# QCD Sum Rules Approach to the $X,~Y$ and $Z$ States

## TL;DR

This paper reviews the experimental discoveries of exotic charmonium-like and bottomonium-like states and discusses their theoretical interpretations using QCD sum rules, comparing results with other methods and data.

## Contribution

It provides a comprehensive review of the application of QCD sum rules to interpret the $X$, $Y$, and $Z$ states, highlighting recent progress and comparisons with experimental findings.

## Key findings

- QCD sum rules offer insights into the structure of exotic states.
- Theoretical results are consistent with some experimental observations.
- Different interpretations like tetraquark and molecular models are discussed.

## Abstract

In the past decade, due to the experimental observation of many charmonium-like states, there has been a revival of hadron spectroscopy. In particular, the experimental observation of charged charmonium-like, $Z_c$ states, and bottomonium-like, $Z_b$ states, represents a challenge since they can not be accommodated within the naive quark model. These charged states are good candidates of either tetraquark or molecular states and their observation motivated a vigorous theoretical activity. This is a rapidly evolving field with enormous amount of new experimental information. In this work, we review the current experimental progress and investigate various theoretical interpretations of these candidates of the multiquark states. The present review is written from the perspective of the QCD sum rules approach, where we present the main steps of concrete calculations and compare the results with other approaches and with experimental data.