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Quarks are the fundamental building blocks from which matter is constructed.

They combine to form hadrons, namely baryons, such as the proton and the neutron, which consist of three quarks, and mesons, which are formed as quark-antiquark pairs.

Several tetraquark particles have been discovered in recent years, but Tcc+ is the first one that contains two charm quarks, without charm antiquarks to balance them. Physicists call this ‘open charm’ (in this case, ‘double open charm’).

Particles containing a charm quark and a charm antiquark have ‘hidden charm’ — the charm quantum number for the whole particle adds up to zero, just like a positive and a negative electrical charge would do. Here the charm quantum number adds up to two, so it has twice the charm.