Analyzing power in elastic scattering of electrons off a spin-0 target

We consider the analyzing power on a spin-0 nuclear target. This observable is related to the imaginary part of the two-photon-exchange (box) diagram. We consider the contributions of elastic and inelastic intermediate states. The former requires knowledge of the elastic nuclear form factor, while the latter uses the optical theorem as input. The elastic contribution scales as the nuclear charge Z, while the inelastic contribution scales as the ratio of the atomic number and nuclear charge, $A/Z$. We provide estimates for ⁴He and ²⁰⁸Pb, in the kinematics of existing or upcoming experiments. In both cases, we predict negative values for the analyzing power of a few parts per million, and the dominant contribution is due to inelastic intermediate states. The analyzing power can contribute a substantial systematic error in parity-violating experiments.