# Nuclear dependence of the transverse-single-spin asymmetry for forward neutron production in polarized $p$$+$$A$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV

## TL;DR

This paper reports on the first measurements of transverse-single-spin asymmetries in forward neutron production during polarized proton collisions with heavy nuclei at RHIC, revealing a surprisingly strong atomic-mass-number dependence.

## Contribution

It presents the first experimental investigation of $A$ dependence of single-spin asymmetries in $p+A$ collisions at RHIC, challenging existing theoretical predictions.

## Key findings

- Asymmetry in $p$+Al is small.
- Asymmetry in $p$+Au is three times larger and opposite in sign.
- Strong $A$ dependence observed in forward neutron asymmetries.

## Abstract

During 2015 the Relativistic Heavy Ion Collider (RHIC) provided collisions of transversely polarized protons with Au and Al nuclei for the first time, enabling the exploration of transverse-single-spin asymmetries with heavy nuclei. Large single-spin asymmetries in very forward neutron production have been previously observed in transversely polarized $p$$+$$p$ collisions at RHIC, and the existing theoretical framework that was successful in describing the single-spin asymmetry in $p$$+$$p$ collisions predicts only a moderate atomic-mass-number ($A$) dependence. In contrast, the asymmetries observed at RHIC in $p$$+$$A$ collisions showed a surprisingly strong $A$ dependence in inclusive forward neutron production. The observed asymmetry in $p$$+$Al collisions is much smaller, while the asymmetry in $p$$+$Au collisions is a factor of three larger in absolute value and of opposite sign. The interplay of different neutron production mechanisms is discussed as a possible explanation of the observed $A$ dependence.