Measurement Of SM Higgs Production Cross-section Via Gluon-gluon Fusion In The H?WW~*?ev?v Decay Channel And Higgs Prospect Study At The HL-LHC With The ATLAS Detector

The Higgs boson was discovered by the ATLAS and CMS experiments at the LHC in 2012.The study of Higgs mechanism and Higgs properties is one of the most important physical topics of the LHC.Precise measurement of Higgs prop-erties with the Run2 data collected by the ATLAS detector is a very important physical project.In this thesis,two measurement results of the Standard Model Higgs pro-duction cross-section via gluon-gluon fusion(ggF)in the H?WW*?ev?v decay channel are presented.The first one is the published result with 36.1 fb-1 data and the second is the result of an ongoing study with full Run2 dataset.The main backgrounds of qq? WW,top and Z+jets productions are nor-malized using the corresponding control region.The W+jets background was estimated by the data-driven method.The cross-section times branching ratio,?ggF·BH?WW*and ?VBF·BH?WW*,were measured to be 11.4-1.1 +1.2(stat.)-1.7 +1.8(syst.)pb and 0.50-0.22 +0.24(stat.)±0.17(syst.)pb with 36.1 fb-1 Run 2 data,respectively.The results agreed with the Standard Model predictions.Then,the gluon-gluon fusion signal strength was measured to be ?ggF obs=1.24-0.17 +0.18 from a global fit using 139 fb-1 Run 2 data.After combining with gluon-gluon fusion(? 2jet)and vector boson fusion analysis,the signal strength of the ?ggF obs and ?VBF obs were measured to be 1.20-0.15 +0.16 and 0.99±0.13,respectively.The results agreed with the Standard Model predictions.Almost identical analysis strategies used in the 36.1 fb-1 paper were implemented to the full Run 2 dataset analysis,therefore,this thesis mainly focuses on the H?WW*?ev?v analysis with full Run 2 dataset.Projections were also performed to measure the Higgs boson signal strengths,cross-sections,branching ratios,and the couplings to individual fermions and gauge bosons,at the HL-LHC with 3000 fb-1 collected at 14 TeV,extrapolating the Run 2 analyses with 36.1 fb-1 or 80 fb-1.This study can also be used to estimate the final precision that can be reached at the end of the HL-LHC programme.The results show that the precision of the measurement of the cross-sections on the five main production modes and the branching ratios of the main decay channels were approximately 2%?8%.Only W(Z)H,H???,V(tt)H,H?ZZ*and the two rare decays(H???,H?Z?),will still be limited by statistical uncertainty.The generic ? model with the beyond Standard Model contributions can give an upper limit of 0.064 on the branching ratio of Higgs decay to beyond the Standard Model(BBSM)contribution which includes both invisible decays and modifications of visible decays at 95%CL.