Dissertation > Mathematical sciences and chemical > Physics > Nuclear physics,high energy physics > High-energy physics > Particle types > Meson

Measurement of the T-channel Single Top-quark Production in ATLAS at Sqrt(s)=7TeV

Author WangJin
Tutor FengCunFeng; Julien Donini; Annick Lleres
School Shandong University
Course Particle Physics and Nuclear Physics
Keywords Standard Model Top Quark LHC ATLAS Single Top t-channel Cross Section Measurement
CLC O572.33
Type PhD thesis
Year 2012
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What are the world’s fundamental building blocks? Which kind of forces hold them together? These are the questions that people have long asked. Par-ticle physics is the study of the basic elements of matter and the interactions between them. From the discovery of the electron about one hundred years ago (1897), the particle physics have developed gradually from the atomic theory to a theory called Standard Model (SM), which describes the three generations of quarks and leptons, as well as the four basic forces via force carrier particles. Standard model successfully explains and predicts a wide variety of experimental results. Although it still remains some questions without answers, such as the reason of the presence of only three generations of quarks, the reason that matters have mass, the problem of the gravity not included in the present formulation of the SM. The top quark, the last and heaviest quark in the SM, is sensitive to physics beyond the SM due to its large mass. The study of top quark is thus very interesting to test the SM and to probe new physics.The Large Hadron Collider (LHC) located at CERN1and Fermilab Tevatron are the only places in the world that can produce top quarks under experiment steering. LHC collides proton and proton beams while Tevatron is a proton-antiproton collider. In both cases, the dominant top-quark production are from top-quark pair processes through strong interactions, while the top quarks are also produced singly via electroweak interactions, named single top-quark pro-duction.The studies on single top-quark production help us to learn the weak in-teractions of the top quark. Its cross section measurement leads to a direct determination of one of the quantities predicted by the SM, the CMK matrix element[Vtb].In addition, the rate of single top-quark production can be signifi-cantly modified by new-physics interactions. Thus the cross section measurement of single top-quark production provides an excellent examination of the SM and deviations from SM prediction give a hint of the presence of new physics. Fur-ther more, the single top-quark productions constitute as backgrounds to many possible new physics. The studies on single top-quark production are important in new-physics searching.Single top-quark production consists of three different processes,t-channel, Wt associated production, and s-channel. This thesis presents a cross section measurement on t-channel single top-quark production using the data collected by ATLAS detector located at the LHC, contributing in one of the most recent results of single top-quark studies. It is a test of the Standard Model and also gives further understandings on the single top-quark productions.Chapter2of this thesis gives an introduction of the Standard Model and the top-quark physics. Then the LHC experiment and ATLAS detector are described in Chapter3, where the reconstruction of the collected data are also explained. Chapter4shows the phenomenology of single top-quark t-channel production and possible background processes, as well as their modeling using Monte Carlo simulations.Single top-quark t-channel cross section measurement starts with applying selections on data to select single top-quark t-channel signal enriched data set, so called pre-selection. A multivariate discriminator is then exploited by using boosted decision trees to further extract signal events. Chapter5of this thesis describes the pre-selection of data and the estimations of the backgrounds in the selected data. Then the application of boosted decision trees and the final cross section measurement are given in Chapter6. Chapter7summarizes the results of this analysis and gives a perspective of the future single top-quark measurements.

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