CERN’s LHCb Spots New ‘Heavy Proton,’ Settling a 20-Year Particle Dispute

CERN’s Large Hadron Collider has produced a new kind of “heavy proton,” and in the process helped settle a 20-year dispute in particle physics.

A new doubly charmed baryon

Physicists with the LHCb experiment announced in mid-March the first confirmed observation of a particle called Ξcc⁺ (Xi sub cc plus). The short-lived baryon—made of two charm quarks and one down quark—is about four times heavier than the proton and is only the second known particle to contain two charm quarks.

“This is the first new particle identified after the upgrades to the LHCb detector that were completed in 2023, and only the second time a baryon with two heavy quarks has been observed,” LHCb spokesperson Vincenzo Vagnoni said in a statement.

The discovery, based on high-energy proton collisions recorded in 2024 and unveiled March 17 at the Rencontres de Moriond conference in Italy, carries a statistical significance greater than seven standard deviations—well above the threshold physicists use to claim a discovery. It completes a long-predicted pair of doubly charmed particles and contradicts an earlier, controversial claim that the same state existed at a much lower mass.

A “quark-upgraded” proton

The new baryon is a close cousin of the proton, the positively charged particle in every atomic nucleus. Like the proton, Ξcc⁺ is a baryon: a composite made of three quarks held together by the strong nuclear force. But where the proton contains two up quarks and one down quark, this new particle replaces the two light up quarks with two much heavier charm quarks.

LHCb scientists describe it as a “quark-upgrade” of the proton: same basic blueprint—three quarks, charge plus one—but with heavy components that make it roughly four times as massive and far more unstable. Theory suggests its lifetime is on the order of tens of femtoseconds, though that lifetime has not yet been measured.

How LHCb saw it

Researchers spotted the particle in data from Run 3 of the Large Hadron Collider, when beams of protons collided at a record energy of 13.6 trillion electronvolts. The upgraded LHCb detector collected about 6.9 inverse femtobarns of data in 2024, a large sample by the standards of heavy-flavor physics.

Within that dataset, the team reconstructed about 900 candidates for the new particle using a specific decay chain. In the signal that clinched the case, a Ξcc⁺ decays into a lighter charmed baryon called Λc⁺ (Lambda-c plus), a negative kaon and a positive pion. The Λc⁺ then decays again into a proton, another kaon and another pion, leaving five charged tracks curving through the detector.

Those tracks originate from a point displaced from the original collision, a pattern that LHCb’s high-precision vertex detector is designed to catch. Software combines kinematic variables, particle-identification data and vertex quality into a multivariate classifier that can tease rare signals from a large background of ordinary collisions.

“The particle is very rare to produce even in the most powerful accelerators like LHC,” said Giovanni Punzi, a professor at the University of Pisa and a leading figure in the Italian contribution to LHCb. Observing it at all, he said, was possible only because of “the special characteristics of the renewed LHCb detector,” which give collaborators “strong optimism for future results.”

A first discovery after LHCb’s upgrade

For LHCb, the finding is also a proof of concept for its recent overhaul. During a long shutdown that ended in 2023, engineers replaced most of the detector’s tracking and readout systems and scrapped the old hardware trigger in favor of a fully software-based system that can analyze every collision in real time.

“This major result is a fantastic example of how LHCb’s unique capabilities play a vital role in the success of the LHC,” CERN Director-General Mark Thomson said. The discovery, he added, “highlights how experimental upgrades at CERN directly lead to new discoveries, setting the stage for the transformative science we expect from the High-Luminosity LHC.”

What its mass says about the strong force

The particle’s measured mass, around 3,620 mega-electronvolts, places it in almost the same spot as Ξcc⁺⁺, a doubly charmed baryon with quark content ccu that LHCb discovered in 2017. The two states form an isospin doublet, differing only by whether the lone light quark is an up or a down, in close analogy to the proton-neutron pair.

The tiny difference in their masses matches long-standing predictions from quantum chromodynamics (QCD), the theory that describes how quarks interact via the strong force. In this picture, the charm-charm pair inside the baryon behaves somewhat like a compact diquark acting as a heavy color source, with the lighter quark orbiting it. Having both members of the doublet established turns this system into a precise testing ground for QCD calculations, including supercomputer-based lattice methods.

“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,” Vagnoni said.

A 2002 claim fades further

The new measurement also addresses a long-running experimental puzzle. In 2002, the SELEX experiment at Fermilab reported evidence for a Ξcc⁺ state decaying to the same Λc⁺ K⁻ π⁺ final state, but at a significantly lower mass—about 3,519 MeV. That claim was never confirmed by other experiments, including BaBar, Belle and earlier runs at LHCb, and its reported properties were difficult to reconcile with most theoretical models.

In its new analysis, LHCb not only finds a clear signal at the higher mass consistent with modern theory and with Ξcc⁺⁺, it also sees no corresponding peak at the SELEX mass. That absence, combined with the strength of the new signal, has led many physicists to conclude that the decades-old claim was likely a statistical fluctuation or an unrecognized background, though the collaboration has avoided making judgments about the earlier experiment.

What comes next

Beyond confirming the particle’s existence and mass, LHCb has not yet published a direct measurement of the Ξcc⁺ lifetime. The unknown lifetime is one factor in the current uncertainty on its mass, and scientists say more data will be needed to pin it down.

Theory predicts a distinctive pattern in the lifetimes of the doubly charmed family: Ξcc⁺ should be the shortest-lived, Ξcc⁺⁺ the longest, and another state, Ωcc⁺, in between. Verifying that pattern would test subtle QCD effects that influence how charm quarks decay within a tightly bound three-quark system.

LHCb collaborators are already looking ahead. With more Run 3 and future Run 4 data, they plan to refine the mass, measure the lifetime and search for additional decay modes of Ξcc⁺. The upgraded detector and planned further improvements, grouped under a proposed Upgrade II, are aimed at handling even higher collision rates and opening searches for rarer doubly heavy and possibly triply heavy baryons.

For Ao Xu, a researcher at Italy’s National Institute for Nuclear Physics and the Scuola Normale Superiore in Pisa who works on LHCb’s real-time selection system, the observation marks a turning point.

The result, Xu said, is “opening a new window onto a very unusual form of matter,” and with the upgraded detector “this truly marks the beginning of a new era for these studies.”

In the fleeting traces of a particle that lives for only a few quadrillionths of a second, physicists see an enduring opportunity: to probe the most powerful force in nature with unprecedented precision, and to show that even mature machines like the LHC still have new pieces of the subatomic puzzle to reveal.