UC Santa Cruz

A measurement oftt¯production is presented in the invariant-mass region near the pair production threshold,mtt¯∼345 GeV, in final states with two charged leptons and multiple jets. The measurement is based on140fb-1of proton-proton collision data collected ats=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 reconstructedmtt¯distributions, in bins of two angular observables sensitive to spin-correlations in thett¯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-singletS-wave quasi-boundtt¯states, as predicted by non-relativistic QCD, and corresponds to an observed cross-section of9.3-1.3+1.4 pb.