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

A search for resonances in top quark pair (tt-) 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 ats=13TeV, corresponding to 138 fb. The analysis explores the invariant mass of thett-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 kinematictt-threshold compared to the non-resonant production predicted by fixed-order perturbative quantum chromodynamics (pQCD). The observed enhancement is consistent with the production of a color-singlet pseudoscalar (1S0[1]) quasi-bound toponium state, as predicted by non-relativistic quantum chromodynamics. Using a simplified model for1S0[1]toponium, the cross section of the excess above the pQCD prediction is measured to be8.8-1.4+1.2pb.