Single Transverse-Spin Asymmetry in Very Forward and Very Backward Neutral Particle Production for Polarized Proton Collisions at sqrt{s} = 200 GeV

TL;DR

This paper reports the measurement of single transverse-spin asymmetries in neutral particle production at RHIC, revealing a significant asymmetry for forward neutrons, indicating interference effects in the underlying production mechanisms.

Contribution

First measurement of transverse-spin asymmetries for neutral particles at 200 GeV in forward and backward regions at RHIC, highlighting a large neutron asymmetry.

Findings

Large forward neutron asymmetry observed (A_N ≈ -0.09)

Photon and pi^0 asymmetries consistent with zero

Indicates interference between spin-flip and non-flip amplitudes

Abstract

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) x (1.0^{+0.47}_{-0.24}) 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 pi^0, and backward neutron, photon, and pi^0 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.