Measurement of the transverse single-spin asymmetry for forward neutron production in a wide 𝑝T range in polarized 𝑝 +𝑝 collisions at √𝑠=510 GeV

Transverse single-spin asymmetries AN of forward neutrons at pseudorapidities larger than 6 had only been studied in the transverse momentum range of pT<0.4 GeV/c. The RHICf Collaboration has extended the previous measurements up to 1.0 GeV/c in polarized p+p collisions at s=510 GeV, using an electromagnetic calorimeter installed in the zero-degree area of the STAR detector at the Relativistic Heavy Ion Collider. The resulting AN values increase in magnitude with pT in the high longitudinal momentum fraction xF range, but they reach a plateau at lower pT for lower xF values. For low transverse momenta, the AN's show little xF dependence and level off from intermediate values. For higher transverse momenta, the AN's also show a tendency to reach a plateau at increased magnitudes. The results are consistent with previous measurements at lower collision energies, suggesting no s dependence of the neutron asymmetries. A theoretical model based on the interference of π and a1 exchange between two protons could partially reproduce the current results; however, an additional mechanism is necessary to describe the neutron AN's over the whole kinematic region measured.