The ATLAS experiment at the Large Hadron Collider has reported new findings in the search for top-quarkonium, a bound state formed by a top quark and an anti-top quark, with significant contributions to the analysis from a research team in Shandong University (SDU).

Published in Reports on Progress in Physics, the study observed an excess near the top-antitop production threshold with a significance of eight standard deviations, aligning with theoretical predictions for top-quarkonium. The bound state measures about 0.01 femtometers, or 10^-17meters, making it the smallest bound state discovered so far.

The top quark is the heaviest known quark, with a mass about 184 times that of a hydrogen nucleus and a lifetime of about 10^-25seconds. Due to its extremely short lifetime, it was widely believed that the top quark could not form a bound state.

Professor Li Haifeng's group from SDU made a full analysis and produced the final results, with doctoral student Liu Xinyan serving as the main analyst. Li was invited to present the findings on behalf of the ATLAS Collaboration at the 2025 European Physical Society Conference on High Energy Physics (EPS-HEP). CERN later described the finding as the discovery of top quarkonium and selected it as one of its 2025 research highlights.

An artistic illustration of top quarkonium. [Photo/Shandong University]