Measurements of the electron-helicity asymmetry in the quasi-elastic ${\rm A}(\vec{e},e' p)$ process

TL;DR

This study measures the electron helicity asymmetry in quasi-elastic proton knockout from deuterium and carbon nuclei, revealing its dependence on residual nuclear states and providing insights into final-state interactions.

Contribution

It provides new experimental data on helicity asymmetries across different residual states and compares these with theoretical models, highlighting areas of agreement and discrepancy.

Findings

Helicity asymmetry depends on residual nuclear state.

Asymmetry is larger for p-shell knockout than s-shell.

Data for deuterium agree well with theory, carbon data show discrepancies.

Abstract

We present measurements of the electron helicity asymmetry in quasi-elastic proton knockout from $^{2}$H and $^{12}$C 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 $\phi_{pq}$, with a polar angle $\theta_{pq}$ coverage up to about 8 degrees. The missing energy resolution enabled us to determine the asymmetries for knock-out resulting in different states of the residual $^{11}$B system. We find that the helicity asymmetry for $p$-shell knockout from $^{12}$C depends on the final state of the residual system and is relatively large (up to $\approx 0.16$), especially at low missing momentum. It is considerably smaller (up to $\approx 0.01$) for $s$-shell knockout from both $^{12}$C and $^2$H. The data for $^2$H are in very good agreement with theoretical calculations, while the predictions for $^{12}$C exhibit differences with respect to the data.