Observation of a cross-section enhancement near the t tbar production threshold in root s = 13 TeV pp collisions with the ATLAS detector

A measurement of t tbar production is presented in the invariant-mass region near the pair production threshold, m(t tbar)∼345 GeV, in final states with two charged leptons and multiple jets. The measurement is based on 140 fb-1of 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 oftt¯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 thett¯threshold. The agreement between the data and the models is quantified via a profile-likelihood fit to the reconstructed m(t tabr) distributions, in bins of two angular observables sensitive to spin-correlations in the t tbar 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 spin-singlet S-wave quasi-bound t tbar states, as predicted by non-relativistic QCD, and corresponds to an observed cross-section of 9.3-1.3+1.4 pb.