The CMS collaboration at CERN’s Large Hadron Collider has observed an unexpected excess of top quark-antiquark pairs that could indicate the existence of toponium - potentially the smallest composite particle yet discovered^[1].

The excess was first detected in 2019 but gained significance with analysis of the full 2016-2018 dataset. In April 2025, CMS reported a cross section of 8.8 picobarns with 15% uncertainty, surpassing the five-sigma threshold for discovery^[1:1][2].

The ATLAS experiment has now confirmed seeing the same effect, measuring a cross section of 9.0 ± 1.3 picobarns^[3]. The excess appears right at the minimum energy needed to produce top quark pairs, with angular patterns matching predictions for a pseudoscalar particle.

While evidence points to toponium - a fleeting union between a top quark and its antimatter partner - scientists remain cautious about the interpretation. “The toponium hypothesis is very exciting as we previously did not expect to be able to see it at the LHC,” said CMS physics coordinator Andreas Meyer^[4].

If confirmed, toponium would complete the quarkonium family after charmonium (1974) and bottomonium (1977), marking the smallest hadron ever observed due to the top quark’s high mass^[1:2].