The top quark is a unique particle that decays almost instantly after formation. However, under certain conditions, physicists predict that it can form a quasi-bound state with its anti-top quark, exchanging gluons before decaying independently.
Researchers at the ATLAS Collaboration have observed this phenomenon in top-pair events produced near the production threshold. The production threshold is the energy required for the top-quark and anti-top quark to be produced almost at rest, resulting in a quasi-bound state with a mass slightly below it.
The previous assumption that measuring this effect was experimentally unfeasible has been overturned due to advances in analysis techniques and increased proton-proton data recorded during Run 2 of the LHC. The ATLAS Collaboration divided the dataset into nine subsets based on angular information used to discover entanglement in top-pair events and employed sophisticated simulations.
Their new result revealed an excess of events when compared to predictions that ignore the quasi-bound state, with a measured cross-section of 9.0 ± 1.3 pb. This measurement is consistent with previous results from the CMS Collaboration. The ongoing Run 3 of the LHC will deliver significantly more data, allowing ATLAS physicists to deepen their exploration of the strong force in this special kinematic regime.
The study highlights the importance of precise theoretical modeling of top-pair production and reinforces the interplay between theory and experiment in precision studies. Improved models will be essential to fully understand this intriguing corner of the Standard Model.
Source: https://atlas.cern/Updates/Briefing/Quasi-Bound-Tops