Observation of a pseudoscalar excess at the top quark pair production threshold

A search for resonances in top quark pair (tt̄) production in final states with two charged leptons and multiple jets is presented, based on proton-proton collision data collected by the CMS experiment at the CERN LHC at √s = 13 TeV, corresponding to 138 fb⁻¹. The analysis explores the invariant mass of the system and two angular observables that provide direct access to the correlation of top quark and antiquark spins. A significant excess of events is observed near the kinematic tt̄ threshold compared to the nonresonant production predicted by fixed-order perturbative quantum chromodynamics (pQCD). The observed enhancement is consistent with the production of a color-singlet pseudoscalar (¹S[1]₀) quasi-bound toponium state, as predicted by nonrelativistic quantum chromodynamics. Using a simplified model for ¹S[1]₀ toponium, the cross section of the excess above the pQCD prediction is measured to be 8.8+1.2-1.4 pb.