Possible Discovery of Toponium Could Reveal the Smallest Known Composite Particle

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The CMS collaboration at CERN has identified an unexpected feature in data from the Large Hadron Collider (LHC) that may signal the existence of a never-before-seen particle known as toponium—a fleeting, composite state made from a top quark and its antimatter counterpart. This finding, unveiled at the Rencontres de Moriond conference in the Italian Alps, suggests that top quarks—the heaviest and shortest-lived of the known elementary particles—might briefly bind with their antimatter partners to form the smallest composite particle ever observed.

While the evidence is compelling, researchers caution that other explanations remain possible, and further analysis by the ATLAS experiment is needed to confirm the result. The formation of toponium has long been considered extremely difficult to detect in high-energy collisions, making this potential observation particularly noteworthy.

Top quark–antiquark pairs (tt-bar) are regularly produced in high-energy proton collisions at the LHC. Measuring the rate of these events, known as the cross section, provides a crucial test of the Standard Model of particle physics and a gateway to discovering particles beyond the current theory. Many hypothetical particles—including new types of Higgs bosons—would most strongly interact with top quarks and could decay into tt-bar pairs.

Between 2016 and 2018, CMS researchers were analyzing data from tt-bar events while searching for such exotic Higgs-like particles. Unexpectedly, they noticed an excess of top–antitop pairs appearing precisely at the minimum energy needed to produce them. This curious signature hinted at a different explanation: a short-lived bound state of a top quark and top antiquark—toponium.

Though tt-bar pairs don’t usually form stable bound states, quantum chromodynamics—the theory governing the strong nuclear force—predicts that temporary enhancements in production can occur near the energy threshold where the pairs form. This behavior, observed in other heavy quark combinations, led the CMS team to model the signal as possible toponium production.

Their analysis yielded a cross section of 8.8 picobarns with a 15% margin of uncertainty—enough to surpass the five-sigma threshold required to claim a discovery in particle physics, meaning it is extremely unlikely to be a statistical fluke.

If confirmed, toponium would complete the set of known quark–antiquark bound states, known collectively as quarkonium. Earlier discoveries in this family include:

Charmonium (charm–anticharm), discovered in 1974 during the "November Revolution" at SLAC and Brookhaven National Laboratory.

Bottomonium (bottom–antibottom), discovered in 1977 at Fermilab.

These bound states are extremely small—charmonium measures about 0.6 femtometers, bottomonium about 0.4 femtometers (a femtometer is one quadrillionth of a meter). Given the top quark’s much larger mass, toponium would be even smaller—making it the smallest composite hadron ever detected.

Unlike other quarkonia, toponium would decay not through mutual annihilation, but because the top quark itself decays too quickly—within a distance of about 0.1 femtometers, far shorter than the size of the particle it forms. This makes toponium uniquely transient and challenging to detect in proton–proton collisions.

Still, this glimpse of toponium—if upheld by further studies—would not only mark a milestone in the study of matter but could help deepen our understanding of the strong force and the behavior of quarks at the highest energies.