Abstract
We report the first measurement of the target single-spin asymmetry, , in quasielastic scattering from the inclusive reaction on a gas target polarized normal to the lepton scattering plane. Assuming time-reversal invariance, this asymmetry is strictly zero for one-photon exchange. A nonzero 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 , 0.46, and . These measurements demonstrate, for the first time, that the asymmetry is clearly nonzero and negative at the level. Using measured proton-to- cross-section ratios and the effective polarization approximation, neutron asymmetries of were obtained. The neutron asymmetry at high is related to moments of the generalized parton distributions (GPDs). Our measured neutron asymmetry at 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
© 2015 American Physical Society