Search for avor-changing neutral current top quark decays t—›Hq, with H—›bb, in pp collisions at √s = 8 TeV with the ATLAS detector

摘要

The results presented in this dissertation fill a gap in the program of searches for t → Hq decays during Run 1 of the LHC by considering the dominant decay mode H → b¯b, which has BR(H → b¯b) ' 58%. The search is focused on the tt¯ → W bHq (q = u, c) process, with W → `ν (` = e, µ, τ ) and H → b¯b, resulting in a leptonplus-jets final state with high b-jet multiplicity, which can be effectively exploited to suppress the overwhelming tt¯ background. The lepton-plus-jets final state also allows the kinematic reconstruction of the final state and in particular the dijet invariant mass spectrum from the H → b¯b decay, providing additional handles that would help in detecting tt¯ → W bHq events. Most of this dissertation is devoted to the discussion of this particular search, for which background estimation techniques, systematic uncertainties and statistical treatment closely follow those used in ATLAS searches using the same final-state signature [8, 9]. The results obtained have been published by the ATLAS collaboration in Ref. [10]. A combination of the three ATLAS searches for tt¯ → W bHq, probing the H → b¯b, H → W W ∗ , τ τ , and H → γγ decay modes, is performed and bounds are set on BR(t → Hc) and BR(t → Hu), as well as on the corresponding non-flavor-diagonal Yukawa couplings.