Search For The Associated Production Of A Higgs Boson With A Single Top Quark In Same Sign Dilepton Final State With The ATLAS Detector At S^1/2=13TeV

The discovery of a Higgs boson by the ATLAS and CMS experiments in 2012 opened a new field for exploration in particle physics that is the precise measurement of Higgs,it is crucial to study Higgs coupling to other elementary particles,which can help us further understand where the mass of elementary particles is from.In the SM,the coupling of Higgs boson to fermions is proportional to the mass of fermion,this coupling is called Yukawa Coupling.Of particular interest is the Yukawa coupling of the Higgs boson to top quark,because the top quark is the heaviest fermion by far,and it is wide-ly believed that the top quark plays a special role in the electroweak symmetry breaking mechanism.The deviation of Yukawa coupling from the SM prediction would unambigu-ously indicate the presence of physics beyond the SM.which this thesis will focus on and discuss.The Higgs boson production in association with top quark pairs(ttH)is one of main Higgs production channels,the measurement of the ttH permits a model-independent determination of the magnitude of the Yukawa coupling,but not of its sign,because it depends on the square of the Yukawa coupling,the sign of it need to be determined by studying the associated production of a Higgs boson with a single top quark(tH).The cross sections of three main tH production channels from large to small in order are:t-channel(tHq),tHW and s-channel.Only t-channel(tHq)is taken into consideration in this thesis.Meanwhile,the concept coupling modifier Ki is introduced into this analysis,while i is the type of coupling,in this analysis,only the simple model is considered here,e.g.the coupling modifier of Higgs boson to fermions is Ki,and of Higgs boson to gauge bosons is Kv,different coupling modifier hypothesis would lead to different cross section,this thesis mainly focus on the tHq production under the SM hypothesis.In this thesis,we process the pp collision data produced in LHC at(?)=13 TeV and collected by ATLAS detector during 2015-2018 with an integrated luminosity of 138965.16pb-1 and the Monte Carlo simulation samples produced by SingleTop group to search the associated production of a Higgs boson with a single top quark in same sign dilepton final state and verify the SM prediction of the Yukawa coupling by measuring tHq cross section.In this thesis,the analysis is mainly carried out in the pre-MVA signal region,it's defined by jet and b jet multiplicity after a series of pre-selections,specifically,the definition of pre-MVA signal region is at least three jets and exact one b jet which sat-isfy 70%WP requirement.However,the purity and significance of pre-MVA is too low to measure tHq,so a multivariate analysis method based on Gradient Boosted Decision Tree(BDTG)is used to further separate signal and background,in the meantime,the usage of Deep Neural Network in this analysis also is explored.Finally,the binary BDT is used to construct new variables named Combined BDTScore which would be used in fit,and a new signal region is defined based on multiclass BDT output.Note that the analysis of this thesis is far away from the final stage of whole tH analysis,the real data cannot be used in measurement of this thesis,so the real data would be replaced with Asimov dataset in the process of fit,which is called Asimov Fit,and its performance would be used to evaluate the performance of different combined BDTs in signal regions defined in different ways.Eventually,we find out the best combination that combined BDT variable CbBDTS core₆₂₂ in the signal region defined by Multiclass BDT likelihood ratio more than 0.1.However,even if in the new signal region,there's no significant signal.There-fore,the final measurement result of this thesis is the Asymptotic limit of signal strength?,while the signal strength is the ratio of observed cross section to one predicted by SM,so the Expected Upper Limit of signal strength at 95%CL.is:Medium Value:7,30,±1?confidence interval:[5.26,11.57],±2?confidence interval:[3.92,15.62].