XYZ: four-quark states?
The observation of many unexpected states decaying into heavy quarkonia has challenged the usual Q-Qbar interpretation. We will discuss the nature of some of the charmonium-like resonances recently observed by BES III and LHCb, and their identication according to the compact tetraquark model. We also commment the production of light nuclei in hadron collisions and the relevance for the physics of the X(3872).