CERN Discovers An Extremely Rare Particle… Four Times Heavier Than A Proton

If you follow scientific news at all, you have inevitably already heard of CERN (European Organization for Nuclear Research), this giant laboratory buried under the Franco-Swiss border, which houses the legendary LHC (Large Hadron Collider), a 27-kilometer underground ring. Inside, protons (the particles that make up the nucleus of atoms) are propelled at a speed close to that of lightthen violently collided with each other. Thanks to the LHC, physicists are trying, among other things, to reproduce the extreme conditions that reigned in the Universe a fraction of a second after the Big Bang.

It was thanks to him, in 2012, that we confirmed the existence of the Higgs boson, a historic discovery that revolutionized our understanding of the physical world. And it is again thanks to him that in 2025, an international team of more than 1,000 researchers spread across 20 countries (led by the University of Manchester) has just announced the discovery of a particle that no one had ever managed to confirm: the Ξcc⁺ (pronounce “ Xi-cc-plus “). A cousin of the proton, but four times heavier: in the infinitely small, it is a chasm and above all it is a particle that we have been looking for for more than 20 years.

What is this strange particle?

To understand what the discovery of Ξcc⁺ represents, it is necessary to return to the basics of physics: all matter is made up of molecules (for example water, H₂O), which are agglomerates of atoms (in the case of water, one oxygen atom and two hydrogen atoms). These atoms all contain a nucleus, surrounded by electrons, itself composed of protons and neutrons. And the protons? Well, they too are still divisible, into even smaller particles, quarks (whose size has never been measured, but which is known to be less than 10^-19 meters).

Three quarks, precisely, whose combination entirely defines the nature of the particle. In the case of the proton: two quarks up and a quark down.

Except that in reality, there are more than two types of quarks; iThere are, according to our current knowledge, six. All baptized with rather eccentric nicknames: we have, in all, quarks uples quarks down, les quarks strange, les quarks charm, les quarks bottom et les quarks top. Their nicknames come from work in the 1960s and 70s, where researchers adopted pictorial names to popularize their discoveries.

Between each of these quarks, the mass gaps are gigantic. Let’s take a quark charmFor example : it weighs about 500 times heavier than a quark up. The heaviest quarks give rise to particles that are generally very unstable and whose lifetime is extremely short.

Once this hierarchy is established, we can return to Ξcc⁺. It is made up of two quarks charm and a quark down, a combination that had never been formally confirmed before its observation in the LHC tunnel. Its structure is that of an ordinary proton, but in which the two quarks up were replaced by two quarks charmwith a much greater mass.

This substitution has a consequence on its mass. To measure it, we use MeV/c² (megaelectronvolt divided by the speed of light squared). The electronvolt is the energy acquired by an electron accelerated under a potential difference of one volt, the prefix “ mega » means that we are talking about it in millions. The c² comes directly from E=mc², Einstein’s equation which establishes that mass and energy are equivalent and convertible into each other. In particle physics, it is impossible to measure such tiny masses with ordinary units, so we measure them in energy.

Thus, a proton weighs approximately 938 MeV/c² when the Ξcc⁺ reaches 3,620 MeV/c², or almost four times more: it is considerable on this scale. With such a high mass, it is extremely unstable: it can only exist for an infinitesimal moment before disintegrating into three lighter particles.

What the LHCb physicists have therefore captured are not the Ξcc⁺ itself, but these three particles resulting from its decay. The LHC detector functions like a very fast camera, capable of taking 40 million images per second: it records the trajectories and properties of all particles produced during collisions.

By analyzing this data, physicists can go back to the parent particle and reconstruct its characteristics. From the corpus of all collisions between protons recorded in 2024, 915 of these decay events have been identified, all pointing towards the same mass: 3,620 MeV/c². A result consistent with theoretical predictions, and consistent with the properties of its sister particle, Ξcc⁺⁺, discovered in 2017.

Why is it important?

Researchers had in fact believed they were observing the Ξcc⁺ at the beginning of the 2000s, but their results had never survived the test of reproducibilitya fundamental requirement that a discovery be accepted only if other teams, with other instruments, reach the same conclusions. Their measurements also did not correspond to theoretical predictions, which left the question of its existence unresolved for twenty years. Today we have the answer, unequivocally.

It is therefore a discovery of primary importance, because when we confirm the existence of a particle predicted by the theory, we at the same time confirm that our standard model of the universe is correct. At least, he is a little more than before, despite his imperfections and his gray areas.

Now we’ll have to dig some more, because confirmation of the existence of Ξcc⁺ is a new beginning which will take us into full terra incognita ; we actually know very little about two-quark particles charm. Studying them is a rare opportunity to test thestrong nuclear interaction, the most powerful of the four fundamental forces of the universe (with electromagnetic interaction, weak nuclear interaction and gravitational interaction) which holds quarks bound together inside protons and neutrons. Without her, you wouldn’t be here to read this articleand the universe as we know it would simply never have appeared. Difficult, in these conditions, to not finding the company a bit dizzyingalthough you may not talk about the Ξcc⁺ at your next family meal or when you meet your neighbor on the stairs.