The LHCb Collaboration has released new results which can be interpreted as the discovery of a new type of tetraquark particle, made from two charm quarks and two charm antiquarks. This is the first time an exotic hadron has been found made exclusively of heavy quarks of the same flavour.
The tetraquark candidate was found by its decay into two J/ψ mesons, each of which is made from a charm quark and charm antiquark. The plot shows a narrow structure peaking near 6900 MeV/c² in the invariant mass of the pair of J/ψ mesons which is interpreted as the new particle.
The decay of the new particle into the J/ψ pair suggests a minimum quark content of two charm quarks and two anticharm quarks, consistent with a tetraquark interpretation.

This news follows the discovery of the first two pentaquark particles by the LHCb collaboration last year.

More information on the tetraquark results is available on the LHCb website and in the two submitted scientific papers (see here and here).