Measurement of the Target-Normal Single-Spin Asymmetry in Quasielastic Scattering from the Reaction [superscript 3]He[superscript ↑](e,e')

Abstract

We report the first measurement of the target single-spin asymmetry, A[subscript y], in quasielastic scattering from the inclusive reaction [superscript 3]He[superscript ↑](e,e′) on a [superscript 3]He gas target polarized normal to the lepton scattering plane. Assuming time-reversal invariance, this asymmetry is strictly zero for one-photon exchange. A nonzero A[subscript y] can arise from the interference between the one- and two-photon exchange processes which is sensitive to the details of the substructure of the nucleon. An experiment recently completed at Jefferson Lab yielded asymmetries with high statistical precision at Q[superscript 2] =0.13, 0.46, and 0.97 GeV[superscript 2]. These measurements demonstrate, for the first time, that the [superscript 3]He asymmetry is clearly nonzero and negative at the 4σ–9σ level. Using measured proton-to-[superscript 3]He cross-section ratios and the effective polarization approximation, neutron asymmetries of -(1–3)% were obtained. The neutron asymmetry at high Q[superscript 2] is related to moments of the generalized parton distributions (GPDs). Our measured neutron asymmetry at Q[superscript 2] = 0.97 GeV[superscript 2] agrees well with a prediction based on two-photon exchange using a GPD model and thus provides a new, independent constraint on these distributions.