Measurement of the target-normal single-spin asymmetry in deep-inelastic scattering from the reaction He 3 ↑ (e,e′)X

We report the first measurement of the target-normal single-spin asymmetry in deep-inelastic scattering from the inclusive reaction He3↑(e,e′)X on a polarized He3 gas target. Assuming time-reversal invariance, this asymmetry is strictly zero in the Born approximation but can be nonzero if two-photon-exchange contributions are included. The experiment, conducted at Jefferson Lab using a 5.89 GeV electron beam, covers a range of 1.72GeV, which is nonzero at the 2.89σ level. Our measured asymmetry agrees both in sign and magnitude with a two-photon-exchange model prediction that uses input from the Sivers transverse momentum distribution obtained from semi-inclusive deep-inelastic scattering. © 2014 American Physical Society.