Abstract
A search for the associated production of the Higgs boson with a top quark pair ($t\bar t H$) 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 $\sqrt{s} = 13$ TeV at the Large Hadron Collider. Higgs boson decays to $WW^*$, $\tau\tau$, and $ZZ^*$ 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 $t\bar t H$ production cross section is $\sigma(t\bar t H) = 790^{+230}_{-210}$ fb, in agreement with the Standard Model prediction of $507^{+35}_{-50}$ fb. The combination of this result with other $t\bar t H$ searches from the ATLAS experiment using the Higgs boson decay modes to $b\bar b$, $\gamma\gamma$ and $ZZ^* \to 4\ell$, has an observed significance of 4.2 standard deviations, compared to an expectation of 3.8 standard deviations. This provides evidence for the $t\bar t H$ production mode.
Highlights
The study of the origin of electroweak symmetry breaking is one of the key goals of the Large Hadron Collider (LHC) [1]
Data-driven methods are used to estimate the backgrounds with nonprompt light leptons and fake τhad candidates, defining control regions enriched in such backgrounds and extrapolating the observed yields to the signal regions
This fake factor is derived in a control region defined in Table VII, which differs from the signal region by looser lepton requirements and lower jet multiplicity
Summary
The study of the origin of electroweak symmetry breaking is one of the key goals of the Large Hadron Collider (LHC) [1]. The term “light lepton,” denoted l, refers to either electrons or muons and is understood to mean both particle and antiparticle as appropriate These signatures are primarily sensitive to the decays H → WWÃ (with subsequent decay to lνlν or lνqq), H → τþτ− and H → ZZÃ (with subsequent decay to llνν or llqq), and their selection is designed to avoid any overlap with the ATLAS searches for ttH production with H → bb [36], H → γγ [37] and H → ZZÃ → 4l [38] decays.
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