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

Asearch 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 at √s =13TeV,corresponding to 138fb−1. The analysis explores the invariant mass of the tt 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 tt 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 (1S[1]0 ) quasi-bound toponium state, as predicted by non-relativistic quantum chromodynamics. Using a simplified model for 1S[1] 0 toponium, thecross section of the excess above the pQCD prediction is measured to be 8.8+1.2 −1.4 pb.