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

The top-quark Yukawa coupling is extracted from the distribution of the top-quark pair (tt¯) invariant mass in proton-proton collisions using 140 fb⁻¹ of data at s=13 TeV collected in 2015–2018 by the ATLAS experiment at the Large Hadron Collider. In the region near the production threshold, the tt¯ 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 tt¯ 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 b-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 Y_t< 2.1 relative to the Standard Model prediction is observed at 95% confidence level, consistent with the expected sensitivity.