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ohwilleke
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- TL;DR Summary
- CMS in a paper published on August 23, 2025, and ATLAS in a paper that will imminently be published, have discovered a meson that is a bound state of a valance top quark and a valence antitop quark.
Toponium is a hadron which is the bound state of a valance top quark and a valance antitop quark.
Oversimplified presentations often state that top quarks don't form hadrons, because they decay to bottom quarks extremely rapidly after they are created, leaving no time to form a hadron. And, the vast majority of the time, this is true.
But, the lifetime of a top quark is only an average lifetime. Sometimes it decays faster and sometimes it decays slower. In the highly improbable case that a top quark and a top anti-quark are created at the same time and both last much longer than the average lifetime before decaying, they can form a hadron which is called toponium, and it is fairly elementary to determine how likely this is to happen at a given energy scale.
In the paper below, the CMS collaboration at the Large Hadron Collider (LHC) claims to have discovered a resonance which appears to be ground state toponium, which has a highly distinctive signature in a collider, because toponium is profoundly more massive (at more than 344 GeV) than any other meson. The background that has to be distinguished from the signal is therefore pretty modest.
Another paper, whose preprint was released today, in the course of considering the possibility of a hadron which is a baryon with three top quarks (a profoundly more difficult to form hadron since three top quarks or three antitop quarks need to be formed within about 3 x 10-25 seconds in essentially the same place), asserts that the ATLAS collaboration at the LHC has also discovered a toponium resonance, although the citation in the preprint does not include any arXiv or journal reference. This citation is to:
Discovering this vanishingly rare and incredibly short lived meson, which is the heaviest possible meson (and has a mass about 70% greater than a uranium-235 atom confined to a space on the order of 1000 times smaller than a proton) is a remarkable accomplishment in and of itself, and also with more detections, could make it possible to measure the top quark mass to a precision of about ten times as great as current measurements (from ± 0.3 GeV currently to ± 0.03 GeV).
Oversimplified presentations often state that top quarks don't form hadrons, because they decay to bottom quarks extremely rapidly after they are created, leaving no time to form a hadron. And, the vast majority of the time, this is true.
But, the lifetime of a top quark is only an average lifetime. Sometimes it decays faster and sometimes it decays slower. In the highly improbable case that a top quark and a top anti-quark are created at the same time and both last much longer than the average lifetime before decaying, they can form a hadron which is called toponium, and it is fairly elementary to determine how likely this is to happen at a given energy scale.
In the paper below, the CMS collaboration at the Large Hadron Collider (LHC) claims to have discovered a resonance which appears to be ground state toponium, which has a highly distinctive signature in a collider, because toponium is profoundly more massive (at more than 344 GeV) than any other meson. The background that has to be distinguished from the signal is therefore pretty modest.
Another paper, whose preprint was released today, in the course of considering the possibility of a hadron which is a baryon with three top quarks (a profoundly more difficult to form hadron since three top quarks or three antitop quarks need to be formed within about 3 x 10-25 seconds in essentially the same place), asserts that the ATLAS collaboration at the LHC has also discovered a toponium resonance, although the citation in the preprint does not include any arXiv or journal reference. This citation is to:
Presumably the authors have received advance word of this paper from someone in the ATLAS collaboration (or are members of the collaboration themselves) and plan to update the reference in their own paper when it is released. Their paper slightly overstates what the papers actually claim (which is that the resonance is consistent with toponium, but not that it definitely is toponium), but only modestly so.
Discovering this vanishingly rare and incredibly short lived meson, which is the heaviest possible meson (and has a mass about 70% greater than a uranium-235 atom confined to a space on the order of 1000 times smaller than a proton) is a remarkable accomplishment in and of itself, and also with more detections, could make it possible to measure the top quark mass to a precision of about ten times as great as current measurements (from ± 0.3 GeV currently to ± 0.03 GeV).
CMS Collaboration, "Observation of a pseudoscalar excess at the top quark pair production threshold" arXiv:2503.22382v2 (March 28, 2025, published version released on August 23, 2025).
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