Parity-Violating Electron Scattering from ${}^4\mathrm{He}$ and the Strange Electric Form Factor of the Nucleon

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.

Figure 1

Raw detector asymmetry $A_{\mathrm{raw}}$ for both spectrometers, broken down by data set. The step pattern represents the insertion/removal of a $\lambda /2$ plate at the beam source, which should flip the sign of the measured asymmetry.

Figure 2

Measured momentum difference from the central momentum ($p_0$) of one spectrometer, at the focal plane (filled histogram). Quasielastic scattering from ${}^4\mathrm{He}$ dominates the data at low momenta. The vertical lines indicate the edges of the Cherenkov detector acceptance.