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  1. (, Reports on Progress in Physics)
    Abstract A search for resonances in top quark pair ( t t ― ) 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 t t ― 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 t t ― 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 ( 1 S 0 [ 1 ] ) quasi-bound toponium state, as predicted by non-relativistic quantum chromodynamics. Using a simplified model for 1 S 0 [ 1 ] toponium, the cross section of the excess above the pQCD prediction is measured to be 8.8 − 1.4 + 1.2 pb . 
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