Amazing stuff!
"An independent measurement by the CMS experiment at the LHC confirms the existence of toponium, a bound state of a top quark and its antiquark, with a statistical significance exceeding five standard deviations. This finding establishes toponium as the most massive composite particle observed and enhances understanding of the strong nuclear force."
"... The first hints of toponium appeared in searches for heavy Higgs-boson-like particles that could decay into a top quark–antiquark pair. An unexpected excess of collision events was observed at a mass close to twice the mass of the top quark, which is more characteristic of a bound state rather than a new fundamental particle. Detailed studies by the CMS and ATLAS experiments confirmed this excess using events in which both top quarks decay into leptons (electrons or muons).
The new CMS study approaches the problem from a different angle, examining events in which one top quark decays into a bottom quark, a charged lepton and a neutrino while the other decays into quarks that produce sprays, or "jets," of particles. ...
These new techniques proved highly effective. They resulted in the observation of an excess with a statistical significance of more than five standard deviations ... The result provides a new, statistically independent confirmation of toponium production.
"Toponium is heavier than the heaviest known atomic nucleus, oganesson, making it the most massive bound state ever observed," ..."
From the abstract:
"A search is presented for top quark-antiquark (t¯t) bound states near the t¯t
production threshold, in final states with a single electron or muon and jets. The study uses proton-proton collision data at √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 t¯t system. A significant excess of events is observed relative to the standard model prediction for t¯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 ± 0.9pb 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."
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