Abstract
A measurement of t (t) over bar production is presented in the invariant-mass region near the pair production threshold, m(t (t) over bar) similar to 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 root s = 13 TeV with the ATLAS detector at the Large Hadron Collider. The data are compared to two models of t (t) over bar production: a baseline model including only perturbative quantum chromodynamics (pQCD) predictions for the hard process at approximate next-to-next-to leading order accuracy in the strong coupling, and an extended model that, in addition, incorporates non-relativistic QCD simulations that also include the formation of colour-singlet quasi-bound-states near the t (t) over bar threshold. The agreement between the data and the models is quantified via a profile-likelihood fit to the reconstructed m(t (t) over bar) distributions, in bins of two angular observables sensitive to spin-correlations in the t (t) over bar system. An excess of events is observed over the baseline pQCD prediction, with an observed significance over 8 standard deviations. This excess is consistent with the formation of colour-singlet and spinsinglet S-wave quasi-bound t((t) over bar) states, as predicted by non-relativistic QCD, and corresponds to an observed cross-section of 9.3- (+1.4)(1.3) pb.