Single transverse-spin asymmetry in very forward and very backward neutral particle production for polarized proton collisions at sqrt(s) = 200   GeV

In the 2001-2002 running period of the Relativistic Heavy Ion Collider (RHIC), transversely polarized protons were accelerated to 100 GeV for the first time, with collisions at sqrt(s) = 200   GeV. We present results from this run for single transverse-spin asymmetries for inclusive production of neutral pions, photons and neutrons of the energy region 20-100 GeV for forward and backward production for angles between 0.3 mrad and 2.2 mrad with respect to the polarized proton direction. An asymmetry of A_N = (- 0.090 ± 0.006 ± 0.009) × (1.0_-0.24^+0.47) was observed for forward neutron production, where the errors are statistical and systematic, and the scale error is from the beam polarization uncertainty. The forward photon and π⁰, and backward neutron, photon, and π⁰ asymmetries were consistent with zero. The large neutron asymmetry indicates a strong interference between a spin-flip amplitude, such as one pion exchange which dominates lower energy neutron production, and remaining spin-non-flip amplitudes such as reggeon exchange.