We report the first measurement of the target single-spin asymmetry, $A_y$, in quasielastic scattering from the inclusive reaction ${{}^3\mathrm{He}}^{\uparrow }(e,e^{\prime })$ on a ${}^3\mathrm{He}$ gas target polarized normal to the lepton scattering plane. Assuming time-reversal invariance, this asymmetry is strictly zero for one-photon exchange. A nonzero $A_y$ 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 $Q^2=0.13$, 0.46, and $0.97{\mathrm{GeV}}^2$. These measurements demonstrate, for the first time, that the ${}^3\mathrm{He}$ asymmetry is clearly nonzero and negative at the $4\sigma –9\sigma$ level. Using measured proton-to-${}^3\mathrm{He}$ cross-section ratios and the effective polarization approximation, neutron asymmetries of $-(1–3)\%$ were obtained. The neutron asymmetry at high $Q^2$ is related to moments of the generalized parton distributions (GPDs). Our measured neutron asymmetry at $Q^2=0.97{\mathrm{GeV}}^2$ agrees well with a prediction based on two-photon exchange using a GPD model and thus provides a new, independent constraint on these distributions.

• Received 5 February 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.172502