Inclusive cross section and single transverse spin asymmetry for very forward neutron production in polarized p+p collisions at √s=200 GeV

The energy dependence of the single-transverse-spin asymmetry, A(N), and the cross section for neutronproduction at very forward angles were measured in the PHENIX experiment at the Relativistic Heavy Ion Colliderfor polarized p + p collisions at root s = 200 GeV. The neutronswere observed in forward detectors covering an angular range of up to 2.2 mrad. We report results for neutronswith a momentum fraction of x(F) = 0.45 to 1.0. The energy dependence of the measured cross sections were consistent with x( F) scaling, compared to measurements by an experiment at the Intersecting Storage Ring, which measured neutronproduction in unpolarized p + p collisions at root s = 30.6-62.7 GeV. The cross sections for large x(F) neutronproduction for p + p collisions, as well as those in e + p collisions measured at the Hadron-Electron Ring Accelerator, are described by a pionexchange mechanism. The observed forward neutron asymmetrieswere large, reaching A(N) = -0.08 +/- 0.02 for x(F) = 0.8; the measured backward asymmetries, for negative x(F), were consistent with zero. The observed asymmetryfor forward neutronproduction is discussed within the pionexchange framework, with interference between the spin-flipamplitude due to the pionexchange and nonflip amplitudes from all Reggeon exchanges. Within the pionexchange description, the measured neutron asymmetryis sensitive to the contribution of other Reggeon exchanges even for small amplitudes.