They found a particle four times more massive than the proton — and it almost can’t be detected

The Large Hadron Collider (LHC) at CERN has just added another landmark discovery to its already impressive record. The LHCb collaboration announced on March 17, 2026, during the ongoing Moriond conference, the observation of a new subatomic particle with a structure similar to the proton — but built from far heavier quarks, making it four times more massive than the ordinary proton. The discovery, confirmed with a statistical significance of 7 sigma — well above the 5-sigma threshold required to claim a discovery — opens a new testing ground for quantum chromodynamics, the theory that describes the strong force.

What exactly was discovered?

The new particle is a baryon, meaning a particle made of three quarks. Its composition is two charm quarks and one down quark. This structure makes it directly comparable to the proton, which also contains three quarks — but in the proton’s case, two are up quarks and one is a down quark.

The critical difference is that charm quarks are considerably more massive than up quarks. By replacing two up quarks with two charm quarks, the new particle accumulates a mass roughly four times greater than that of the proton. The particle belongs to the family of doubly charmed baryons, an extremely rare and difficult-to-observe category.

A very rare family of particles

This is not the first discovery of its kind, but it is only the second in the history of particle physics. In 2017, the same LHCb experiment reported the existence of a particle containing two charm quarks and one up quark, known as Ξcc++ (Xi-cc++). The new particle differs from that one in a single respect: the third quark is down instead of up.

That seemingly minor difference carries deep physical consequences. Due to complex quantum effects, the new particle has a predicted lifetime up to six times shorter than its 2017 counterpart, making its detection significantly more challenging.

How the detection works

Quarks are confined inside hadrons and cannot be observed in isolation. To study unstable particles like this one, physicists accelerate protons in the LHC to speeds approaching that of light and smash them together. The collisions produce a cascade of particles, including exotic hadrons that decay almost instantly.

Although the particle itself vanishes in a fraction of a second, the products of its decay leave detectable traces in the LHCb detector. By analyzing those signatures, researchers can reconstruct the properties of the original baryon, including its mass, charge, and lifetime.

The LHCb collaboration used data from proton-proton collisions recorded during the third run of the LHC to carry out this analysis.

First particle identified after the detector upgrade

This result also carries institutional significance: it is the first new particle discovered following the LHCb detector upgrades completed in 2023, a modernization process that substantially increased its data collection capacity and resolution.

With this finding, the total number of hadrons discovered by LHC experiments now stands at 80.

Significance for theoretical physics

LHCb Spokesperson Vincenzo Vagnoni noted that the result will allow theorists to test models of quantum chromodynamics (QCD) — the theory that describes how the strong force binds quarks inside hadrons — in both conventional baryons and mesons, as well as more exotic structures such as tetraquarks and pentaquarks.

CERN Director-General Mark Thomson emphasized that the discovery is a direct example of how technological improvements in experiments translate into new science, and that it sets the stage for the results expected from the High-Luminosity LHC, currently under development.

Why this discovery matters

The proton is the most studied stable particle in the history of physics. Yet the exact nature of the force that holds its internal quarks together — the strong force, mediated by gluons — remains one of the most mathematically difficult problems in all of physics.

Doubly charmed baryons are ideal tools for testing that theory in a different regime than usual, because the two heavy charm quarks create an internal system that is slower and more calculable than that of the ordinary proton. The more varied the family of hadrons physicists can study, the more precise and robust our understanding of the strong force becomes.

The newly discovered baryon is, in that sense, far more than an addition to a catalog. It is a new precision instrument for understanding how matter works at its deepest levels.

Source: CERN – LHCb Collaboration discovers new proton-like particle (March 17, 2026). https://home.cern/news/news/physics/lhcb-collaboration-discovers-new-proton-particle

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