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Study of the Higgs boson decay into b-quarks with the ATLAS experiment - run 2

Abstract : In this thesis, three aspects of my work within the ATLAS experiment are described. The first one is dedicated to the extraction of correction factors to be applied on the simulation to match the data. The second is the analysis of the Higgs boson decay to a pair of b-quarks. The last one is a search for BSM in the interaction between a top quark and the Higgs boson.My implication in the ATLAS collaboration started with updates of the pseudo-continuous calibration of b-jets flavour identification. The pseudo-continuous calibration of b-Tagging in the ATLAS experiment was developed during the run 1 of the LHC. An update of the framework was performed, and calibration results have been produced. The uncertainties on the pseudo-continuous b-Tagging calibration are dominated by systematic sources.The second topic of the thesis is the search for a Higgs boson decaying into beauty quarks, produced in association with a vector boson. I was especially involved in the channel looking for a Z boson decaying into neutrinos. The analysis was optimised with respect to the experience acquired during the run 1 of the LHC. Many ideas have been explored in order to improve the multivariate analysis. The evidence for the VH(bb) process was presented using data collected in 2015 and 2016. The signal strength is measured to ˆμV H = 1.20+0.24 −0.23(stat.)+0.34 −0.28(syst), which corresponds to an observed significance of 3.5 standard deviations. The uncertainty on this result is dominated by systematic sources. Hence, several studies have been performed in order to better understand specific aspects of this result. A special attention was given to the modelling of background processes to improve the generation efficiency of Monte-Carlo simulation. Also, the use of coulourflow in the VH(bb) search appears to provide a limited gain in the rejection of background processes with the current jet reconstruction performances. The analysis was updated adding data collected in 2017, and lead to the observation of the Higgs boson decay into beauty quarks.At last, the final study presented in this work is a major revision of the search for a top quark decaying into an up-type ( = , ) quark and a Higgs boson decaying into two photons. This process is forbiden at tree level in the SM and highly supressed at loop level, with an expected branching ratio of 10−15, making it a very good probe for BSM physics. The analysis is now designed in order to disentangle the and couplings by targeting both the single- and pair-production modes of top quarks, and exploiting c-tagging in the categorisation. The use of kinematic variables was developped, and multivariate methods have been implemented in the analysis. Preliminary prospect results are presented. With the set of data accumulated by the end of the LHC run 2, the analysis will be sensitive to branching ratio below the permil level and could provide the first constraints on realistic BSM models.
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Charles Delporte. Study of the Higgs boson decay into b-quarks with the ATLAS experiment - run 2. High Energy Physics - Experiment [hep-ex]. Université Paris-Saclay, 2018. English. ⟨NNT : 2018SACLS404⟩. ⟨tel-02459260⟩

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