The proton is a basic building block of matter and consists of three light quarks – elementary particles that are held together by the strong nuclear force. Now physicists have discovered a “heavy brother” of the proton. Like the proton, this short-lived particle contains a down quark, but also two heavy charm quarks. This makes the particle called Ξcc⁺ (Xi‑cc‑plus) four times heavier than a normal proton. The existence of this particle has been predicted for a long time, but because it decays within fractions of a second, conclusive proof has so far failed. This has now been achieved for the first time with the help of the Large Hadron Collider (LHC) particle accelerator at the CERN research center near Geneva. The newly discovered particle can now help to test fundamental theories of particle physics.
Protons and neutrons consist of three quarks that are held together by the strong nuclear force and their carrier particles, the gluons. In the proton, two up quarks and one down quark are combined with each other and form the stable core building block. But there are four other types of quarks – charm and strange quarks and top and bottom quarks. In principle, these heavier quarks can also come together to form combinations of two or three – mesons or baryons. However, these particles are not stable and decay very quickly. They can therefore only be detected using decay patterns in particle accelerators.
Heavy baryon variants wanted
As early as the 1970s, physicists predicted that there would have to be baryons with two charm quarks and a lighter up or down quark – in a sense, the heavier brothers of protons and neutrons. Such particles are considered particularly exciting because its unequal quark combination offers new insights into the interactions of elementary particles. But it was only in 2017 that physicists at the CERN research center near Geneva managed to detect one of these particles – a positively charged particle with two charm quarks and an up quark and a mass of 3621 megaelectron volts. It was revealed by a significant excess of certain decay products in the LHCb detector of the LHC particle accelerator. The physicists in the LHCb collaboration have now also demonstrated the second variant of such a baryon with two charm quarks: the Ξcc⁺ – a particle made up of two charm quarks and a down quark.
The physicists discovered the new particle during the collision of protons accelerated to almost the speed of light in the LHC particle accelerator. Its detectors were expanded in 2023 and upgraded to higher sensitivity. For their analyses, the physicists evaluated the data from the first year of the accelerator’s third term, which started in 2024. They analyzed the frequency of certain decay products of the theoretically predicted particle in the LHCb detector. In fact, there was a clear “hump” in the curve of decay products – an excess of particles of a certain mass occurred in 915 events. From this, the researchers determined that the original, short-lived baryon must have a mass of around 3619.97 megaelectron volts. It is therefore around four times heavier than a proton. “This is consistent with expectations for the Ξcc⁺,” said the team.
Mass matches predictions
The physicists in the LHCb collaboration have finally proven the long-sought “heavy brother” of the proton. At seven sigma, the statistical significance of their detection is well above the threshold of five sigma, which is the criterion for a discovery in particle physics. “This is only the second baryon with two heavy quarks that has been detected – and the last one was almost ten years ago,” says LHCb spokesman Vincenzo Vagnoni. The new particle only differs from its predecessor by having a down quark instead of an up quark and therefore has a very similar mass. Despite this similarity, its lifespan is much shorter. Because of the slightly different quantum interactions, the heavy proton variant decays after one sixth of the time compared to the heavy neutron analogue.
The newly discovered heavy baryon can now help test fundamental predictions of the Standard Model of particle physics. “The new result will help theorists test models of quantum chromodynamics – the theory that explains the strong nuclear force,” says Vagnoni. This fundamental force links quarks to the stable basic building blocks of matter, but also to more exotic particles, for example the particles made up of four, five or six quarks, which have only been discovered in recent years. By studying how different quarks interact with each other and with the strong nuclear force, the structure of such particles can be better understood.
Source: LHCb Collaboration, CERN, Conference Rencontres de Moriond, 2026