In a striking and unexpected observation from new studies aimed at an understanding of the anomalous Y(4260) particle, the international team that operates the Beijing Spectrometer (BESIII) experiment at the Beijing Electron–Positron Collider (BEPCII) has reported that it decays to a new, and perhaps even more mysterious, particle that they have named the Zc(3900).

The Y(4260) has mystified researchers since its discovery by the BaBar collaboration at SLAC in 2005. While other particles with certain similarities have long been successfully explained as bound states of a charmed quark and anticharmed quark, attempts to incorporate the Y(4260) into this model have failed and its underlying nature remains unknown. In December 2012, the BESIII team embarked on a programme to produce large numbers of Y(4260) particles by annihilating electrons and positrons with a total energy tuned to the particle’s mass. Previous studies had used electron–positron collisions at a higher energy, where the Y(4260) mesons were produced via the relatively rare process in which either the original electron or positron particle first radiated a high-energy photon, thereby lowering the total annihilation energy to the mass region of the Y(4260). By contrast, by tuning the beam energies to the particle’s mass, BEPCII can produce the Y(4260) directly and more efficiently. During the first two weeks of the programme, BESIII already collected the world’s largest sample of Y(4260) decays and by the end of the first month there was strong evidence pointing to the existence of the Zc(3900).

The anomalous charmonium particles – such as the Y(4260) and, now, the Zc(3900) – appear to be members of a new class of recently discovered particles. Called the XYZ mesons, they are adding new dimensions to the study of the strong force. QCD, the theory of the strong force, allows more possibilities for charmonium mesons than simply a charmed quark bound to an anticharmed quark. One possibility is that gluons may exist inside mesons in an excited state, a configuration referred to as "hybrid charmonium". An alternative is that more than just a charmed and anticharmed quark may be bound together to form a "tetraquark" or a molecule-like meson.

Some progress has been made recently in using lattice QCD to account for the existence of the Y(4260) as a state of hybrid charmonium. However, the hybrid picture cannot explain the newly discovered Zc(3900), which decays into a charged pion plus a neutral J/ψ. To decay in this way, the Zc(3900) must contain a charmed quark and an anticharmed quark (to form the J/ψ) together with something that is charged, so therefore cannot be a gluon. To have nonzero charge, the Zc(3900) cannot be a hybrid, but must also contain lighter quarks. Different theoretical models have been proposed that attempt to explain how this could come about. The positively charged Zc(3900) particle could be a tightly bound four-quark composite of a charmed and anticharmed quark pair plus an additional up quark and antidown quark. Or, perhaps, the Zc(3900) is a molecule-like structure comprising two mesons, each of which contain a charmed quark (or anticharmed quark) bound to a lighter antiquark (or quark). Another scenario is that the Zc(3900) is an artefact of the interaction between these two mesons.

Whatever the explanation, the appearance of such an exotic state in the decay of another exotic state was not anticipated by most researchers. Now, the ball is clearly in the experimenters’ courts and there is much hope – by theorists and experimenters alike – that with more data, the veil that continues to shroud these mysterious particles can be lifted.

• The Beijing Spectrometer (BESIII) collaboration has some 350 members from 50 institutions in 11 countries.