University of Manchester physicists played a key role in the LHCb collaboration's discovery of a new heavy relative of the proton: a baryon containing two charm quarks and one down quark, known as the Xi-cc-plus. The particle was identified through its decay into a Lambda-c baryon, a kaon and a pion, in proton-proton collision data recorded during 2024, the first full year of operation of the upgraded LHCb detector at CERN's Large Hadron Collider.
A clear peak of about 915 events appeared at a mass of 3,619.97 MeV/c2, closely consistent with predictions based on the particle's previously discovered partner, the doubly charged Xi-cc, and incompatible with an earlier unconfirmed claim from another experiment. The result, presented at the Rencontres de Moriond Electroweak conference by a collaboration of more than 1,000 scientists across 20 countries, resolves a question that had remained open for more than two decades.
Manchester's contribution was tangible hardware: the university's groups built the silicon pixel detector modules at the core of LHCb's upgraded tracking system, the instrument that makes reconstructing such rare decays possible. Doubly charmed baryons are prized because they offer a clean laboratory for studying how quarks bind, and the new state gives theorists a second firm data point.