The LHCb experiment at CERN has today announced the discovery of a new baryon: the quark content of the Ξcc++ is ccu — in other words it contains two charm quarks and one up quark.
The commonest baryons contain combinations of u (up) and d (down) quarks. The proton and the neutron, for example, are uud and udd respectively. Baryons containing s (strange) quarks have long been known: the Ω–, which was discovered in 1964, has a quark content of sss. But up until now the heavier quarks (charm (c), bottom (b), top (t)) have only ever appeared singly in baryons. The Ξcc++ baryon contains two heavy quarks.
It’s important to note that the LHCb experiment hasn’t discovered a new fundamental particle. The Ξcc++ baryon is a permissible collection of bound quarks. But it is the first time that anyone has seen a baryon containing two heavy quarks. The Ξcc++ should allow physicists to explore the theories behind the Standard Model in ever more detail.