Measurement of the Target-Normal Single-Spin Asymmetry in Deep-Inelastic Scattering from the Reaction ^3He^↑(e,e′)X

Abstract

We report the first measurement of the target-normal single-spin asymmetry in deep-inelastic scattering from the inclusive reaction ^3He^↑ (e,e′) X on a polarized ^3He 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 1.7 < W < 2.9  GeV, 1.0 < Q^2 < 4.0  GeV^2 and 0.16 < x < 0.65. Neutron asymmetries were extracted using the effective nucleon polarization and measured proton-to-^3He cross-section ratios. The measured neutron asymmetries are negative with an average value of (−1.09±0.38) × 10^(−2) for invariant mass W > 2  GeV, 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.

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