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

Abstract

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 (Formula presented.), 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 (Formula presented.) quasi-bound toponium state, as predicted by non-relativistic quantum chromodynamics. Using a simplified model for (Formula presented.) toponium, the cross section of the excess above the pQCD prediction is measured to be 8.8 − 1.4 + 1.2 pb .