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

A measurement of 𝑡𝑡¯ production is presented in the invariant-mass region near the pair production threshold, 𝑚_𝑡𝑡¯ ∼ 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 at the Large Hadron Collider. The data are compared to two models of 𝑡𝑡¯ 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 𝑡𝑡¯threshold. The agreement between the data and the models is quantified via a profile-likelihood fit to the reconstructed 𝑚_𝑡𝑡¯ distributions, in bins of two angular observables sensitive to spin-correlations in the 𝑡𝑡¯ 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 𝑆-wave quasi-bound 𝑡𝑡¯states, as predicted by non-relativistic QCD, and corresponds to an observed cross-section of 9.3^+1.4_−1.3 pb.