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.
