Measurements at the ttbar threshold with the ATLAS detector

A measurement is presented of tt̄ production in the invariant-mass region near the pair production threshold, mtt̄ ~ 345\,GeV, in final states with two charged leptons and multiple jets. The measurement is based on 140\,fb⁻¹ of proton–proton collision data collected at √s = 13\,TeV with the ATLAS detector. The data are compared to two models of tt̄ production: a baseline model including only perturbative QCD (pQCD) predictions for the hard process, and an extended model that, in addition to the pQCD predictions, incorporates Monte Carlo simulations of color-singlet quasi-bound-state formation near the tt̄ threshold. An excess of events is observed over the baseline pQCD prediction, with an observed significance of $7.7$ standard deviations. This excess is consistent with the formation of colour-singlet, S-wave, quasi-bound tt̄ states as predicted by non-relativistic QCD, and corresponds to a measured cross-section of 9.0 ± 1.3\,pb.