Evidence for the associated production of the Higgs boson and a top quark pair with the ATLAS detector

TL;DR

This paper reports evidence for the production of the Higgs boson in association with a top quark pair using ATLAS data, showing an excess over background expectations with a significance of over 4 standard deviations.

Contribution

It provides the first evidence for the associated production of the Higgs boson with a top quark pair at the LHC, combining multiple decay channels and datasets.

Findings

Observed significance of 4.1 standard deviations for $t\bar t H$ production

Measured cross section consistent with Standard Model predictions

Combined analysis yields an overall significance of 4.2 standard deviations

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.