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

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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