Observation of a cross-section enhancement near the ¯ production threshold in √ = 13 TeV collisions with the ATLAS detector

A measurement of ¯ production is presented in the invariant-mass region near the pair production threshold, _¯ ∼ 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 √ = 13 TeV with the ATLAS detector at the Large Hadron Collider. The data are compared to two models of ¯ production: a baseline model including only perturbative QCD predictions for the hard process, and an extended model that, in addition, incorporates non-relativistic QCD simulations of colour-singlet quasi-bound-state formation near the ¯threshold. The agreement between the data and the models is quantified via a profile-likelihood fit to the reconstructed _¯ distributions, in bins of two angular observables sensitive to spin-correlations in the ¯ system. An excess of events is observed over the baseline perturbative QCD prediction, with an observed significance over 8 standard deviations. This excess is consistent with the formation of colour-singlet and spin-singlet -wave quasi-bound ¯states, as predicted by non-relativistic QCD, and corresponds to an observed cross-section of 9.3^+1.4_−1.3 pb.