Z_c(3900) - what is inside?

Understand

The models for the internal structure of the newly found four-quark charmonium-like resonance $Z_c(3900)$ are discussed: the molecular model as well as the hadro-charmonium and tetraquark schemes.

• It is argued that it would be possible to resolve between these models by combining measurements of the quantum numbers of the resonance and of its decay rates into yet unseen channels $\pi \psi'$, $\pi h_c$, $\rho \eta_c$ and into pairs of heavy mesons $D^* \bar D$ and $D \bar D^*$.
• The models also predict different related four-quark states, which can be sought for in the existing and future data.