Four-Quark Hadrons: an Updated Review

TL;DR

This review discusses recent experimental discoveries of exotic four-quark states, highlighting the evidence for tetraquarks and the need for a unified theoretical understanding of these complex particles.

Contribution

It provides a comprehensive overview of experimental findings and theoretical challenges regarding XYZ particles, emphasizing the potential existence of compact tetraquarks.

Findings

Experimental evidence supports the existence of charged exotic states.

Conflicting theories highlight the need for a unified model.

Tetraquarks are promising candidates for explaining XYZ particles.

Abstract

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. At this state of progress, XYZ phenomenology speaks in favour of the existence of compact four-quark particles (tetraquarks) and we believe that realizing this instructs us in the quest for a firm theoretical framework.