# Photoproduction of charged final states in ultra-peripheral collisions and electroproduction at an electron-ion collider

## TL;DR

This paper explores the potential of ultra-peripheral collisions and electron-ion colliders to study charged final states via photoproduction, including exotic states like Zc(4430), expanding the physics program for nuclear and particle physics research.

## Contribution

It proposes a new approach to study charged final states and exotic mesons through photoproduction in UPCs and EICs, with detailed rate calculations and kinematic predictions.

## Key findings

- Rates are sufficient for detailed experimental studies.
- Final state rapidity distributions depend on beam energies.
- Production is accessible at various collider energies.

## Abstract

Ultra-peripheral collisions (UPCs) of relativistic ions are an important tool for studying photoproduction at high energies. Vector meson photoproduction is an important tool for nuclear structure measurements and other applications. A future electron-ion collider (EIC) will allow additional studies, using virtual photons with a wide range of $Q^2$. We propose a significant expansion of the UPC and EIC photoproduction physics programs to include charged final states which may be produced via Reggeon exchange. We consider two examples: $a_2^+(1320)$, which is a conventional $q\overline q$ meson, and the exotic $Z_c^+(4430)$ state (modeled here as a tetraquark). The $Z_c^+(4430)$ cross-section depends on its internal structure, so photoproduction can test whether the $Z_c^+(4430)$ is a tetraquark or other exotic object. We calculate the rates and kinematic distributions for $\gamma p\rightarrow X^+n$ in $pA$ UPCs and $ep$ collisions at an EIC and in UPCs. The rates are large enough for detailed studies of these final states. Because the cross-section for Reggeon exchange is largest near threshold, the final state rapidity distribution depends on the beam energies. At high-energy colliders like the proposed LHeC or $pA$ collisions at the LHC, the final states are produced at far forward rapidities. For lower energy colliders, the systems are produced closer to mid-rapidity, within reach of central detectors.