Elusive romance of top-quark pairs observed at the LHC The CMS and ATLAS experiments at CERN’s Large Hadron Collider have observed an unforeseen feature in the behavior of top quarks that suggests that these heaviest of all elementary particles form a fleeting union of top quarks and antiquarks, the so-called toponium. Leader of the JINR program of physics research in the CMS collaboration, MLIT Director Sergei Shmatov commented on this phenomenon. As Sergei Shmatov noted, MLIT scientists, as well as all JINR physicists participating in the CMS and ATLAS experiments, are actively working to search for physics beyond the Standard Model and, in particular, new states predicted by non-minimal Higgs models. One of the areas of research is the search for new physics in channels with a pair of top quarks in the final state. “I would say that this is more a triumph of experimental physics than theory. This state was previously predicted within non-relativistic quantum chromodynamics, but it was not observed before, since the probability of non-resonant formation of a pair of top quarks at the kinematic threshold is much greater than the probability of formation of their bound state. For a long time, the existence of such a quasi-stable pair was considered impossible – the large mass of top quarks led to their rapid decay before a gluon bond was established between them. The observation became possible primarily due to the unprecedented statistics of experimental data collected during Run 2 at the LHC. We, as the Laboratory and as the Joint Institute as a whole, consider it our success that our Tier1 CMS data center ranked first among all experimental sites worldwide in terms of CPU time spent on data processing in 2024. In addition, methods for event reconstruction and physics analysis have been and continue to be actively developed. Now, for the first time, the CMS experiment has detected an excess of events indicating a phenomenon in an extremely narrow energy region from 338 to 350 GeV, just on the threshold of the production of two top quarks. The ATLAS experiment has confirmed this observation. We actually have two hypotheses, either this is a color singlet, a quasi-stable state of the top quark-antiquark pair, or we have obtained an indication of new physics, which, judging by the available data, seems less likely. To answer this, we need more accurate theoretical predictions and an analysis of the new data that the both experiments have already collected during Run 3. The first observation of any phenomenon in human history is always intriguing. Every time we hope that such a phenomenon may have new properties that are not described by the Standard Model, and this gives us hope that we can talk about indications of new physics,” Sergei Shmatov said. Artist’s impression of the short-lived union of a top quark and a top antiquark formed by the exchange of gluons. (Image: D. Dominguez/CERN) Candidate events for the formation of a quasi-bound state of a pair of top quarks in the experiment (left) ATLAS and (right) CMS.(Image: ATLAS/CMS/CERN) Links ATLAS Collaboration 2025 (ATLAS-CONF-2025-008): “Observation of a cross-section enhancement near the t¯t production threshold in √s=13 TeV pp collisions with the ATLAS detector”. CMS Collaboration 2024 (TOP-24-007): "Observation of a pseudoscalar excess at the top quark pair production threshold". Source: CERN press release, 8 July 2025. Scientists from the Joint Institute for Nuclear Research are actively involved in the ATLAS and CMS experiments at the Large Hadron Collider (LHC). JINR started participating in the both experiments at the preliminary research and development stages and subsequently became one of the major participants in ATLAS and CMS.
