CERN: Upgraded LHCb detector discovers heavy proton counterpart

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Particle physics has cataloged a new building block in the zoo of subatomic matter: At the Rencontres de Moriond Electroweak conference, the LHCb collaboration at CERN announced the discovery of the Ξ_cc⁺ (pronounced Xi-cc-plus).

This is a proton-like particle that contains two heavy charm quarks instead of two light up quarks – and thus weighs about four times as much as a normal proton. It is the first new discovery with the LHCb detector upgraded in 2023, which is part of the world's largest particle accelerator.

Most of the matter in the universe ultimately consists of quarks, fundamental building blocks that cannot be further decomposed. The Standard Model of particle physics knows six types of quarks, organized into three generations. The first generation includes down (d) and up quarks (u), from which protons (two up, one down, i.e., uud) and neutrons (udd) are built. For a proton with a single positive electric charge and a neutral neutron to form, the quarks must each carry fractional elementary charges: the down quark -1/3 e (where e is the elementary charge of the electron) and the up quark +2/3 e. The same applies to the quarks of the further generations. The second generation contains strange (s) and charm quarks (c), the third bottom (b) and top quarks (t). The mass increases significantly from generation to generation – a charm quark is the significantly heavier relative of the up quark.

Quarks do not occur individually but always in bound states, the so-called hadrons. The strong nuclear force, mediated by gluons, holds the quarks together. Protons, neutrons, and the newly discovered Ξ_cc⁺ belong to the hadron subgroup of baryons, which consist of three quarks each; mesons, on the other hand, consist of a quark and an antiquark.

Two charm quarks instead of two up quarks

According to the researchers, the newly discovered Ξ_cc⁺ can be understood as a proton with a dramatic quark swap: in its ccd composition, two heavy charm quarks replace the two light up quarks of the proton (uud), while the down quark remains. Since charm quarks bring significantly more mass than their light relatives, the Ξ_cc⁺ reaches a measured mass of 3619.97 ± 0.83 MeV/c² – about four times that of a normal proton.

Physicists detected the short-lived particle via its characteristic decay: Ξ_cc⁺ ⇀ Λ_c⁺ K⁻ π⁺, where the Λ_c⁺ in turn decays into a proton, a K⁻, and a π⁺. By reconstructing the so-called invariant mass of these decay products from proton-proton collision data from 2024 (LHC Run 3, where highly complex nuclear transformations such as the generation of gold nuclei from lead are also observed), the researchers observed a clear peak at around 3620 MeV/c² with approximately 915 events. The statistical significance is 7 sigma, far above the 5-sigma threshold commonly used in particle physics, above which a discovery is considered confirmed.

Solution to a 20-year-old riddle

The Ξ_cc⁺ is the so-called isospin partner of the Ξcc⁺⁺ discovered by LHCb in 2017 with the composition ccu. Since up and down quarks behave almost identically in their properties, theory predicts nearly identical masses for both particles. The new measurement confirms exactly that – and thus resolves a controversy that has lasted for over 20 years.

The SELEX experiment reported a signal in the early 2000s that indicated a significantly lighter Ξ_cc⁺. However, subsequent searches at the FOCUS, BaBar, and Belle experiments, as well as in earlier LHCb datasets, were unable to reproduce this result. Only the improved performance of the upgraded LHCb detector has now made it possible to detect the particle at a mass that aligns with theoretical expectations and its Ξ_cc⁺⁺ partner.

Significance for the Standard Model

The discovery is not only a milestone for the LHCb experiment as its first particle finding after the detector upgrade, but also an important test for quantum chromodynamics (QCD) – the theory that describes how quarks and gluons interact via the strong interaction. The precise agreement of the measured mass with the predictions validates the theoretical understanding of hadrons with multiple heavy quarks.

Furthermore, the detection demonstrates that the modernized detector is sensitive enough to detect even rarer exotic particle states such as pentaquarks or tetraquarks. For the search for even rarer phenomena, CERN is already planning the construction of a significantly larger successor accelerator.

The collaboration presented detailed results in their Moriond talk; a full scientific publication is expected shortly. Further information is provided by a CERN press release.

(vza)