Transverse single spin asymmetry measurement of very forward neutron production in the RHICf experiment

The RHICf collaboration has installed an electromagnetic calorimeter in the very forward zero-degree region of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) to measure the transverse single spin asymmetry (TSSA) of neutrons over a wide transverse momentum (p_T) range with 0 < p_T < 1 GeV/c in high-energy polarized p+p collisions with collision energy √s = 510 GeV. The finite TSSA of the very forward neutron production in high-energy polarized p+p collisions was first measured at √s = 200 GeV in the IP12 experiment at RHIC, and combined measurements at several collision energies in the subsequent PHENIX experiment showed a tendency to increase with p_T in the range p_T < 0.4 GeV/c. The RHICf experiment allows us to extend the measurement range to the higher p_T region and to investigate whether there is a √s dependence of the neutron TSSA. In this talk we will present the experimental results and discuss theoretical attempts to understand the results.