CERN Confirms Toponium and Discovers New Doubly Charmed Baryon in a Landmark Week for Particle Physics
Back-to-back results at Moriond 2026 confirm the most massive bound state ever observed and resolve a two-decade search for the doubly charmed baryon.
Overview
The Rencontres de Moriond conference in March 2026 delivered two major results from CERN’s Large Hadron Collider: an independent confirmation of toponium by the CMS experiment and the discovery of a new doubly charmed baryon by LHCb. Together, the announcements complete the catalogue of quark-antiquark bound states and expand the roster of known baryonic particles, giving theorists new tools to test the strong nuclear force at its extremes.
CMS Confirms Toponium With Five-Sigma Significance
The CMS Collaboration has confirmed the existence of toponium — a fleeting bound state formed when a top quark pairs with its antimatter counterpart — with a statistical significance exceeding five standard deviations, the gold standard for discovery in particle physics, according to CERN.
Toponium is the most massive composite particle ever observed, heavier than oganesson, the heaviest known atomic nucleus. Its existence completes the family of quark-antiquark bound states, known as quarkonia, across all six quark flavors.
The top quark is the heaviest and most short-lived elementary particle known. It decays so quickly that physicists long questioned whether it could form bound states at all. The new CMS result provides independent confirmation of an initial observation made last year through a different decay channel, as reported by Phys.org.
Rather than reconstructing the top quark-antiquark pair mass directly, the CMS team focused on the relative velocity between the particles. As graduate student Yu-Heng Yu explained, “If they form a bound state, their relative velocity should be much smaller than when they are produced independently,” according to CERN.
The analysis examined collision events in which one top quark decayed into a bottom quark, a charged lepton, and a neutrino, while the other decayed into quarks producing particle jets. Otto Hindrichs at the University of Rochester developed an AI-assisted technique to reconstruct these collision events, isolating the toponium signal from vast quantities of collision data, as reported by Phys.org.
Regina Demina, who leads the CMS group at Rochester, said the discovery “deepens our understanding of the strong nuclear force and its ability to bind the fundamental constituents of matter,” according to CERN.
LHCb Discovers the Doubly Charmed Baryon
In a separate result presented at Moriond on March 17, the LHCb Collaboration announced the observation of a new subatomic particle designated Xi-cc-plus, composed of two charm quarks and one down quark, according to CERN.
The particle has a structure analogous to the proton, but with two heavy charm quarks replacing the proton’s two up quarks, resulting in approximately four times the proton’s mass. Researchers measured its mass at 3,619.97 MeV/c-squared, detecting a clear signal of roughly 915 events with a statistical significance of seven sigma, well above the five-sigma threshold required for discovery, according to ScienceDaily.
The particle was identified through its decay into three lighter particles and has an extremely short lifetime of roughly 45 femtoseconds — about six times shorter than the related Xi-cc-plus-plus particle discovered by LHCb in 2017, as reported by ScienceDaily.
The discovery resolves a question that had remained open for more than two decades. An earlier unconfirmed claim of observation placed the particle at a mass incompatible with the new measurement, which instead aligns with theoretical predictions based on its partner particle, according to Phys.org.
The result marks the first particle discovery made using the upgraded LHCb detector, which images particles produced at the LHC and captures data at a rate of 40 million frames per second. The observation was made using proton-proton collision data collected in 2024 during LHC Run 3, according to Phys.org.
LHCb Spokesperson Vincenzo Vagnoni said the result “will help theorists test models of quantum chromodynamics, the theory of the strong force that binds quarks into not only conventional baryons and mesons but also more exotic hadrons such as tetraquarks and pentaquarks,” according to CERN.
The discovery brings the total number of hadrons found by LHC experiments to 80.
What We Don’t Know
While toponium has now been independently confirmed, detailed measurements of its properties — including precise mass and decay width — remain limited by available statistics. Larger datasets from future LHC runs will be needed to characterize the state fully.
For the doubly charmed baryon, the mechanism governing its extremely short lifetime relative to its partner particle is not yet well understood theoretically. Further measurements of its production rate and other decay channels will be needed to constrain QCD models.
CERN’s ATLAS experiment, which presented 45 new analyses at Moriond 2026, did not report any new particle discoveries but continued to set increasingly stringent limits on physics beyond the Standard Model.
Analysis
The back-to-back announcements underscore the scientific output of CERN’s upgraded detectors and the continued productivity of the LHC in its third operational run. The toponium confirmation validates a long-standing theoretical prediction and demonstrates that even the heaviest and most transient quarks can form bound states, a finding that constrains models of the strong force at extreme energy scales.
The doubly charmed baryon discovery, meanwhile, opens a new window onto how heavy quarks bind within baryonic matter. As researcher Ao Xu at INFN noted, the upgraded detector’s sensitivity means the collaboration is “opening a new window onto a very unusual form of matter,” according to ScienceDaily.
This follows prior coverage by The Machine Herald on CMS results revealing quark wakes in primordial plasma, continuing a productive stretch for CERN’s experimental program.