Charming and strange exotic hadrons
Hadrons are bound states of collections of quarks and antiquarks, held together by the strong interaction. In the decades following the development of the quark model, proposed independently by Murray Gell-Mann and George Zweig in 1964, two types of hadrons were seen: baryons, which are composed of three quarks, and mesons, composed of a quark and an antiquark. But in recent years an increasing number of exotic hadrons have been observed that do not fit into this scheme since they have minimal quark content of either two quarks and two antiquarks ("tetraquarks") or four quarks and one antiquark ("pentaquarks"). This raises questions about how precisely these exotic hadrons are bound together, and provides the exciting possibility of learning more about the behaviour of the strong interaction. Until now all exotic hadrons discovered contained either a charm-anticharm, or a beauty-antibeauty, quark-antiquark pair. In such cases, the flavour (charm or beauty) of the quarks involved is said to be "hidden", since the quark and antiquark cancel each other out. Theorists have argued, however, that if exotic hadrons exist then states with "open", i.e. unhidden, flavour should exist. The new discovery by LHCb appears to be the first discovery of such a state. The discovery was made through a detailed study of the beauty meson decay process B+D+D−K+. Conventional resonances in this process can only occur in the D+D− pair, but after removing most such decays a clear peak in the D−K+ mass around 2.9 GeV/c2 is seen. The minimal quark content of a resonance causing this structure is anticharm, antistrange, up and down, so this appears to be the first tetraquark composed of four quarks of different flavours. The ball is now in the theorists court to explain this unexpected discovery.
- Caption to figure: Distribution of the D-K+ invariant mass observed in B+ -> D+D-K+ decays, with a clear peak near 2.9 GeV/c2 due to the new particle.