Parity-Violating Electron Scattering fromHe4and the Strange Electric Form Factor of the Nucleon

Abstract

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from ${}^4\mathrm{He}$ at an average scattering angle $⟨{\theta }_{\mathrm{lab}}⟩=5.7°$ and a four-momentum transfer $Q^2=0.091{\mathrm{GeV}}^2$. From these data, for the first time, the strange electric form factor of the nucleon $G_E^s$ can be isolated. The measured asymmetry of $A_{\mathrm{PV}}=(6.72\pm {0.84}_{(\mathrm{stat})}\pm {0.21}_{(\mathrm{syst})})\times {10}^{-6}$ yields a value of $G_E^s=-0.038\pm {0.042}_{(\mathrm{stat})}\pm {0.010}_{(\mathrm{syst})}$, consistent with zero.