The Higgs boson interacts more strongly with more massive particles, so the coupling between the top quark and the Higgs boson (the top-quark Yukawa coupling) is expected to be large. The coupling can be directly probed by measuring the rate of events in which a Higgs boson is produced in association with a pair of top quarks (ttH production). Using the 13 TeV LHC data set collected in 2015 and 2016, several ATLAS analyses targeting different Higgs boson decay modes were performed. The combination of their results, released in late October, provides the strongest single-experiment evidence to date for ttH production.
The H → bb decay channel offers the largest rate of ttH events, but extracting the signal is hard because of the large background of top quarks produced in association with a pair of bottom quarks. The analysis relies on the identification of b-jets and multivariate analysis techniques to reconstruct the events and determine whether candidates are more likely to arise from ttH production or from background processes.
The probability for the Higgs boson to decay to a pair of W bosons or a pair of τ leptons is smaller, but the backgrounds to ttH searches with these decays are also smaller and easier to estimate. These decays are targeted in searches for events with a pair of leptons carrying the same charge or three or more charged leptons (including electrons, muons, or hadronically decaying τ leptons). In total, seven different final states were probed in the latest ATLAS analysis.
Higgs boson decays to a pair of photons or to a pair of Z bosons with subsequent decays to lepton pairs (giving a four-lepton final state) are also considered. These decay channels have very small rates, but provide a high signal-to-background ratio.
In the combination of these ttH analyses, an excess with a significance of 4.2 standard deviations with respect to the “no-ttH-signal” hypothesis is observed, compared to 3.8 standard deviations expected for a Standard Model signal. This constitutes the first direct evidence for the ttH process occurring at ATLAS. A cross-section of 590+160–150 fb is measured, in good agreement with the Standard Model prediction of 507+35–50 fb. This measurement, when combined with other Higgs boson production and decay studies, will shed more light on the possible presence of physics beyond the Standard Model in the Higgs sector.