE BOSONS; LUMINOSITY; MESONS; PAIR PRODUCTION; PRODUCTION; PROTONS; QUANTUM CHROMODYNAMICS; QUARKS; SILICON; STANDARD MODEL; T QUARKS; Experiment-HEP

Citation Formats

Blekman, Freya. Top quark pair production in proton anti-proton collisions. United States: N. p., 2005. Web. doi:10.2172/15017276.
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Blekman, Freya. Top quark pair production in proton anti-proton collisions. United States. doi:10.2172/15017276.
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Blekman, Freya. Fri . "Top quark pair production in proton anti-proton collisions". United States. doi:10.2172/15017276. https://www.osti.gov/servlets/purl/15017276.
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@article{osti_15017276,
title = {Top quark pair production in proton anti-proton collisions},
author = {Blekman, Freya},
abstractNote = {This thesis presents a measurement of the t$\bar{t}$ cross section in the all-jets channel, measured in p$\bar{p}$ collisions at a center of mass energy of 1.96 TeV, using data collected with the D0 detector. The dataset used for this analysis has an integrated luminosity equivalent to L = 162.5 ± 10.6 pb-1. A t$\bar{t}$ cross section measurement is a test of the Standard Model predictions for heavy quark production, and the first step towards measurements of the mass and other properties of the top quark. The presented measurement of the cross section for the process p$\bar{p}$ → t$\bar{t}$ uses the decay channel where both top quarks decay to quarks. The top quark first decays to a b quark and a W boson, and then, for this particular channel, the W boson decays hadronically. Hence, events with six energetic quarks are expected, which ideally leads to events with six jets. These so called all-jets events have a significantly larger branching fraction than other t$\bar{t}$ decay channels. The large branching fraction in the all-jets channel means that a significant sample of t$\bar{t}$ candidates can be extracted, which can subsequently be used for studies of top quark properties, like the top mass. The background, multijet production through Quantum Chromo Dynamics (QCD) has a cross section three to four orders of magnitude larger than expected for t$\bar{t}$ production. The analysis presented in this thesis uses the decay vertices of long-lived b-flavored mesons to identify the b jets. With the silicon detector installed at the start of Run 2 of the Tevatron, the D0 experiment is now able to use this method for b identification. The presence of b quarks in the event makes it possible to reduce the background to a few percent of the original sample, while only rejecting around 45% of the t$\bar{t}$ content in the sample.},
doi = {10.2172/15017276},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Fri Apr 01 00:00:00 EST 2005},
month = {Fri Apr 01 00:00:00 EST 2005}
}
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Thesis/Dissertation:
View Thesis/Dissertation (14.44 MB)
DOI:  10.2172/15017276
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  • A measurement of the top anti-top quark pair production cross section in pmore » $$\bar{p}$$ collisions at a center-of-mass energy of 1.96 TeV is presented. The measurement is made using data with integrated luminosity of 109.4 pb -1, obtained at the Collider Detector at Fermilab between August 2002 and May 2003. A search is made for the dilepton decay channel of top anti-top production, t$$\bar{t}$$→l+νbl-νl$$\bar{b}$$. The Standard Model dilepton decay channel of the top anti-top quark pair production involves two high transverse momentum leptons, a large missing energy from the undetected neutrinos, two jets from the b and b¯ quark fragmentations. Various Standard Model processes can mimic t$$\bar{t}$$ production in this decay channel. These backgrounds are estimated to be 0.65 ± 0.13 events in an integrated luminosity of 109.4 pb -1. In the data, one ee, three μμ, and two eμ dilepton candidates were observed. From these data, the t$$\bar{t}$$ production cross section of σ t$$\bar{t}$$ =11.4 $$+6.2\atop{-4.6}$$ pb has been measured. This measurement is consistent with the theoretical prediction of the Standard Model, σ t$$\bar{t}$$ =6.70 $$+71\atop{0.88}$$ pb assuming a top quark mass of 175 GeV/c 2.« less

  • This dissertation presents a new measurement of pmore » $$\bar{p}$$ → t$$\bar{t}$$X production at √s = 1.96 TeV using 974.2 pb -1of data collected with the D0 detector between 2002 and 2006. We focus on the final state where the W boson from one of the top quarks decays into a τ lepton and its associated neutrino, while the other W boson decays into a quark-antiquark pair. We aim to select those events in which the τ lepton subsequently decays hadronically, meaning to one or three charged hadrons, zero or more neutral hadrons and a tau neutrino (the charge conjugate processes are implied in all of the above). The observable signature thus consists of a narrow calorimeter shower with associated track(s) characteristic of a hadronic tau decay, four or more jets, of which two are initiated by b quarks accompanying the W's in the top quark decays, and a large net missing momentum in the transverse plane due to the energetic neutrino-antineutrino pair that leave no trace in the detector media. The preliminary result for the measured cross section is: σ(t$$\bar{t}$$) = 6.9 -1.2 +1.2(stat) -0.7 +0.8(syst) ± 0.4 (lumi) pb. This indicates that our finding is consistent with the Standard Model prediction.« less

  • Two alternative measurements of the t¯t production cross section at √s = 1.96 TeV in proton-antiproton collisions in the lepton+jets channel are presented. The t¯t production cross section is extracted by combining the kinematic event information in a multivariate discriminant. The measurement yields σ p¯p → t¯t + x = 5.13 -1.57 +1.76(stat) -1.10 +0.96(syst) ± 0.33 (lumi) pb in the muon+jets channel, using 229.1 pb -1, and in the combination with the electron+jets channel (226.3 pb -1) σ p¯p → t¯t + x = 6.60 -1.28 +1.37(stat) -1.11 +1.25(syst) ± 0.43 (lumi) pb. The second measurement presented reconstructs explicitlymore » secondary vertices to d lifetime b-tagging. The measurement combines the muon+jets and the electron+jets channel, using 158.4 pb -1 and 168.8 pb -1, respectively: σ p¯p → t¯t + x = 8.24 -1.25 +1.34(stat) -1.63 +1.89(syst) ± 0.54 (lumi) pb.« less

  • Of the six quarks in the standard model the top quark is by far the heaviest: 35 times more massive than its partner the bottom quark and more than 130 times heavier than the average of the other five quarks. Its correspondingly small decay width means it tends to decay before forming a bound state. Of all quarks, therefore, the top is the least affected by quark confinement, behaving almost as a free quark. Its large mass also makes the top quark a key player in the realm of the postulated Higgs boson, whose coupling strengths to particles are proportional to their masses. Precision measurements of particle masses for e.g. the top quark and the W boson can hereby provide indirect constraints on the Higgs boson mass. Since in the standard model top quarks couple almost exclusively to bottom quarks (t → Wb), top quark decays provide a window on the standard model through the direct measurement of the Cabibbo-Kobayashi-Maskawa quark mixing matrix element V tb. In the same way any lack of top quark decays into W bosons could imply the existence of decay channels beyond the standard model, for example charged Higgs bosons as expected in two-doublet Higgs models: t → H +b. Within the standard model top quark decays can be classified by the (lepton or quark) W boson decay products. Depending on the decay of each of the W bosons, tmore » $$\bar{t}$$ pair decays can involve either no leptons at all, or one or two isolated leptons from direct W → e$$\bar{v}$${sub e} and W → μ$$\bar{v}$$ μ decays. Cascade decays like b → Wc → e$$\bar{v}$$ ec can lead to additional non-isolated leptons. The fully hadronic decay channel, in which both Ws decay into a quark-antiquark pair, has the largest branching fraction of all t$$\bar{t}$$ decay channels and is the only kinematically complete (i.e. neutrino-less) channel. It lacks, however, the clear isolated lepton signature and is therefore hard to distinguish from the multi-jet QCD background. It is important to measure the cross section (or branching fraction) in each channel independently to fully verify the standard model. Top quark pair production proceeds through the strong interaction, placing the scene for top quark physics at hadron colliders. This adds an additional challenge: the huge background from multi-jet QCD processes. At the Tevatron, for example, t$$\bar{t}$$ production is completely hidden in light q$$\bar{q}$$ pair production. The light (i.e. not bottom or top) quark pair production cross section is six orders of magnitude larger than that for t$$\bar{t}$$ production. Even including the full signature of hadronic t$$\bar{t}$$ decays, two b-jets and four additional jets, the QCD cross section for processes with similar signature is more than five times larger than for t$$\bar{t}$$ production. The presence of isolated leptons in the (semi)leptonic t$$\bar{t}$$ decay channels provides a clear characteristic to distinguish the t$$\bar{t}$$ signal from QCD background but introduces a multitude of W- and Z-related backgrounds.« less

  • The subject of this thesis is the measurement of the rate at which a top-antitop quark pair is produced in pp collisions at a center of mass energy of ps = 1:96 TeV. The data are collected with the D detector, a multi-purpose detector operating at the Fermilab Tevatron collider during Run II. A measurement of the top quark pair (tmore » $$\bar{t}$$) production cross section ( σ t$$\bar{t}$$) in p$$\bar{p}$$ collisions at a center of mass energy of 1.96 TeV is presented. The measurement is based on data recorded by the D0 Detector at the Fermilab Tevatron Collider, and preselected in the e+jets (366 pb -1) and μ+jets (363 pb -1) channels. The cross section is extracted by applying a lifetime-tagging technique to the data, and yields σ t$$\bar{t}$$ = 6.96$$+1.07\atop{-0.98}$$(stat + syst) ± 0.45(lumi) pb; for a top quark mass m t =175 GeV, in good agreement with the Standard Model prediction.« less