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Author: Yun-Tse Tsai Publisher: ISBN: Category : Languages : en Pages : 295
Book Description
"We present a new model-independent measurement of the electroweak single top-quark production cross section in proton-antiproton collisions at [radical]s = 1:96 TeV in 9.7 fb-1 of integrated luminosity collected with the D[Zero] detector. Top quarks can be produced singly through the electroweak interaction when an off-sell (virtual) W boson in the s-channel decays to tb quarks ("tb" final state), or through the fusion of a virtual W boson with an incident virtual b quark in the t-channel to produce a top quark associated with a b and a first-generation quarks ("tqb" final state). We select signal-like events, containing one energetic electron or muon, an imbalance in transverse momentum, and two or three jets, with one or two identified as candidates for originating from the fragmentation of b quarks. A discriminant based on the "Matrix Element" method is used to separate the signal from background, and a Bayesian approach is utilized to extract the cross sections for signals. The s and t-channel cross sections are extracted simultaneously, providing a way to measure both modes without assuming the Standard Model prediction for either. The s and t-channel cross sections for producing a single top quark are measured to be [Formulas would not render]. The probabilities to measure these values or larger of cross section in absence of signal are 4.0 x 10-4 for the s-channel and 3.1 x 10-6 for the t-channel, corresponding, respectively, to 3.4 and 4.5 standard deviation significance. These results are among the most accurate measurements to date, and are consistent with the expectations of the Standard Model of 1.04 [plus or minus] 0.08 and 2.26 [plus or minus] 0.2 pb, respectively"--Abstract.
Author: Yun-Tse Tsai Publisher: ISBN: Category : Languages : en Pages : 295
Book Description
"We present a new model-independent measurement of the electroweak single top-quark production cross section in proton-antiproton collisions at [radical]s = 1:96 TeV in 9.7 fb-1 of integrated luminosity collected with the D[Zero] detector. Top quarks can be produced singly through the electroweak interaction when an off-sell (virtual) W boson in the s-channel decays to tb quarks ("tb" final state), or through the fusion of a virtual W boson with an incident virtual b quark in the t-channel to produce a top quark associated with a b and a first-generation quarks ("tqb" final state). We select signal-like events, containing one energetic electron or muon, an imbalance in transverse momentum, and two or three jets, with one or two identified as candidates for originating from the fragmentation of b quarks. A discriminant based on the "Matrix Element" method is used to separate the signal from background, and a Bayesian approach is utilized to extract the cross sections for signals. The s and t-channel cross sections are extracted simultaneously, providing a way to measure both modes without assuming the Standard Model prediction for either. The s and t-channel cross sections for producing a single top quark are measured to be [Formulas would not render]. The probabilities to measure these values or larger of cross section in absence of signal are 4.0 x 10-4 for the s-channel and 3.1 x 10-6 for the t-channel, corresponding, respectively, to 3.4 and 4.5 standard deviation significance. These results are among the most accurate measurements to date, and are consistent with the expectations of the Standard Model of 1.04 [plus or minus] 0.08 and 2.26 [plus or minus] 0.2 pb, respectively"--Abstract.
Author: Publisher: ISBN: Category : Languages : en Pages : 466
Book Description
At particle accelerators the Standard Model has been tested and will be tested further to a great precision. The data analyzed in this thesis have been collected at the world's highest energetic-collider, the Tevatron, located at the Fermi National Accelerator Laboratory (FNAL) in the vicinity of Chicago, IL, USA. There, protons and antiprotons are collided at a center-of-mass energy of √s = 1.96 TeV. The discovery of the top quark was one of the remarkable results not only for the CDF and D0 experiments at the Tevatron collider, but also for the Standard Model, which had predicted the existence of the top quark because of symmetry arguments long before already. Still, the Tevatron is the only facility able to produce top quarks. The predominant production mechanism of top quarks is the production of a top-antitop quark pair via the strong force. However, the Standard Model also allows the production of single top quarks via the electroweak interaction. This process features the unique opportunity to measure the.
Author: Publisher: ISBN: Category : Languages : en Pages : 334
Book Description
We present a new model-independent measurement of the electroweak single top-quark production cross section in proton-antiproton (p- $\bar{p}$) collisions at √s = 1.96 TeV in 9.7 fb-1 of integrated luminosity collected with the DØ detector.
Author: Dag Gillberg Publisher: Springer Science & Business Media ISBN: 1441977996 Category : Science Languages : en Pages : 149
Book Description
The top quark is by far the heaviest known fundamental particle with a mass nearing that of a gold atom. Because of this strikingly high mass, the top quark has several unique properties and might play an important role in electroweak symmetry breaking—the mechanism that gives all elementary particles mass. Creating top quarks requires access to very high energy collisions, and at present only the Tevatron collider at Fermilab is capable of reaching these energies. Until now, top quarks have only been observed produced in pairs via the strong interaction. At hadron colliders, it should also be possible to produce single top quarks via the electroweak interaction. Studies of single top quark production provide opportunities to measure the top quark spin, how top quarks mix with other quarks, and to look for new physics beyond the standard model. Because of these interesting properties, scientists have been looking for single top quarks for more than 15 years. This thesis presents the first discovery of single top quark production. It documents one of the flagship measurements of the D0 experiment, a collaboration of more than 600 physicists from around the world. It describes first observation of a physical process known as “single top quark production”, which had been sought for more than 10 years before its eventual discovery in 2009. Further, his thesis describes, in detail, the innovative approach Dr. Gillberg took to this analysis. Through the use of Boosted Decision Trees, a machine-learning technique, he observed the tiny single top signal within an otherwise overwhelming background. This Doctoral Thesis has been accepted by Simon Fraser University, Burnaby, BC, Canada.
Author: Publisher: ISBN: Category : Languages : en Pages : 280
Book Description
Until now, the top quark has only been observed produced in pairs, by the strong force. According to the standard model, it can also be produced singly, via an electroweak interaction. Top quarks produced this way provide powerful ways to test the charged-current electroweak interactions of the top quark, to measure.
Author: Publisher: ISBN: Category : Languages : en Pages : 6
Book Description
An overview of recent top quark measurements using the full Run II data set of CDF or D0 at the Tevatron is presented. Results are complementary to the ones at the LHC. Recent measurements of the production cross section of top quarks in strong and electroweak production and of top quark production asymmetries are presented. The latter includes the measurement of the tt-bar production asymmetry by D0 in the dilepton decay channel. Within their uncertainties the results from all these measurements agree with their respective Standard Model expectation. Finally latest updates on measurements of the top quark mass are discussed, which at the time of the conference are the most precise determinations.
Author: Publisher: ISBN: Category : Languages : en Pages : 6
Book Description
The top quark was discovered in 1995 by the CDF and D0 experiments at the Fermilab Tevatron during the Run I operation. Since the start of the Tevatron Run II in 2001, both experiments have collected (almost equal to)2 fb−1 data samples, which are over twenty times larger than that used in the Run 1 discovery. This larger data sample allows more precise studies of top-quark properties; differences between observed top-quark properties and the Standard Model (SM) prediction may give hints to possible physics beyond the SM. Here we present the latest results on the measurements of top-quark properties and the search for electroweak (EW) single top quark production from the CDF and D0 collaborations. The integrated luminosity used for the measurements corresponds to about 1 fb−1.
Author: Publisher: ISBN: Category : Languages : en Pages : 180
Book Description
The standard model of elementary particle physics (SM) predicts, besides the top-quark pair production via the strong interaction, also the electroweak production of single top-quarks [19]. Up to now, the Fermilab Tevatron proton-antiproton-collider is the only place to produce and study top quarks emerging from hadron-hadron-collisions. Top quarks were directly observed in 1995 during the Tevatron Run I at a center-of-mass energy of √s = 1.8 TeV simultaneously by the CDF and D0 Collaborations via the strong production of top-quark pairs. Run II of the Tevatron data taking period started 2001 at √s = 1.96 TeV after a five year upgrade of the Tevatron accelerator complex and of both experiments. One main component of its physics program is the determination of the properties of the top quark including its electroweak production. Even though Run II is still ongoing, the study of the top quark is already a successful endeavor, confirmed by dozens of publications from both Tevatron experiments. A comprehensive review of top-quark physics can be found in reference. The reasons for searching for single top-quark production are compelling. As the electroweak top-quark production proceeds via a Wtb vertex, it provides the unique opportunity of the direct measurement of the CKM matrix element.
Author: Publisher: ISBN: 9781109413762 Category : Electroweak interactions Languages : en Pages : 360
Book Description
The top quark is generally produced in quark and anti-quark pairs. However, the Standard Model also predicts the production of only one top quark which is mediated by the electroweak interaction, known as 'Single Top'. Single Top quark production is important because it provides a unique and direct way to measure the CKM matrix element V{sub tb}, and can be used to explore physics possibilities beyond the Standard Model predictions. This dissertation presents the results of the observation of Single Top using 2.3 fb−1 of Data collected with the D0 detector at the Fermilab Tevatron collider. The analysis includes the Single Top muon+jets and electron+jets final states and employs Boosted Decision Tress as a method to separate the signal from the background. The resulting Single Top cross section measurement is: (1) [sigma](p{bar p} → tb + X, tqb + X) = 3.74{sub -0.74}{sup +0.95} pb, where the errors include both statistical and systematic uncertainties. The probability to measure a cross section at this value or higher in the absence of signal is p = 1.9 x 10−6. This corresponds to a standard deviation Gaussian equivalence of 4.6. When combining this result with two other analysis methods, the resulting cross section measurement is: (2) [sigma](p{bar p} → tb + X, tqb + X) = 3.94 ± 0.88 pb, and the corresponding measurement significance is 5.0 standard deviations.
Author: Publisher: ISBN: Category : Languages : en Pages : 254
Book Description
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.
Author: Yun-Tse Tsai
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Author: Dag Gillberg
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