CMS-PAS-TOP-25-002

CMS-PAS-TOP-25-002
Observation of a pseudoscalar excess at the top quark pair production threshold in the single lepton channel
CMS Collaboration
2026-03-22

Abstract: A search is presented for top quark-antiquark ($ \mathrm{t\bar{t}} $) bound states near the $ \mathrm{t\bar{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{t}} $ system. A significant excess of events is observed relative to the standard model prediction for $ \mathrm{t\bar{t}} $ production calculated at next-to-next-to-leading order in perturbative quantum chromodynamics. The excess corresponds to an observed cross section of 5.1 $ \pm $ 0.9 $ \mathrm{pb} $ and is consistent with a simplified model of a color-singlet pseudoscalar toponium motivated by nonrelativistic quantum chromodynamics. The result provides an independent confirmation of the excess reported in the dilepton channel.
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to ROPP. The superseded preliminary plots can be found here.

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