Measurement of the single-spin asymmetry $A_y^0$ in quasi-elastic ${}^3$He${}^{\uparrow }$($e,e^{\prime }n$) scattering at $0.4<Q^2<1.0$ GeV$/c^2$

Abstract

Due to the lack of free neutron targets, studies of the structure of the neutron are typically made by scattering electrons from either ${}^2$H or ${}^3$He targets. In order to extract useful neutron information from a ${}^3$He target, one must understand how the neutron in a ${}^3$He system differs from a free neutron by taking into account nuclear effects such as final state interactions and meson exchange currents. The target single spin asymmetry $A_y^0$ is an ideal probe of such effects, as any deviation from zero indicates effects beyond plane wave impulse approximation. New measurements of the target single spin asymmetry $A_y^0$ at $Q^2$ of 0.46 and 0.96 (GeV/$c{)}^2$ were made at Jefferson Lab using the quasi-elastic ${}^3{\mathrm{He}}^{\uparrow }(e,e^{\prime }n)$ reaction. Our measured asymmetry decreases rapidly, from $>20\%$ at $Q^2=0.46$ (GeV/$c{)}^2$ to nearly zero at $Q^2=0.96$ (GeV$/c{)}^2$, demonstrating the fall-off of the reaction mechanism effects as $Q^2$ increases. We also observed a small $\hat{I}\mu$-dependent increase in $A_y^0$ compared to previous measurements, particularly at moderate $Q^2$. This indicates that upcoming high $Q^2$ measurements from the Jefferson Lab 12 GeV program can cleanly probe neutron structure from polarized ${}^3$He using plane wave impulse approximation.

6 pages, 5 figures