Search for the associated production of the Higgs boson with a top quark pair in multilepton final states with the ATLAS detector

A search for the associated production of the Higgs boson with a top quark pair is performed in multilepton final states using 20.3 fb(-1) of proton-proton collision data recorded by the ATLAS experiment at root s = 8 TeVat the Large Hadron Collider. Five final states, targeting the decays H -> WW*,tau tau, and ZZ*,are examined for the presence of the Standard Model (SM) Higgs boson: two same-charge light leptons (e or mu) without a hadronically decaying tau lepton;three light leptons;two same-charge light leptons with a hadronically decaying tau lepton;four light leptons;and one light lepton and two hadronically decaying tau leptons. No significant excess of events is observed above the background expectation. The best fit for the t (t) over barH production cross section, assuming a Higgs boson mass of 125 GeV, is 2.1(-1.2)(+1.4) times the SM expectation, and the observed (expected) upper limit at the 95% confidence level is 4.7 (2.4) times the SM rate. The p-value for compatibility with the background-only hypothesis is 1.8s;the expectation in the presence of a Standard Model signal is 0.9 sigma. (C) 2015 CERN for the benefit of the ATLAS Collaboration. Published by Elsevier B.V