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| Author | : Sarah Kristina Wolinski |
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| Total Pages | : 524 |
| Release | : 2002 |
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| Release | : 2016 |
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A search is presented for single top quark production in the s channel in proton-proton collisions with the CMS detector at the CERN LHC in decay modes of the top quark containing a muon or an electron in the final state. The signal is extracted through a maximum-likelihood fit to the distribution of a multivariate discriminant defined using boosted decision trees to separate the expected signal contribution from background processes. Data collected at centre-of-mass energies of 7 and 8 TeV yield cross sections of 7.1 +/- 8.1 pb and 13.4 +/- 7.3 pb, respectively, and a best fit value of 2.0 +/- 0.9 for the combined ratio of the measured and expected values. The signal significance is 2.5 standard deviations, and the upper limit on the rate relative to the standard model expectation is 4.7 at 95% confidence level.
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| Release | : 2017 |
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A search for the production of a single top quark in association with a Z boson is presented, both to identify the expected standard model process and to search for flavour-changing neutral current interactions. The data sample corresponds to an integrated luminosity of 19.7 inverse-femtobarns recorded by the CMS experiment at the LHC in proton-proton collisions at $\sqrt{s}$ = 8 TeV. Final states with three leptons (electrons or muons) and at least one jet are investigated. An events yield compatible with tZq standard model production is observed, and the corresponding cross section is measured to be sigma(p p -> t Z q -> l nu b l+ l- q) = 10 +8/-7 fb with a significance of 2.4 standard deviations. No presence of flavour-changing neutral current production of tZq is observed. Exclusion limits at 95% confidence level on the branching fractions of a top quark decaying to a Z boson and an up or a charm quark are found to be B(t -> Z u) 0.022% and B(t - Z c)
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| Release | : 2011 |
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A search for pair-produced bottom-like quarks in pp collisions at sqrt(s) = 7 TeV is conducted with the CMS experiment at the LHC. The decay b' to tW is considered in this search. The b' b'-bar to tW-̂ t-bar W+̂ process can be identified by the distinctive signature of trileptons and same-sign dileptons. With a data sample corresponding to an integrated luminosity of 34 inverse picobarns, no excess above the standard model background predictions is observed and a b' quark with a mass between 255 and 361 GeV/c2̂ is excluded at the 95% confidence level.
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| Release | : 2010 |
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We report a search for single top quark production with the CDF II detector using 2.1 fb−1 of integrated luminosity of p{bar p} collisions at √s = 1.96 TeV. The data selected consist of events characterized by large energy imbalance in the transverse plane and hadronic jets, and no identified electrons and muons, so the sample is enriched in W → [tau][nu] decays. In order to suppress backgrounds, additional kinematic and topological requirements are imposed through a neural network, and at least one of the jets must be identified as a b quark jet. We measure an excess of signal-like events in agreement with the standard model prediction, but inconsistent with a model without single top quark production by 2.1 standard deviations ([sigma]), with a median expected sensitivity of 1.4 [sigma]. Assuming a top quark mass of 175 GeV/c2 and ascribing the excess to single top quark production, the cross section is measured to be 4.9{sub -2.2}{sup +2.5} (stat+syst) pb, consistent with measurements performed in independent datasets and with the standard model prediction.
| Author | : John Joseph Walsh |
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| Total Pages | : 212 |
| Release | : 1990 |
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| Author | : David J. Cox |
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| Release | : 2012 |
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| ISBN | : 9781267967893 |
We present the results of a search for pair production of a heavy top-like (t') quark decaying to Wq final states using data corresponding to an integrated luminosity of 5.6 fb−1 collected by the CDF II detector in proton-antiproton collisions at [square root s] = 1.96 TeV center of mass energy. A searche for t' --> Wb in events containing a lepton and four or more jets is conducted. By performing a fit to the two-dimensional distribution of total transverse energy versus reconstructed t' quark mass, we set upper limits on the t't̄' pair production cross section and exclude a standard model fourth-generation t' quark decaying to Wb with mass below 358 GeV/c2 at 95% CL.
| Author | : Christopher Matthew Holck |
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| Total Pages | : 207 |
| Release | : 1999 |
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| Author | : Avik Roy (Ph. D.) |
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| Total Pages | : 0 |
| Release | : 2021 |
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This thesis presents the search for a heavy, up-type vector-like quark (VLQ), or Top Partner, with charge +2/3. Top Partners are predicted in well motivated models of new physics beyond the Standard Model. Having collected data with the ATLAS detector during 2015-2018 run of the LHC at a center-of-mass energy of 13 TeV, I describe in this dissertation an analysis searching for Top Partners produced by the fusion of a W boson and a bottom quark and decaying into a Z boson and a top quark with a kinematic signature of three leptons. Since the search performed by analyzing the data collected in 2015-16 does not provide sufficient evidence in favor of VLQ discovery, we set limits on Top Partner mass as a function of its coupling constants. In order to extend this search for the entire dataset collected by ATLAS in Run 2 and facilitate a general interpretation and combination of all single VLQ searches in ATLAS, a generalized, semi-analytical framework for model-independent interpretation of the results of a generic search for the single production of VLQs in terms of the free parameters in the VLQ Lagrangian is presented. Additional studies are performed to facilitate computationally manageable signal generation for a fine grid of VLQ mass and couplings, probe interference between VLQ and standard model processes, and quantify contribution of sub-dominant diagrams. Finally, an updated analysis strategy to expand the previously mentioned search for the entire Run 2 dataset is described with initial results for kinematic distributions as well as expected limits on cross-sections, Top Partner masses, and couplings
| Author | : Dag Gillberg |
| Publisher | : Springer Science & Business Media |
| Total Pages | : 149 |
| Release | : 2011-01-22 |
| Genre | : Science |
| ISBN | : 1441977996 |
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.
