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

TL;DR

This paper investigates the analyzing power in elastic electron scattering off spin-0 nuclei, focusing on the imaginary part of two-photon exchange contributions, with estimates for helium-4 and lead-208 relevant to current experiments.

Contribution

It provides a detailed analysis of elastic and inelastic intermediate state contributions to the analyzing power, including estimates for specific nuclei and implications for experiments.

Findings

Inelastic contributions dominate the analyzing power.

Predicted negative analyzing power of a few ppm.

Elastic contribution scales with nuclear charge Z.

Abstract

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 as the ratio of the atomic number and nuclear charge, $A/Z$. We provide estimates for $^4$He and $^{208}$Pb, in the kinematics of existing or upcoming experiments. In both cases, we predict negative values of a few parts per million, and the dominant contribution is due to inelastic contributions. The analyzing power can contribute a substantial systematic error in parity-violating experiments.