Transverse single spin asymmetry for very forward neutron production in polarized $p+p$ collisions at $\sqrt{s} = 510$ GeV

TL;DR

This paper reports on measurements of transverse single spin asymmetry for very forward neutron production in polarized proton-proton collisions at 510 GeV, testing theoretical models and exploring kinematic dependencies.

Contribution

It provides new experimental data on neutron asymmetry at higher transverse momentum and energy, and discusses theoretical interpretations beyond Reggeon exchange models.

Findings

Neutron asymmetry increases with transverse momentum in the measured range.

Data allows testing of $ ext{pi}$ and $a_1$ exchange models at higher $p_T$.

Comparison with previous measurements explores energy dependence.

Abstract

In the high-energy $p+p$ collisions, the transverse single spin asymmetry for very forward neutron production has been interpreted by an interference between $\pi$ (spin flip) and $a_1$ (spin non-flip) exchange with a non-zero phase shift. The $\pi$ and $a_1$ exchange model predicted the neutron asymmetry would increase in magnitude with transverse momentum ($p_{\scriptsize{\textrm{T}}}$) in $p_{\scriptsize{\textrm{T}}} < 0.4$ GeV/$c$. In June 2017, the RHICf experiment installed an electromagnetic calorimeter at the zero-degree area of the STAR experiment at the Relativistic Heavy Ion Collider and measured the neutron asymmetry in a wide $p_{\scriptsize{\textrm{T}}}$ range of $0 < p_{\scriptsize{\textrm{T}}} < 1$ GeV/$c$ from polarized $p+p$ collisions at $\sqrt{s} = 510$ GeV. The RHICf data allows us to investigate the kinematic dependence of the neutron asymmetry in detail, which not only can test the $\pi$ and $a_1$ exchange model in the higher $p_{\scriptsize{\textrm{T}}}$ range but also can study the $\sqrt{s}$ dependence by comparing with the previous measurements. We present the preliminary result and analysis status of the neutron asymmetry measured by the RHICf experiment. In order to understand the RHICf result, a theoretical calculation other than Reggeon exchange will also be discussed.