Speaker
Description
In the high-energy polarized $p+p$ collisions, finite transverse single spin asymmetry for forward neutron production was first observed at the IP12 experiment at Relativistic Heavy Ion Collider and it has been described by an interference between $\pi$ (spin flip) and $a_1$ (spin non-flip) exchange. The $\pi$ and $a_1$ exchange model predicted the neutron asymmetry would increase in magnitude with transverse momentum ($p_{\scriptsize{\textrm{T}}}$) with little longitudinal momentum fraction ($x_{\scriptsize{\textrm{F}}}$) dependence in $p_{\scriptsize{\textrm{T}}} < 0.4$ GeV/$c$. Recently, the PHENIX collaboration precisely extracted the neutron asymmetry as functions of $x_{\scriptsize{\textrm{F}}}$ and $p_{\scriptsize{\textrm{T}}}$ at center of mass energy of $\sqrt{s} = 200$ GeV and showed the validity of the $\pi$ and $a_1$ exchange model in $p_{\scriptsize{\textrm{T}}} < 0.2$ GeV/$c$. In June 2017, the RHICf collaboration measured the neutron asymmetry in a wide $p_{\scriptsize{\textrm{T}}}$ range of $0 < p_{\scriptsize{\textrm{T}}} < 1$ GeV/$c$ at $\sqrt{s} = 510$ GeV by installing an electromagnetic calorimeter at the zero-degree area of the STAR experiment. Using the RHICf data, one can test the validity of the $\pi$ and $a_1$ exchange model in the higher $p_{\scriptsize{\textrm{T}}}$ region and also can study if there is any $\sqrt{s}$ dependence in the neutron asymmetry. We present the final result of the neutron asymmetry measured by the RHICf experiment. A theoretical trial to understand the RHICf result other than $\pi$ and $a_1$ exchange mechanism will also be discussed.
