# Single-Transverse-Spin Asymmetries in Exclusive Photo-production of J/psi in Ultra-Peripheral Collisions in the Fixed-Target Mode at the LHC and in the Collider Mode at RHIC

## TL;DR

This paper explores the potential of measuring single-transverse-spin asymmetries in exclusive J/psi photo-production during ultra-peripheral collisions at LHC and RHIC, aiming to access proton GPD E_g(x,xi,t).

## Contribution

It introduces a novel approach to study proton GPDs through exclusive J/psi production in ultra-peripheral collisions at fixed-target and collider modes, with detailed expected measurement precisions.

## Key findings

- Expected counting rates for J/psi production at LHC and RHIC.
- Projected precision on single-transverse-spin asymmetries (A_N).
- Feasibility of using polarized deuterium and helium targets.

## Abstract

We investigate the potentialities offered by the study of J/psi exclusive photo-production in ultra-peripheral collisions at a fixed-target experiment using the proton and lead LHC beams (generically denoted as AFTER@LHC) on hydrogen targets and at RHIC in the collider mode. We compare the expected counting rates in both set-ups. Studying Single-Transverse-Spin Asymmetries (A_N) in such a process provides a direct path to the proton Generalised Parton Distribution (GPD) E_g(x,xi,t). We evaluate the expected precision on A_N for realistic conditions with the LHCb detector in pH(pol) and PbH(pol) collisions. We also discuss prospects with polarised deuterium and helium targets in the case of AFTER@LHC.