Experimental Road to a Charming Family of Tetraquarks ... and Beyond

TL;DR

This paper reviews the history and recent developments in exotic tetraquark states, especially all-charm candidates, discussing their models, structure, and potential for revealing new insights in heavy quarkonium physics.

Contribution

It provides a comprehensive review of the experimental and theoretical progress on all-charm tetraquarks, highlighting their potential as a family of radial excitations and exploring models of their structure.

Findings

All-charm tetraquarks form a Regge trajectory.

Evidence suggests these states are a family of radial excitations.

A possible threshold excess hints at a fourth family member.

Abstract

Discovery of the X(3872) meson in 2003 ignited intense interest in exotic (neither $q\bar{q}$ nor $qqq$) hadrons, but a $c\bar{c}$ interpretation of this state was difficult to exclude. An unequivocal exotic was discovered in the $Z_c(3900)^+$ meson -- a charged charmonium-like state. A variety of models of exotic structure have been advanced but consensus is elusive. The grand lesson from heavy quarkonia was that heavy quarks bring clarity. Thus, the recently reported triplet of all-charm tetraquark candidates -- $X(6600)$, $X(6900)$, and $X(7100)$ -- decaying to $J/\psi\,J/\psi$ is a great boon, promising important insights. We review some history of exotics, chronicle the road to prospective all-charm tetraquarks, discuss in some detail the divergent modeling of $J/\psi\,J/\psi$ structures, and offer some inferences about them. These states form a Regge trajectory and appear to be a family of radial excitations. A reported, but unexplained, threshold excess could hint at a fourth family member. We close with a brief look at a step beyond: all-bottom tetraquarks.