Measurement of the transverse single spin asymmetry for forward neutron production in a wide transverse momentum range

In the high-energy polarized $p+p$ collisions, a spin-involved diffractive particle production mechanism can be understood by measuring transverse single-spin asymmetries A_N of forward particles that are produced at pseudorapidity larger than 6. Since A_N of forward neutron had only been studied in a narrow transverse momentum range of p_T<0.4~GeV/c, the RHICf Collaboration has extended the previous measurements up to 1.0~GeV/c at √s=510~GeV to study the kinematic dependence of the neutron A_N in more detail. The resulting A_Ns reach a plateau in the low longitudinal momentum fraction x_F range, but explicitly increase in magnitude with p_T in the high x_F range. The A_Ns show little x_F dependence in the low p_T range. A clear x_F dependence is observed for higher p_T range in the intermediate x_F region. The results are consistent with the previous measurements at √s=200~GeV, which suggests no √s dependence of the neutron A_N. A theoretical model based on π and a₁ exchange between two protons could reproduce the current results only in a limited kinematic region. An additional mechanism is necessary to understand the measured neutron A_Ns over the whole kinematic region.