Search for the Higgs boson in the $WH \to ℓνb\overline{b}$ channel with the ATLAS detector - Development of high performance $b$-jet identification algorithms

In July 2012, the ATLAS and CMS collaborations announced the discovery of a new particle at a mass of about 125 GeV, compatible with the Higgs boson predicted by the Standard Model (SM) of particle physics. Although all measurements as of Summer 2015 show a full consistency with the SM predictions, the fermionic decay channels (H → bb and ττ) have not been seen yet as clearly as the bosonic decay channels. This thesis presents a search for the Higgs boson in the WH → ℓνbb channel, using proton-proton collision data at √s = 8 TeV taken with the ATLAS detector in the year 2012, corresponding to an integrated luminosity of 20.3 fb−1. This document details in particular one of the contributions made by the author in this search: the additional analysis region with muon events triggered by missing transverse momentum. An excess over the background-only hypothesis has been found with a significance of 1.8 σ while 1.5 σ was expected. The observed (expected) upper limit on the cross-section times branching ratio for WH → ℓνbb at 95% confidence level is found to be 2.35 (1.37) times the SM prediction at mH = 125 GeV. This search highly relies on the identification of jets originating in b-quark fragmentation, so-called b-tagging. In order to improve the H → bb search and other physics analyses using b-tagging in ATLAS, the development of high performance b-tagging algorithms has been performed and is presented also in this thesis. A new b-tagging algorithm, called MV2, is introduced: it improves the b-tagging performance significantly and is now the baseline b-tagging algorithm in ATLAS for the Run-2 analyses.