Measurement of the top-quark Yukawa coupling from <i>tt</i> production in the lepton+jets final state using <i>pp</i> collisions at √<i>s</i> = 13 TeV with the ATLAS detector

The top-quark Yukawa coupling is extracted from the distribution of the top-quark pair (<i style="background-color: rgb(255, 255, 238);">tt̄</i>) invariant mass in proton-proton collisions using 140 fb^-1 of data at √<i>s</i> = 13 TeV collected in 2015-2018 by the ATLAS experiment at the Large Hadron Collider. In the region near the production threshold, the <i style="background-color: rgb(255, 255, 238);">tt̄</i> invariant mass spectrum is sensitive to electroweak virtual corrections, including contributions from Higgs boson exchange, thereby providing sensitivity to the top-quark Yukawa coupling. This is the first measurement in ATLAS that aims to obtain this coupling exploiting this approach. The <i style="background-color: rgb(255, 255, 238);">tt̄</i> system is reconstructed in the single-lepton final state, requiring exactly one isolated electron or muon and at least four jets with at least two identified as originating from <i>b</i>-quarks. The measured Yukawa coupling is found to be in good agreement with the Standard Model prediction. An upper limit on the top-quark Yukawa coupling strength of <i>Y_t</i> &lt; 2.1 relative to the Standard Model prediction is observed at 95% confidence level, consistent with the expected sensitivity.