I Toponium Discovered

[ohwilleke]

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:

ATLAS Collaboration, “Observation of a cross-section enhancement near the t¯t production threshold in √s =13 TeV pp collisions with the ATLAS detector.”

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).

A search for resonances in top quark pair (tt¯) production in final states with two charged leptons and multiple jets is presented, based on proton-proton collision data collected by the CMS experiment at the CERN LHC at s√ = 13 TeV, corresponding to 138 fb−1. The analysis explores the invariant mass of the tt¯ system and two angular observables that provide direct access to the correlation of top quark and antiquark spins. A significant excess of events is observed near the kinematic tt¯ threshold compared to the nonresonant production predicted by fixed-order perturbative quantum chromodynamics (pQCD). The observed enhancement is consistent with the production of a color-singlet pseudoscalar (1S[1]0) quasi-bound toponium state, as predicted by nonrelativistic quantum chromodynamics. Using a simplified model for 1S[1]0 toponium, the cross section of the excess above the pQCD prediction is measured to be 8.8 +1.2−1.4 pb.

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).

 

[sbrothy]

I may be too quick to like this as it's based on preprints, right? Also I'm not a real physicist, but it sure sounds exciting.

 

[ohwilleke]

I may be too quick to like this as it's based on preprints, right? Also I'm not a real physicist, but it sure sounds exciting.

The CMS paper was published last week, with some modest revisions from the original preprint in March. If I were to hazard a guess, I'd guess that the ATLAS paper will be published more or less simultaneously with the release of its preprint in the very near future.

As far as "exciting" goes, on one hand, it is something that was possible to predict from shortly after the time that the top quark was discovered in 1995 at Fermilab, thirty years ago. So, it isn't all that surprising. On the other hand, it is an incredible technical achievement and if you'd asked me last Christmas when I though it would be achieved, I would have told you "probably sometime around 2035" at a next generation collider.

 

[sbrothy]

Yeah, as usual I was too fast. Coming from the CMS and ATLAS collaborationS I think it's a foregone conclusion that it will hold up.

 

[mfb]

Internal reviews in the big collaborations are far more stringent than the official peer review process. I have never heard of a paper that would have been rejected for having problems in its science content. Some get rejected because the reviewers think it's not notable enough for that journal, or because reviewers and authors can't agree on how to present things in a clear way.

Money plot from the paper:

Only up to 8% difference, but nicely fitting to the expected distribution.

 

[sbrothy]

That’s actually not surprising when you look at the number of authors on their papers. I think I mentioned this elsewhere.

 

[pines-demon]

How many times has this been claimed before?

 

[ohwilleke]

How many times has this been claimed before?

I'm not aware of any such claims before March of 2025.

 

[pines-demon]

I'm not aware of any such claims before March of 2025.

Check this article on how it has been hard to claim that toponium IS a thing: Don’t call it toponium

 

[ohwilleke]

Check this article on how it has been hard to claim that toponium IS a thing: Don’t call it toponium

The main discussion, after a lot of background for readers unfamiliar with the topic (the language in bold is the core substance of the matter, the language is italics is charming language):

While the CMS scientists were exploring the top quark’s secrets with a collider, members of the theoretical physics community were debating a key tenet of the top quark’s personality: whether it could momentarily bond with other top quarks.

When the LHC started up in 2008, theorist Fabio Maltoni read a paper about a theoretical top-anti-top bound state called toponium. “I remember thinking, toponium at the LHC is a fun thing, but it will never be seen,” says Maltoni, a professor at Università di Bologna and Université Catholique de Louvain.

As a PhD student, Maltoni studied the bound states of quarks with a focus on heavy quarks like the bottom and charm. Quarks are social particles and will always clump up and form hadrons like protons and neutrons (which are made from light quarks) or mesons that consist of a quark anti-quark pair. The one exception was the most unstable quark, the top quark.

“Even when I was doing my PhD, people were talking about toponium, but we all knew that the top quark decays so quickly that it cannot form a bound state,” he says.

Nonetheless, the idea wedged itself into Maltoni’s head, and as more people started to talk about toponium in both the experimental and theoretical physics communities, Maltoni became excited again about top quark bound states.

The theory was that if two top quarks were produced simultaneously and moved very slowly, they might see each other, exchange a gluon and start to orbit.

“It’s the Romeo and Juliet of particle physics,” Maltoni says. “As soon as they see each other by exchanging a gluon, one of them disappears.”

In January 2024, Maltoni and his colleagues proposed that if top quarks could dance with each other—even just for half a twirl—the LHC experiments might be able to detect it. It would appear as an excess of top quark pairs materializing at the lowest possible energy with very specific quantum spin states. This happened to be exactly the region Schwanenberger and Grohsjean had been studying since 2016.

“When we started the analysis, we did not think about toponium,” Schwanenberger says. “If you look at the textbooks, they say that there is no toponium.”

But as other analysis groups also started to see excesses in the same region as Schwanenberger and Grohsjean, the toponium debate gained momentum, and experimentalists and theorists came together to confer.

“I talked to one theorist who said he would never call it toponium,” Schwanenberger says. “But then his collaborator was putting all their results into a folder called ‘toponium.’ Even people within the same working group have different opinions.”

There is no question that Grohsjean, Schwanenberger and their colleagues on CMS have seen something unexpected.

“This enhancement looks like toponium, walks like toponium and quacks like toponium,” Maltoni says.

However, scientists still cannot rule out other possible explanations for the excess.

“Experimentally, the observed excess could be from a top quark quasi-bound state or from a new heavy Higgs boson,” Grohsjean says. “But the location of the excess is curious, since it is right where toponium should be. It would be really surprising to see a new particle in the exact same location and with exactly the same quantum properties.”

For Schwanenberger, while the semantics are still important, toponium by any other name would still be just as sweet.

“If I go to bed and start thinking about toponium, I cannot sleep,” Schwanenberger says. “It’s probably not a Nobel Prize winning discovery, but it’s still something exciting. As a scientist we want to do something that will survive our careers, even if it’s just a small update to the physics textbooks.”

A new heavy Higgs boson is very unlikely IMHO. This doesn't mean that it is truly a bound state, but if it is some sort of top physics, it gives the toponium characterization a boost.