Abstract
Abstract:
A search for the associated production of the Higgs boson with a top quark pair (tt¯H) is
reported. The search is performed in multilepton final states using a dataset corresponding to an
integrated luminosity of 36.1 fb−1 of proton–proton collision data recorded by the ATLAS
experiment at a center-of-mass energy √s = 13 TeV at the Large Hadron Collider. Higgs
boson decays to WW ∗, ττ, and Z Z∗ are targeted. Seven final states, categorized by the number and
flavor of charged-lepton candidates, are examined for the presence of the Standard Model Higgs
boson with a mass of 125 GeV and a pair of top quarks. An excess of events over the expected
background from Standard Model processes is found with an observed significance of 4.1 standard
deviations, compared to an expectation of 2.8 standard deviations. The best fit for the tt¯H
production cross section is σ tt¯H = 790+230 fb, in agreement with the Standard
−210
Model prediction of 507+35 fb. The combination of this result with other tt¯H searches from
the ATLAS experiment using the Higgs boson decay modes to bb¯, γγ and Z Z∗ → 4l, has an
observed significance of 4.2 standard deviations, compared to an expectation of 3.8 standard
deviations. This provides evidence for the tt¯H production mode.