Search For Z(v(?))H(b(?))and V(q(?))H(invisible) Decays At The ATLAS Experiment

In this thesis,the analyses of the Standard Model(SM)Higgs boson produced in association with a vector boson and decays into a pair of b-quarks or invisible particles are performed.The first one is the search for the Higgs boson decaying to a pair of b-quarks,while the vector boson decays into leptons,based on the Large Hadron Collider(LHC)proton-proton(pp)collision data at(?),collected by ATLAS detector in 2015,2016 and 2017 with an integrated luminosity of 79.8 fb-1.According to the number of charged lepton in the final state,the analysis is categorised in 0-,1-and 2-lepton channels,targeting the Higgs production and decay of ZH ? vvbb,WH?lvbb and ZH?l+lbb,where l represents an electron or a muon.This thesis focus on the 0-lepton channel,noted as Z(vv)H(bb)where the decay products are reconstructed as a large missing transverse momentum(ETmiss)and a pair of b-tagged jets.A multivariate analysis(MVA)is performed with machine learning method to sep-arate the Higgs boson signal from the large V+jets and top quark backgrounds,where the invariant dijet mass(mbb)is the most discriminating input variable.To enhance the sensitivity to signal,the selected events are subdivided into different categories accord-ing to the number of jets reconstructed in the event and the transverse momentum of the vector boson(pTV).A binned likelihood fit is then performed to the MVA output discriminant to extract the signal.Z(vv)H(bb)analysis gives an observed significance of 3.34 a derivation w.r.t background-only hypothesis,comparing to the expected 2.70 a.With the 0-,1-and 2-lepton channels combination,the observed(expected)significance is 4.9(4.3)? while the signal strength is measured to be 1.16-0.25+0.27.A combination with previous VH analysis with 2012 and 2013 data,as well as other production mode(vector boson fusion VBF and top quark associated production ttH)that is sensitive to the H? bb signal is also performed.The significance is ob-served(expected)to be 5.4(5.5)a derivation w.r.t background-only hypothesis,which claims the discovery of H? bb decay.The measured H? bb signal strength with the combination is 1.01-0.20+0.20.A search of invisible decay of Higgs boson produced in association with a hadron-ically decaying W or Z boson(V(qq)H(invisible))is also presented,using 36.1 fb-1 of data recorded during 2015 and 2016.Events are characterised by large ETmiss and a hadronically decaying vector boson reconstructed as either a pair of small-radius jets in the resolved category,or as a single large-radius jet with substructure in the merged category.The merge category is added aiming at the high pTV region where the W/Z boson is highly boosted.With the ETmiss plus j ets final state,signals from gluon-gluon fusion(ggH)or VBF also presence in our signal regions.We include those signals as well in the analysis,by designing a mass window for the reconstructed resonant of mjj(resolved)or mJ distribution(merged)near the W/Z boson mass to highlight the VH production mode,while keeping the window sideband for other signal production modes,for which the mjj or mJ shows only a smooth continuous distribution.Based on the profile likelihood fit applied on the discriminant variable ETmiss,no ex-cess beyond the Standard Model expectation is observed.The observed(expected)up-per limit at 95%CL on the H?invisible branching ratio is then set to 0.83(0.5-0.16+0.23).Comparing to the previous analysis in 2013,the expected limit is improved by 30%.With a combination to the Z(l+l)H(invisible)and VBF channel using data from 2012 to 2016,an exclusion limit on the H? invisible branching ratio of 0.26(0.17-0.05+0.07)is observed(expected).