Exotic hadron states

TL;DR

This paper reviews the recent experimental discoveries of exotic hadron states like XYZ resonances, discusses their potential interpretations, and summarizes theoretical approaches such as QCD sum rules to understand their mass spectra.

Contribution

It provides a concise review of the mass spectra of quarkonium-like tetraquark states and hybrids using QCD sum rules, highlighting their role in interpreting XYZ resonances.

Findings

Mass spectra of tetraquark states analyzed

Heavy quarkonium hybrids discussed

Possible interpretations of XYZ resonances considered

Abstract

Many charmonium-like and bottomonium-like $XYZ$ resonances have been observed by the Belle, Babar, CLEO and BESIII collaborations in the past decade. They are difficult to fit in the conventional quark model and thus are considered as candidates of exotic hadrons, such as multi-quark states, meson molecules, and hybrids. In this talk, we first briefly introduce the method of QCD sum rules and then provide a short review of the mass spectra of the quarkonium-like tetraquark states and the heavy quarkonium hybrids in the QCD sum rules approach. Possible interpretations of the $XYZ$ resonances are briefly discussed.