Measurement Of The Cross-section Of The Associated Production Of A Top Quark And A W Boson With ATLAS Detector

For a long time, it has been a human endeavor to explore and understand what the universe is made of. In the1960’s and1970’s, physicists develope-d Standard Model theory of particle physics which successfully explained the fundamental particles constituting the universe and their fundamental interac-tions. Standard Model has not only been confirmed by numerous of precise measurements, but also made many successful predictions. Top quark, the heav-iest fundamental particle in Standard Model, is discovered at Tevatron collider at Fermilab in1995. Its discovery validated the three generation quark structure of Standard Model. Single top quark is produced via weak interaction and only one top quark is produced in the process. Single top serves both as an important tool to test Standard Model and a window to search for beyond Standard Mod-el theory. However, it is not easy to measure the single top production whose observation is14years later than that of top pair production.LHC, located at CERN in Switzerland, is a particle collider with the highest ceter-of-mass energy in the world. In2011and2012, LHC has been running with√s＝7TeV and√s=8TeV respectively and produced lots of collisions so far. These collisions provide a good opportunity to measure single top produc-tion and its properties. ATLAS is a large particle detector installed around an interaction point at LHC. It can detect, filter and record the outcome of particle collisions. The structure and size of ATLAS detector are determined by the need to reconstruct the properties of the produced particles.We process the pp collision data collected by ATLAS detector at√s=8TeV to measure the cross-section of associated production of a top quark and a W boson (Wt-channel) for the first time. The integrated luminosity of the collision data is20.3ft-1. According to the dilepton final state of Wt-channel, a series of event selections are applied. The event selections require there are exactly two opposite signed leptons with high transverse momentum(pT>25GeV), and at least one high transverse momentum jet (pr>30GeV). The events are then divided into different categories according to the jet multiplicity. The1-jet and2-jet regions are used as Wt signal region and it, control region separately, and then at least one b-tagged jet are required in the two regions to suppress Diboson, Z+jets and fake lepton backgrounds. The tt, Diboson and Z+jets are estimated with Monte Carlo samples, while the yield and shape of fake lepton background is estimated with a data-driven Matrix method. After event selection and background estimation, data is well modeled by the yields and shapes of signal and backgrounds.For the tt contribution is dominant after event selections with80.3%in1-jet region and93.4%in2-jet region, a multivariate method based on BDT is used to separate Wt and tt to reduce the tt uncertainties on the Wt production cross-section measurement. A few powerful discriminant variables based on the event properties and the correlation between decay products are constructed to train BDT. In addition, BDT parameters are carefully studied to avoid overtraining and improve the BDT performance.The theoretical uncertainties as well as the experimental uncertainties are carefully studied in the measurement. Most of the theoretical uncertainties are evaluated with two different MC samples with different theory models, like gen-erator, parton shower, ISR/FSR and DR/DS uncertainties. PDF uncertainties are evaluated by reweighting default samples to different PDF sets. Most of the experimental uncertainties are estimated with shifts on the efficiencies, scales or resolutions of objects. Luminosity uncertainty is provide by ATLAS luminosity group. All the systematic effects are propagated to BDT templates separately.The Wt production cross section is extracted from data with a binned max-imum likelihood fitting on BDT distributions in1-jet and2-jet regions. The cross section uncertainty is measured with frequentist method based on pseu-do experiments, including systematic rate and shape uncertainties. The sig-nificance of data over background hypothesis is also evaluated with an ensem-ble test of pseudo experiments. The measured Wt production cross section is27.2±2.8(stat)±5.4(syst) pb, which is consistent with Standard Model predi-cation, and corresponding observed significance is4.2σ。 As the CKM matrix element|Vtb|can be directly determined by the single top production cross-section,|Vtb|is derived with the Wt cross section measured in the thesis.The result is|Vtb|=1.10±0.12,with a95%CL lower limit of0.72.