What particle, containing two charm quarks and one down quark, was reported discovered by CERN's LHCb experiment in March 2026?

The particle reported discovered by CERN's LHCb experiment in March 2026, composed of two charm quarks and one down quark, is indeed the Ξcc⁺ (Xi-cc-plus). This designation correctly identifies a baryon, which is a type of hadron made of three quarks. In this specific case, the "cc" subscript indicates the presence of two charm quarks, while the absence of a second subscript implies the third quark is a down quark. The superscript "⁺" denotes its positive electric charge, stemming from the combination of the two charm quarks and one down quark.

This discovery is significant as the Ξcc⁺ is a heavy cousin of the everyday proton. While a proton is made of two up quarks and one down quark, the Ξcc⁺ replaces the two lighter up quarks with two much heavier charm quarks, making it roughly four times as massive as a proton. The observation of this particle, with its specific quark composition, marks the first new particle identified using the upgraded LHCb detector and provides a crucial testing ground for quantum chromodynamics (QCD), the theory that describes the strong force binding quarks together.

The existence of such a "doubly charmed" baryon was predicted by the Standard Model of particle physics, and a similar particle, the Ξcc⁺⁺ (containing two charm quarks and one up quark), was previously discovered by LHCb in 2017. However, the Ξcc⁺ itself remained elusive for decades, with earlier hints from the SELEX experiment in 2002 suggesting a much lighter particle that was never confirmed. The recent detection, based on proton-proton collision data collected in 2024, finally resolves this long-standing mystery and confirms theoretical predictions regarding its mass and properties. By studying particles like the Ξcc⁺, physicists gain deeper insights into the fundamental forces that govern the universe and how matter is assembled at its most basic level.