Measurement of the transverse single-spin asymmetry for forward neutron production in a wide p_T range in polarized p plus 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 p(T) < 0.4 GeV/c. The RHICf Collaboration has extended the previous measurements up to 1.0 GeV/c in polarized p + p collisions at root s = 510 GeV, s using an electromagnetic calorimeter installed in the zero-degree area of the STAR detector at the Relativistic Heavy Ion Collider. The resulting A(N) values increase in magnitude with p(T) in the high longitudinal momentum fraction x(F) range, but they reach a plateau at lower p(T) for lower x(F) values. For low transverse momenta, the A(N)'s show little x(F) dependence and level off from intermediate values. For higher transverse momenta, the A(N)'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 root s dependence of the neutron asymmetries. A theoretical model based on the interference of pi and a(1) exchange between two protons could partially reproduce the current results; however, an additional mechanism is necessary to describe the neutron A(N)'s over the whole kinematic region measured.