Four-Quark Hadrons: an Updated Review

Understand

The past decade witnessed a remarkable proliferation of exotic charmonium-like resonances discovered at accelerators.

• In particular, the recently observed charged states are clearly not interpretable as q-qbar mesons.
• Notwithstanding the considerable advances on the experimental side, conflicting theoretical descriptions do not seem to provide a definitive picture about the nature of the so-called XYZ particles.
• We present here a comprehensive review about this intriguing topic, discussing both those experimental and theoretical aspects which we consider relevant to make further progress in the field.