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

We report the first measurement of the target single-spin asymmetry, A sub(y), in quasielastic scattering from the inclusive reaction He super(3)[arrowup](e,e') on a He super(3) 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 sub(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 super(2)=0.13, 0.46, and 0.97 GeV super(2). These measurements demonstrate, for the first time, that the He super(3) asymmetry is clearly nonzero and negative at the 4[sigma]-9[sigma] level. Using measured proton-to-He super(3) cross-section ratios and the effective polarization approximation, neutron asymmetries of -(1-3)% were obtained. The neutron asymmetry at high Q super(2) is related to moments of the generalized parton distributions (GPDs). Our measured neutron asymmetry at Q super(2)=0.97 GeV super(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.