Measurements of the electron-helicity asymmetry in the quasi-elastic A(e→,e^′p) process

We present measurements of the electron helicity asymmetry in quasi-elastic proton knockout from ²H and ¹²C nuclei by polarized electrons. This asymmetry depends on the fifth structure function, is antisymmetric with respect to the scattering plane, and vanishes in the absence of final-state interactions, and thus it provides a sensitive tool for their study. Our kinematics cover the full range in off-coplanarity angle ϕ_pq, with a polar angle θ_pq coverage up to about 8°. The missing energy resolution enabled us to determine the asymmetries for knock-out resulting in different states of the residual ¹¹B system. We find that the helicity asymmetry for p-shell knockout from ¹²C depends on the final state of the residual system and is relatively large (up to ≈0.16), especially at low missing momentum. It is considerably smaller (up to ≈0.01) for s-shell knockout from both ¹²C and ²H. The data for ²H are in very good agreement with theoretical calculations, while the predictions for ¹²C exhibit differences with respect to the data.