# Large single transverse spin asymmetry for forward neutrons in ultraperipheral polarized proton-nucleus collisions in explanation of measurements at RHIC-PHENIX

## TL;DR

This paper explains the large single transverse spin asymmetry for forward neutrons observed in polarized proton-nucleus collisions at RHIC by proposing a model involving ultraperipheral collisions and hadronic interactions, supported by Monte Carlo simulations.

## Contribution

It introduces a novel explanation for the observed asymmetry using a combined ultraperipheral and hadronic interaction model, validated by detailed Monte Carlo simulations.

## Key findings

- Simulated asymmetries agree with PHENIX measurements.
- Ultraperipheral collisions significantly contribute to the asymmetry.
- The combined model reproduces the experimental data.

## Abstract

The PHENIX experiment at BNL-RHIC recently reported the single transverse spin asymmetry $A_N$ for forward neutrons measured in polarized proton-nucleus collisions at $\sqrt{s_{NN}} = 200\,$GeV; $A_N$ in proton-aluminum and proton-gold collisions are -0.015 and 0.18, respectively, which are clearly different from $A_N=-0.08$ in proton-proton collisions. In this paper, we propose large $A_N$ for forward neutrons in ultraperipheral polarized proton-nucleus collisions as an explanation of the PHENIX measurements. The proposed model is demonstrated using two Monte Carlo simulations organized as follows. In the ultraperipheral collision simulation, we use the STARlight event generator for the simulation of the virtual photon flux and then use the MAID2007 unitary isobar model for the simulation of the neutron production in the interactions of a virtual photon with a polarized proton. In the proton-nucleus hadronic interaction simulation, the differential cross sections of forward neutron production are predicted by a simple one-pion exchange model and the Glauber model. The simulated $A_N$ values accumulating the both contribution of ultraperipheral collisions and hadronic interactions are in good agreement with the PHENIX results.