Measurements of the induced polarization in the quasi-elastic A(e,e^′p→) process in non-coplanar kinematics

We report measurements of the induced polarization P→ of protons knocked out from ²H and ¹²C via the A(e,e^′p→) reaction. We have studied the dependence of P→ on two kinematic variables: the missing momentum p_miss and the “off-coplanarity” angle ϕ_pq between the scattering and reaction planes. For the full 360° range in ϕ_pq, both the normal (P_y) and, for the first time, the transverse (P_x) components of the induced polarization were measured with respect to the coordinate system associated with the scattering plane. P_x vanishes in coplanar kinematics, however in non-coplanar kinematics, it is on the same scale as P_y. We find that the dependence on ϕ_pq is sine-like for P_x and cosine-like for P_y. For carbon, the magnitude of the induced polarization is especially large when protons are knocked out from the p_3/2 shell at very small p_miss. For the deuteron, the induced polarization is near zero at small |p_miss|, and its magnitude increases with |p_miss|. For both nuclei such behavior is reproduced qualitatively by theoretical results, driven largely by the spin-orbit part of the final-state interactions. However, for both nuclei, sizeable discrepancies exist between experiment and theory.