Hadronic Spectrum - Multiquark States

TL;DR

This paper reviews the experimental evidence for exotic multiquark states, such as tetraquarks, in the charmonium, bottom, and strange quark sectors, highlighting their complex substructures beyond conventional mesons.

Contribution

It provides a comprehensive overview of the experimental discoveries and status of exotic multiquark states across different quark flavors.

Findings

Evidence for mesons with non-zero charge indicating multiquark content

Observation of states with properties inconsistent with traditional quark-antiquark models

Summary of experimental status across charm, bottom, and strange sectors

Abstract

Many newly discovered mesons behave like $\ccbar$ charmonium states in that they preferentially decay into final states that contain a $c$- and a $\bar{c}$-quark, but do not fit expectations for any of the unfilled levels of the conventional $\ccbar$ spectrum. There is a growing suspicion that at least some of these states are {\it exotic}, {\it i.e.} have a substructure that is more complex than the quark-antiquark mesons of the classical constituent quark model. Some of these candidate states have a non-zero electric charge and, thus, a minimal quark content of $\ccbar u\bar{d}$ or $\ccbar d\bar{u}$. In addition, states with similar properties have been observed in the $b$- and $s$-quark sectors. In this report, the experimental situation is briefly reviewed.