The CMS and ATLAS Collaborations at the Large Hadron Collider at CERN discovered a feature in the top quark behaviour that suggests that these heaviest and shortest-lived of all the elementary particles momentarily form a quasi-bound state of a top-anti-top quark pair, the so-called toponium. These results, obtained by international teams of scientists with the participation of JINR physicists, may provide new insights into the physics of top quarks and the nature of their strong interaction.

Candidate events for the formation of a quasi-bound state of a top quark pair at the CMS (left) and ATLAS (right) Experiments. (Source: ATLAS/CMS/CERN)

Head of the JINR programme of physics studies as part of the CMS Collaboration, Director of the Meshcheryakov Laboratory of Information Technologies at JINR Sergei Shmatov commented on the scientific observation. “This state was predicted in the framework of non-relativistic quantum chromodynamics but had never been observed before, since the probability of non-resonant formation of a top quark pair at the kinematic threshold is much higher than the probability of the formation of their quasi-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 they were bound together by gluons,” Sergei Shmatov explained.

The MLIT Director noted that researchers made the observation primarily thanks to the extensive statistics of experimental data collected during Run 2 at the LHC. “For the first time, the CMS Experiment detected an excess of events, indicating a certain phenomenon in an extremely narrow energy range, from 338 to 350 GeV, on the verge of the birth of two top quarks. The ATLAS Experiment confirmed this observation,” Sergei Shmatov said. According to him, scientists have two hypotheses to explain the new observed phenomenon: either it is a colour singlet (a quasi-stable state of a top-quark-antiquark pair) or an sign of new physics, which, according to available data, seems less likely. Testing the hypotheses requires obtaining refined theoretical predictions and further analysing the data collected by both experiments during the ongoing Run 3 at the LHC.