Do top quarks combine for a fleeting moment to form a new particle called toponium?
- Mateo Cardinal
- 4 days ago
- 3 min read
When researchers from DESY and the University of Hamburg noticed something new in the data from the CMS experiment at CERN's largest particle accelerator, the Large Hadron Collider, last year, they chose their words very carefully, saying that they had ‘seen an interesting effect in the data that could point to something that has never been seen before.’ Now, their colleagues from the competing ATLAS experiment have observed the same effect, they reported this week at a scientific meeting of the European Physical Society. DESY researchers also played a leading role in this analysis. Researchers from the CMS collaboration at DESY and the University of Hamburg have detected an unexpected excess of top quark-antiquark pairs at very low energies. This effect was first observed in the 2016 data and has been further reinforced by the analyses of 2017 and 2018 data. The results suggest that top quarks and their antiparticles may be able to combine into a short-lived bound state known as ‘toponium’. Until now, it was considered virtually impossible that such bonds could be observed at the LHC.
The fact that the ATLAS collaboration has now also been able to confirm a similar effect shows its scientific significance. These findings provide independent support for the CMS team's observations, effectively ruling out the possibility that they could be a measurement error.
"For a long time, it was assumed that it would be impossible to observe quasi-bound top quarks at the LHC," explains Katharina Behr, ATLAS researcher and leader of a Helmholtz Young Investigator Group at DESY. Alexander Grohsjean, CMS scientist from the University of Hamburg, adds: "The new experimental results now impressively refute this assumption and show that, with sophisticated measurement strategies and the abundance and quality of the recorded LHC data, even such extremely subtle interactions of elementary particles can be observed."
The top quark, the heaviest of all elementary particles, usually appears on its own. It is very often produced in collisions at the LHC together with its antimatter counterpart. But while other quarks can combine to form bound states, known as hadrons, the top quark usually decays almost instantly due to its extremely short lifetime – most of the time before it can form a bound state. In rare cases, however, a top quark-antiquark pair is produced at the LHC at very low speeds. The top quark and its antimatter counterpart are then almost at rest relative to each other and can interact with each other for the short duration of their lives via the exchange of force particles called gluons – a brief dance before the first of them decays.
The scientists did not expect the measurements from the experiments to be precise enough to observe this phenomenon. And yet, it has now been confirmed that something happens at low speeds. In addition to the top quark-antiquark connection, other effects could also contribute to the observation, and it is now the task of the experiments and theorists to work together and understand exactly what happens when the top quark and top antiquark combine in a brief dance. To investigate these exciting effects, state-of-the-art predictions of the strong interaction must be used. They deal with how top quarks interact with each other via gluons. Researchers from the Institute for Theoretical Physics and DESY in Hamburg are already playing a leading role in these calculations.
"We now want to provide a comprehensive description of the phenomenon," says CMS scientist Christian Schwanenberger from DESY and the University of Hamburg. “We rely both on new calculations and on new data from the LHC, which we expect to triple with the current run of the accelerator. Then, together with ATLAS, we will take an even closer look at the interactions between top quarks and top antiquarks.”
The excess now discovered by both experiments marks a significant step forward in the quest to understand the fundamental structure of the matter around us. Reference Observation of a pseudoscalar excess at the top quark pair production threshold
CMS Collaboration https://arxiv.org/abs/2503.22382 Deutsches Elektronen-Synchrotron (DESY)
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