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Measurement of Top Quark Properties and Search for the Standard Model Higgs Boson in Proton Anti-Proton Collisions at $\sqrt{s}$

Measurement of Top Quark Properties and Search for the Standard Model Higgs Boson in Proton Anti-Proton Collisions at $\sqrt{s}$
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Release: 2011
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We present a measurement of the top quark mass and of the top-antitop (tf) pair production cross section and a search for the Standard Model Higgs boson with CDF II Detector in pp collisions at $\sqrt{s}$ = 1.96 TeV. The integrated luminosity of 2.9 $fb^{-1}$ is used for top-antitop pair production cross section and top quark mass measurement. We adopt a neural-network algorithm to select candidate events from six or more jets. At least one of these jets should be required to be b jet, as identified by the reconstruction of a secondary vertex inside the jet. The mass measurement is based on a likelihood fit incorporating reconstructed mass distributions representative of signal and background, where the absolute jet energy scale (JES) is measured simultaneously with the top quark mass. The measurement yields a value of 174.8 ± 2.4 (stat + JES) $^{+1.2}_{-1.0}$ (syst) GeV/$c^2$, where the uncertainty from the absolute jet energy scale is evaluated together with the statistical uncertainty. The procedure also measures the amount of signal from which we derive a cross section, $\sigma_{t\bar{t}}$ = 7.2 ± 0.5 (stat) ±1.0 (syst) ± 0.4 (lum) $pb$, for the measured values of top quark mass and JES. Top quark mass and W boson mass constrain the mass of the Standard Model Higgs boson, indirectly. This prediction implies MH = 89 $^{+35}_{-26}$GeV/$c^2$ (68% confidence level) as of July 2010. Therefore, we concentrate on the Standard Model Higgs mass search region with $\le$ 135 GeV / $c^2$ . Then, we search for the Standard Model Higgs boson associated with vector boson using the decay modes consisting of leptons only: Signal processes are $WH \to \ell \nu + \tau\tau$ and $ZH \to l l + \tau \tau$. We simply select 3 or 4 lepton including hadronic T to pick candidate events out. To improve search sensitivity, we adopt Support Vector Machine to discriminate signals from backgrounds. Using about 6.2 $fb^{-1}$ data, there was no clear discrepancy between data and our background estimation. Therefore, we extract cross section upper limit of the Standard Model Higgs production at 95 % confidence level. The observed upper limit on assumption of $M_H$ = 115 GeV/$c^2$ is 25.1 x $\sigma^{SM}$ at 95% confidence level while the expectation is 17.3 x $\sigma ^{SM}$ at 95%.

Measurements and Searches with Top Quarks

Measurements and Searches with Top Quarks
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Total Pages: 254
Release: 2008
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In 1995 the last missing member of the known families of quarks, the top quark, was discovered by the CDF and D0 experiments at the Tevatron, a proton-antiproton collider at Fermilab near Chicago. Until today, the Tevatron is the only place where top quarks can be produced. The determination of top quark production and properties is crucial to understand the Standard Model of particle physics and beyond. The most striking property of the top quark is its mass--of the order of the mass of a gold atom and close to the electroweak scale--making the top quark not only interesting in itself but also as a window to new physics. Due to the high mass, much higher than of any other known fermion, it is expected that the top quark plays an important role in electroweak symmetry breaking, which is the most prominent candidate to explain the mass of particles. In the Standard Model, electroweak symmetry breaking is induced by one Higgs field, producing one additional physical particle, the Higgs boson. Although various searches have been performed, for example at the Large Electron Positron Collider (LEP), no evidence for the Higgs boson could yet be found in any experiment. At the Tevatron, multiple searches for the last missing particle of the Standard Model are ongoing with ever higher statistics and improved analysis techniques. The exclusion or verification of the Higgs boson can only be achieved by combining many techniques and many final states and production mechanisms. As part of this thesis, the search for Higgs bosons produced in association with a top quark pair (t{bar t}H) has been performed. This channel is especially interesting for the understanding of the coupling between Higgs and the top quark. Even though the Standard Model Higgs boson is an attractive candidate, there is no reason to believe that the electroweak symmetry breaking is induced by only one Higgs field. In many models more than one Higgs boson are expected to exist, opening even more channels to search for charged or neutral Higgs bosons. Depending on its mass, the charged Higgs boson is expected to decay either into top quarks or be the decay product of a top quark. For masses below the top quark mass, the top decay into a charged Higgs boson and a b quark can occur at a certain rate, additionally to the decays into W bosons and a b quark. The different decays of W and charged Higgs bosons can lead to deviations of the observed final number of events in certain final states with respect to the Standard Model expectation. A global search for charged Higgs bosons in top quark pair events is presented in this thesis, resulting in the most stringent limits to-date. Besides the decay of top quarks into charged Higgs or W bosons, new physics can also show up in the quark part of the decay. While in the Standard Model the top quark decays with a rate of about 100% into a W boson and a b quark, there are models where the top quark can decay into a W boson and a non-b quark. The ratio of branching fractions in which the top quark decays into a b quark over the branching fractions in which the top quark decays into all quarks is measured as part of this thesis, yielding the most precise measurement today. Furthermore, the Standard Model top quark pair production cross section is essential to be known precisely since the top quark pair production is the main background for t{bar t}H production and many other Higgs and beyond the Standard Model searches. However, not only the search or the test of the Standard Model itself make the precise measurement of the top quark pair production cross section interesting. As the cross section is calculated with high accuracy in perturbative QCD, a comparison of the measurement to the theory expectation yields the possibility to extract the top quark mass from the cross section measurement. Although many dedicated techniques exist to measure the top quark mass, the extraction from the cross section represents an important complementary measurement. The latter is briefly discussed in this thesis and compared to direct top mass measurements. The goal of this thesis is the improved understanding of the top quark sector and its use as a window to new physics. Techniques are extended and developed to measure the top quark pair production cross section simultaneously with the ratio of branching fractions, the t{bar t}H cross section or the rate with which top quarks decay into charged Higgs bosons. Some of the results are then taken to extract more information. The cross section measurement is used to extract the top quark mass, and the ratio of the top quark pair production cross sections in different final states, yielding a limit on non-Standard Model top quark decays.

Measurement of $WZ$ Production and Searches for Anomalous Top Quark Decays and Higgs Boson Production Using Tri-lepton Final States in $p\bar{p}$ Collisions at $\sqrt{s}

Measurement of $WZ$ Production and Searches for Anomalous Top Quark Decays and Higgs Boson Production Using Tri-lepton Final States in $p\bar{p}$ Collisions at $\sqrt{s}
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Total Pages: 211
Release: 2012
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We present the results of three analyses; a $WZ$ production cross section measurement, a search for new physics in anomalous top quark decays, and the search for the standard model Higgs boson, all with final states of three or more leptons -- either electrons or muons -- plus an imbalance of transverse momentum using Tevatron proton and anti-proton collisions at a center--of--mass energy of $\sqrt{s}$ = 1.96 TeV with the D0 detector at the Fermi National Accelerator Laboratory in Chicago, IL. The first analysis reports a measurement of the $WZ \rightarrow \ell^{\prime}\nu\ell\bar{\ell}$ cross section. Using 4.1 $fb^{-1}$ of integrated luminosity, we measure a cross section of 3.90$_{-0.85}^{+1.01}$($stat+syst$) $\pm$ 0.31($lumi$) pb, which is found to be in good agreement with the standard model prediction. The second analysis is an extension of the first, in which we use the same dataset and look for the flavor changing neutral current decay of $t \rightarrow Zq$ in $p\bar {p} \rightarrow t\bar{t} \rightarrow WbZq \rightarrow \ell^{\prime}\nu\ell\bar{\ell} + \rm{jets}$ decays. Here $q$ is considered to be either a $u$ or $c$ quark, and both the $q$ and $b$ quarks decay hadronically. We find no evidence of flavor changing neutral current production and set upper limits on the branching ratio of $BR(t \rightarrow Zq)

Top Quark Mass Measurements

Top Quark Mass Measurements
Author: A. P. Heinson
Publisher:
Total Pages: 5
Release: 2006
Genre:
ISBN:
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First observed in 1995, the top quark is one of a pair of third-generation quarks in the Standard Model of particle physics. It has charge +2/3e and a mass of 171.4 GeV, about 40 times heavier than its partner, the bottom quark. The CDF and D0 collaborations have identified several hundred events containing the decays of top-antitop pairs in the large dataset collected at the Tevatron proton-antiproton collider over the last four years. They have used these events to measure the top quark's mass to nearly 1% precision and to study other top quark properties. The mass of the top quark is a fundamental parameter of the Standard Model, and knowledge of its value with small uncertainty allows us to predict properties of the as-yet-unobserved Higgs boson. This paper presents the status of the measurements of the top quark mass.

Measurement of the Top Quark Pair Production Cross-section in Dimuon Final States in Proton-antiproton Collisions at 1.96 TeV.

Measurement of the Top Quark Pair Production Cross-section in Dimuon Final States in Proton-antiproton Collisions at 1.96 TeV.
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Total Pages: 181
Release: 2008
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Particle physics deals with the fundamental building blocks of matter and their interactions. The vast number of subatomic particles can be reduced to twelve fundamental fermions, which interact by the exchange of spin-1 particles as described in the Standard Model (SM) of particle physics. The SM provides the best description of the subatomic world to date, despite the fact it does not include gravitation. Following the relation [lambda] = h/p, where h is Planck's constant, for the examination of physics at subatomic scales with size [lambda] probes with high momenta p are necessary. These high energies are accessible through particle colliders. Here, particles are accelerated and brought to collision at interaction points at which detectors are installed to record these particle collisions. Until the anticipated start-up of the Large Hadron Collider at CERN, the Tevatron collider at Fermilab near Chicago is the highest energy collider operating in the world, colliding protons and anti-protons at a center-of-mass energy of √s = 1.96 TeV. Its two interaction points are covered by the multi purpose particle detectors D0 and CDF. During the first data-taking period, known as Run I, the Tevatron operated at a center-of-mass energy of 1.8 TeV. This run period lasted from 1992 to 1996. During this period, the long-predicted top quark was discovered. From 1996 and 2001, the accelerator was upgraded to deliver higher instantaneous luminosities at its current center-of-mass energy. At the same time, the experiments were upgraded to take full advantage of the upgraded accelerator complex. The Tevatron is currently the only accelerator in the world with a sufficient energy to produce top quarks. Studying top quark production, decay and properties is an important part of the D0 and CDF physics programs. Because of its large mass, the top quark is a unique probe of the Standard Model, and an interesting environment to search for new physics. In this thesis, a measurement of the production cross-section of top quark pairs decaying to two muons is presented. In addition, a Monte Carlo study of the top quark spin correlation measurement was carried out. This thesis is laid out as follows: chapter two gives a short overview over the Standard Model of particle physics and the theoretical aspects of unpolarized and polarized top quark production and decay, chapter three describes the accelerator complex and the D0 experiment whose data is used in this analysis. The Reconstruction of events recorded with the D0 detector is explained in chapter four and the data and Monte Carlo samples used are presented in chapter five. Finally, the cross-section measurement is described in chapter six and the Monte Carlo study of top quark spin correlations in chapter seven.

Measurement of Quarkonium Polarization to Probe QCD at the LHC

Measurement of Quarkonium Polarization to Probe QCD at the LHC
Author: Valentin Knünz
Publisher: Springer
Total Pages: 179
Release: 2017-01-07
Genre: Science
ISBN: 3319499351
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This thesis discusses in detail the measurement of the polarizations of all S-wave vector quarkonium states in LHC proton-proton collisions with the CMS detector. Heavy quarkonium states constitute an ideal laboratory to study non-perturbative effects of quantum chromodynamics and to understand how quarks bind into hadrons. The experimental results are interpreted through an original phenomenological approach, which leads to a coherent picture of quarkonium production cross sections and polarizations within a simple model, dominated by one single color-octet production mechanism. These findings provide new insights into the dynamics of heavy quarkonium production at the LHC, an important step towards a satisfactory understanding of hadron formation within the standard model of particle physics.

Measurement of the Front Back Asymmetry in Top-antitop Quark Pairs Produced in Proton-antiproton Collisions at Center of Mass Energy

Measurement of the Front Back Asymmetry in Top-antitop Quark Pairs Produced in Proton-antiproton Collisions at Center of Mass Energy
Author: Thomas A. Schwarz
Publisher:
Total Pages: 110
Release: 2006
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Quarks, along with leptons and force carrying particles, are predicted by the Standard Model to be the fundamental constituents of nature. In distinction from the leptons, the quarks interact strongly through the chromodynamic force and are bound together within the hadrons. The familiar proton and neutron are bound states of the light ''up'' and ''down'' quarks. The most massive quark by far, the ''top'' quark, was discovered by the CDF and D0 experiments in March, 1995. The new quark was observed in p{bar p} collisions at 1.8 TeV at the Fermilab Tevatron. The mass of the top quark was measured to be 176 {+-} 13 GeV/c{sup 2} and the cross section 6.8{sub -2.4}{sup +3.6} pb. It is the Q = 2/3, T{sub 3} = +1/2 member of the third generation weak-isospin doublet along with the bottom quark. The top quark is the final Standard Model quark to be discovered. Along with whatever is responsible for electroweak symmetry breaking, top quark physics is considered one of the least understood sectors of the Standard Model and represents a front line of our understanding of particle physics. Currently, the only direct measurements of top quark properties come from the CDF and D0 experiments observing p{bar p} collisions at the Tevatron. Top quark production at the Tevatron is almost exclusively by quark-antiquark annihilation, q{bar q} {yields} t{bar t} (85%), and gluon fusion, gg {yields} t{bar t} (15%), mediated by the strong force. The theoretical cross-section for this process is {sigma}{sub t{bar t}} = 6.7 {+-} 0.8 pb for m{sub t} = 175 GeV/c{sup 2}. Top quarks can also be produced at the Tevatron via q{bar b}{prime} {yields} tb and qg {yields} q{prime}tb through the weak interaction. The cross section for these processes is lower (3pb) and the signal is much more difficult to isolate as backgrounds are much higher. The top quark is predicted to decay almost exclusively into a W-boson and a bottom quark (t {yields} Wb). The total decay width t {yields} Wb is {Lambda} = 1.50 GeV. This corresponds to an incredibly short lifetime of 0.5 x 10{sup -24} seconds. This happens so quickly that hadronization and bound states do not take place, which leads to the interesting consequence that the top quark spin information is passed to the decay products.