CMS Strengthens Case for Toponium Discovery

Experiments at the Large Hadron Collider provide new evidence for the existence of the most massive bound state ever observed.

Mar. 26, 2026 at 12:18am

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Researchers from the CMS Collaboration at CERN have presented new results that strengthen the case for the existence of toponium, a fleeting bound state of the top quark and its antimatter counterpart. This would be the most massive composite particle ever observed, completing the family of quark-antiquark states bound by the strong nuclear force.

Why it matters

The discovery of toponium would deepen our understanding of the strong nuclear force and its ability to bind the fundamental constituents of matter. The top quark is the heaviest and most short-lived elementary particle known, so the ability to form a bound state with its antimatter counterpart challenges previous assumptions.

The details

The CMS team used new AI-assisted techniques to isolate and analyze collision events where one top quark decays into a bottom quark, a charged lepton, and a neutrino, while the other decays into jets of particles. By focusing on the relative velocity of the top quark and antiquark, they were able to observe an excess with a statistical significance of over five standard deviations, providing independent confirmation of toponium production.

  • The new CMS results were presented this week at the Rencontres de Moriond conference.
  • The first hints of toponium appeared in searches for heavy Higgs-boson-like particles at the LHC last year.

The players

CMS Collaboration

An international team of particle physicists conducting experiments at the Large Hadron Collider at CERN.

Otto Hindrichs

A researcher at the University of Rochester who developed a new AI-assisted technique to reconstruct the collision events.

Yu-Heng Yu

A graduate student involved in the CMS analysis.

Regina Demina

The leader of the CMS group at the University of Rochester.

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What they’re saying

“Isolating the signal in this decay channel was challenging.”

— Otto Hindrichs, Researcher, University of Rochester

“If they form a bound state, their relative velocity should be much smaller than when they are produced independently.”

— Yu-Heng Yu, Graduate student

“Toponium is heavier than the heaviest known atomic nucleus, oganesson, making it the most massive bound state ever observed. Its discovery deepens our understanding of the strong nuclear force and its ability to bind the fundamental constituents of matter.”

— Regina Demina, Leader of the CMS group, University of Rochester

What’s next

The CMS and ATLAS experiments plan to continue studying the properties of toponium to further confirm its existence and understand its implications for particle physics.

The takeaway

The potential discovery of toponium, the most massive bound state ever observed, represents a significant advancement in our understanding of the strong nuclear force and the fundamental building blocks of the universe.