Studies Of The Higgs Boson Properties And Search For New Physics Beyond The Standard Model In Multilepton Final States With The ATLAS Detector At LHC

The measurement of the Higgs boson's property is one of the main focuses of the high energy physics.The Yukawa coupling between Higgs and top quark-which is the heaviest Standard Model(SM)fermion-is one of the most interesting constants which can be probed from the cross section measurement of associated production of a Higgs boson with a top-quark pair(ttH).In this thesis,ttH cross section has been measured with proton-proton colli-sions at a center-of-mass energy of(?).The measurement is performed in multilepton final states using a dataset corresponding to an integrated lu-minosity of 79.9 fb-1 data recorded by the ATLAS experiment at the Large Hadron Collider(LHC).Six final states are categorised by the number and fla-vor of charged-lepton candidates to search for the ttH signal.The best-fit value of ?ttH is ?=0.58-0.33+0.36,while the corresponding cross section for ttH produc-tion is ?ttH=294-162+182 fb,in agreement with the Standard Model prediction(?ttHSM=507-50+35 fb).The observed(expected)significance as excess above the background-only hypothesis is 1.8(3.1)standard deviations.The analysis for ttH cross section measurement with the full Run 2 ATLAS dataset of an integrated luminosity of 139.0 fb-1 is still in progress.The expectation results have been presented following the method used in the previous round with only nominal samples considered in 2lMS channel.The expected significance derived from the statistical calculation is 2.2?.The last part of the thesis presents a search for a heavy neutral resonance X,in a channel of X?WW?e??? using data collected by ATLAS in 2015 and 2016.The data was taken at a center-of-mass energy of(?)TeV,and the corresponding luminosity is 36.1 fb-1.Seven hypotheses are tested during the analysis.Two scenarios,narrow width approximation(NWA)and large width as-sumption(LWA),consider the decay widths of a model-independent heavy Higgs boson.Five benchmark models including two-Higgs-doublet models(2HDM),Georgi-Machacek(GM)model,heavy vector triplet(HVT)model with a spin-1 heavy boson,bulk Randall-Sundrum(RS)model with a spin-2 graviton at k/MPl=0.5 or 1,and effective Lagrangian model(ELM)with a VBF spin-2 graviton.No significant excess was found beyond the Standard Model background predic-tion in a mass range between 200 GeV and 5 TeV.Upper limits at 95%CL are set on the resonance production cross section times branching fraction X?WW.