Hunting for tetraquarks in ultra-pheripheral heavy ion collisions

TL;DR

This paper proposes using ultra-peripheral heavy ion collisions at high energies to search for exotic tetraquark states, particularly the $X(6900)$ and partners of the $X(3872)$, to test the compact tetraquark model.

Contribution

It introduces a novel experimental approach to detect specific tetraquark states in ultra-peripheral collisions and estimates their production cross sections.

Findings

The $X(6900)$ is a promising candidate for detection with estimated cross sections of 250-1150 nb.

Ultra-peripheral collisions offer a clean environment for exotic hadron searches.

Observation of predicted tetraquark partners would support the compact tetraquark model.

Abstract

Ultra-peripheral heavy ion collisions constitute an ideal setup to look for exotic hadrons because of their low event multiplicity and the possibility of an efficient background rejection. We propose to look for four-quark states produced by photon-photon fusion in these collisions at center-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}=5.5~{\rm TeV}$. In particular, we focus on those states that would represent a definite smoking gun for the compact tetraquark model. We show that the $X(6900)$, a likely $cc\bar{c}\bar{c}$ compact state, is a perfect candidate for this search, and estimate a production cross section ranging from around $250$ nb to $1150$ nb, depending on its quantum numbers. Furthermore, we discuss the importance of ultra-peripheral collisions to the search for the scalar and tensor partners of the $X(3872)$ predicted by the diquarkonium model, and not yet observed. The completion of such a flavor-spin multiplet would strongly speak in favor of the compact tetraquark model.