In a striking new result, the CMS Collaboration has observed something truly remarkable. They saw a pseudoscalar excess at the top quark pair production threshold, set in 2024, named TOP-24-007. This observation is a historic high-energy physics milestone. It is in agreement with observations by the ATLAS Collaboration, which proved the same phenomenon while studying the large LHC Run-2 data set accumulated from 2015 to 2018.
As their accompanying visualization explains, the research points to a novel phenomenon of toponnium — a “quasi-bound-state” compound formed from two top quarks plus their antimatter counterparts. If confirmed, this finding would substantially alter the story we’ve been telling about quarkonia. Quarkonia denotes the bound states made from heavy quark-antiquark combinations of the same flavor that would classically produce a meson. The top quark is, indeed, the heaviest and the shortest-lived of all elementary particles. For one, it’s greatly responsible for this new discovery.
Our researchers presented their preliminary findings at the European Physical Society’s High‐Energy Physics conference in Marseille. They rejoiced at reaching the “five sigma” level of certainty, a critical threshold for announcing a discovery in particle physics.
Top Quark Pair Production and Pseudoscalar Excess
To achieve this precision, the CMS Collaboration performed a thorough analysis of data corresponding to 4.5 fb −1 of top quark–antiquark production. From 2016-2018, they collected this data. At first, scientists were searching for evidence of new kinds of Higgs bosons when they unexpectedly hit the smelly deposit or excess.
Stéphane Willocq, a leader in the CMS Collaboration, provided background on how this subtle effect was once considered too difficult to measure at the LHC. He pointed out that events just above the production threshold account for only a minuscule sample of the top pairs produced, and so are difficult to observe in the data.
His comments highlight the difficulty researchers have had in trying to identify this effect, especially at such low production rates. Advancements in data processing and analysis techniques have allowed scientists to overturn assumptions that previously limited their investigative horizons.
This is all rapidly being changed by the wealth of proton-proton data that was recorded during Run 2 of the LHC. New developments in analysis techniques are allowing us to reverse this common belief.
The collaboration’s work shows how complicated top quarks can be. We’re thrilled that it opens the door to some really cool new explorations in particle physics.
Confirmation from ATLAS Collaboration
Just days after the initial announcement by the CMS Collaboration, the ATLAS Collaboration validated those same results by looking at their own, full LHC Run-2 dataset. Their findings were recorded in ATLAS-CONF-2025-008, highlighting how solid these observations are from independent research groups.
“The observation of a non-relativistic QCD effect that was thought to be too difficult to detect is a great triumph for the LHC experiment program,” commented Gautier Hamel de Monchenault from the ATLAS Collaboration.
This scientific confirmation only enhances the significance of those original findings. It highlights how collaboration continues to inspire scientific innovation in the rapidly advancing field of high-energy physics. Both collaborations found very comparable results. Researchers are excited about what these findings mean for our understanding of fundamental particles.
“We keenly anticipate further rich interactions with our theory colleagues so that we may learn more about this fascinating corner of the Standard Model,” Hamel de Monchenault expressed.
Implications for Quarkonia Research
Beyond confirming a long-suspected anomaly in particle interactions, the implications of these findings are manifold. The observation of toponnium would challenge our current theories of quarkonia and provide a better understanding of heavy quark interactions.
Charmonium, the combination of charm and anti-charm, was found in 1974. Three years later, scientists discovered bottomonium, a state made of pairs of bottom and antibottom quarks. Both discoveries happened at U.S. laboratories and established the base understanding of how quark pairs are arranged. The possible confirmation of toponnium would add a surprising new chapter to this developing cautionary tale.
“These impressive results from ATLAS and CMS prove that there is still much to learn about the Standard Model of Particle Physics at high energies,” said Joachim Mnich, highlighting the ongoing quest for knowledge in particle physics.