Observation of an excess at the top quark pair production threshold in the single-lepton channel

Abstract

A search is presented for top quark-antiquark ($\mathrm{t}\bar{\mathrm{t}}$) quasi-bound ``toponium'' states, near the kinematic $\mathrm{t}\bar{\mathrm{t}}$ production threshold in final states with a single electron or muon and jets. The study uses proton-proton collision data at $\sqrt{s}$ = 13 TeV, collected by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 138 fb${}^{-1}$. The analysis examines the relative velocity between the top quark and antiquark, along with two angular observables sensitive to the parity and spin of the $\mathrm{t}\bar{\mathrm{t}}$ system. A significant excess of events is observed relative to the standard model prediction for $\mathrm{t}\bar{\mathrm{t}}$ production calculated at next-to-next-to-leading order in perturbative quantum chromodynamics (QCD). The excess corresponds to an observed total cross section of 5.1$\pm$0.9 pb and is consistent with a simplified model of a color singlet pseudoscalar toponium motivated by nonrelativistic QCD, with a resulting observed significance of 6.1 standard deviations. The result provides an independent confirmation of the excess reported in the dilepton channel.

Submitted to Reports on Progress in Physics. All figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/TOP-25-002 (CMS Public Pages)