While searching for new Higgs bosons the CMS experiment at the Large Hadron Collider (LHC) may have just found a surprise. They have observed an excess of events that look to be a new particle, and are reporting high statistical evidence for their claim. The only question is what exactly is this new particle?

The search was initially designed to look for new, heavier, versions of the Higgs boson decaying to a top quark and an anti-top quark. Its well known that the Higgs boson of the Standard Model, discovered jointly by ATLAS and CMS in 2012, underlies the mechanism which gives all fundamental particles their masses. The Higgs boson itself interacts with particles in proportion to their mass, preferring heavier particles over lighter ones. It therefore interacts the most strongly with the heaviest known fundamental particle, the top quark, which has a mass of ~173 GeV. The Higgs boson itself only has a bass of 125 GeV, meaning conservation of energy dictates it can’t decay into a top quark-antiquark pair.

However many theories of physics beyond the the Standard Model predict additional Higgs bosons, heavier cousins of the current one. If these new heavy Higgs bosons had a mass larger than 350 GeV, they would likely decay to a top quark-antiquark pair quite often. CMS therefore was analyzed its data searching for this signature, hoping to find signs of a new Higgs boson. To do so, they had scrutinize very carefully the known production of top quark-antiquark pairs, which are produced copiously at the LHC from other processes. If a new particle was being produced and decaying to top quarks, the mass of the new particle would give the top quarks a characteristic energy. One key sign of a new particle would therefore be an excess of top quark-antiquark events at a particular energy, corresponding to the mass of the new particle. 

When CMS scrutinized their data looking for such an excess they found one. But curiously right ~350 GeV, the minimum energy required to produce the top quark-antiquark pair. It would be quite the coincidence for a new particle to show up right at this minimum threshold, which made CMS consider alternative possibilities.

 

 One unorthodox explanation that seems to fit the bill is ‘toponium’, a short lived bound state of the top quark-antiquark pair is being formed. Toponium would be the heaviest version of ‘quarkonia’ we have seen, bound states of quark antiquark pairs that form bound states similar to atoms. We have observed and measured quarkonia states of the other quarks for decades, however it was long thought that the top quark, whose large mass causes it to decay in just 10^(-25) seconds, would decay too quickly to create observable bound state effects at a hadron collider. Toponium production would happen most often if the top quarks were produced just at the energy threshold, such that they don’t any extra energy. These low energy top quarks would spend more time close to each other than normal, rather than immediately flying away, so they could have time to briefly form a toponium state before decaying. However, once small hints of intriguing excesses started appearing in LHC analyses, updated calculations in the last few years suggested that perhaps such an effect could be observable.

These calculations are approximate, and more work is still being done to refine them. But the preliminary predictions they give for the properties of toponium seem to match well with what CMS is seeing, both in terms of the rate of toponium production and the quantum properties of the toponium state (spin and parity).

Still CMS is being cautious before claiming a discovery of toponium. They claim observation of an ‘excess at the top quark pair production threshold’ which is consistent with toponium. However given the limited present data and incomplete theoretical models of toponium, they cannot rule out that the excess they are seeing is coming from a new Higgs-like particle.

Further work will be needed to develop improved theoretical models of toponium, and detailed studies from CMS assessing the properties of their observed excess. The excess will also need confirmation from CMS’s rival LHC experiment, ATLAS, to ensure it has not merely made a mistake in its analysis.

However, the smart money would say this very likely looks like toponium. Which, while not signaling the long sought overthrow of the standard model, would be an unexpected and cool surprise from the LHC. Understanding the properties of this previously-thought-impossible quasiparticle will spawn much fruitful research in the years to come. Physicists love a surprise!

Paper:

“Observation of a pseudoscalar excess at the top quark pair production threshold” https://arxiv.org/abs/2503.22382

Additional CMS Paper considering Heavy-Higgs interpretation “Search for heavy pseudoscalar and scalar bosons decaying to top quark pairs in proton-proton collisions”

Read more

CERN Courier “CMS observes top–antitop excess“

Symmetry Magazine “Don’t call it toponium“

Discloure: The author is a member of the CMS collaboration but did not directly work on this analysis

Erratum 4/15/2025 : The article was updated to clarify that in the theory literature prior to the LHC toponium was thought possible to form, just that it was thought to be too small an effect to be observable. The article previously incorrectly stated it had been previously thought impossible to form