Measurement of Single Transverse‐spin Asymmetry in Forward Production of Photons and Neutrons in pp Collisions at s = 200 GeV

The Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory (BNL) was commissioned for polarized proton‐proton collisions at the center of mass energy s = 200 GeV during the run in 2001–2002. We have measured the single transverse‐spin asymmetry AN for production of photons, neutral pions, and neutrons at the very forward angle. The asymmetries for the photon and neutral pion sample were consistent with zero within the experimental uncertainties. In contrast, the neutron sample exhibited an unexpectedly large asymmetry. This large asymmetry will be used for the non‐destructive polarimeter for polarized proton beams at the collision points in the RHIC interaction region.