B-tagging Calibration And Observation Of Higgs Boson Decays To A Pair Of Bottom Quarks With The ATLAS Detector

This thesis firstly talks about the b-jet tagging efficiency calibration with the tag-and-probe method in the ATLAS experiment with the data collected in 2015 and 2016.The efficiency measurement method,the object selection,the event and probe jet selection and the statistical and systematic uncertainties are discussed,as well as the results.The b-tagging efficiencies have been measured as a function of the jet transverse momentum,pseudorapidity and of the average number of pile-up collisions.The efficiency measured in data have been compared to those predicted from simulation,and simulation-to-data efficiency scale factors have been determined.The efficiency scale factors are close to unity,with total uncertainty ranging between 2%and 12%.Then the evidence of the Standard Model(SM)Higgs boson decaying to bottom quarks in the associated production mode with vector bosons with 36.1 fb-1 data collected at(?)=13 TeV during the LHC Run-2(until 2016)will be introduced,and followed by the observation of VH,H?bb from the study including also the additional 44 fb-1 data collected in 2017.For the study based on 36 fb-1 data,the event classification,the discriminating technique be-tween signal and background,the main background and their modelling,the signal properties and the interpretation of results are discussed.For the study based on 80 fb-1 data,the small difference from the previous study and the observation result are discussed.Both results provide a strong evidence of such process,with statistical significances of 3.5 and 4.9 standard devia-tions respectively.The signal strength,defined as the ratio between the measured cross section times branching ratio and the SM prediction,is measured with the 2015-2017 dataset to be?VHbb=1.16-0.25+0.27=1.16±0.16(stat.)-0.19+0.21(syst.),indicating good agreement with the SM.The combination of the results of the VH,H?bb results based on 80 fb-1 of 13 TeV collisions with the results with the results of the same analysis performed on data collected at lower(7 and 8 TeV)centre-of-mass energies in 2011 and 2012 and same decay mode with other Higgs boson production modes(vector-boson fusion,associated production with a top-quark pair)have led to the observation of the H?bb decay with a significance of 5.4 standard deviations,and a signal strength ?bb=1.01±0.20 = 1.01±0.12(stat.)-0.15+0.16(syst.).A combination of the results of the VH,H?bb results based on 80 fb-1 of 13 TeV collisions with the results of searches for the same production mode on the same amount of data in the diphoton and ZZ*?4l Higgs boson final states has led to the observation of VH production with a significance of 5.3 stan-dard deviations,and a signal strength ?VH=1.13-0.23+0.24=1.13±0.15(stat.)-0.17+0.18(syst.)in good agreement with the SM.