Recent Results in Parity-Violating Electron Scattering at Jefferson Lab: PREX and HAPPEX-III

The parity-violating asymmetry $A_{PV}$ in electron scattering from the $^{208}$Pb nucleus is cleanly sensitive to the neutron radius $R_n$. A precision measurement of $R_n$ would have important implications for the understanding of nuclear structure, and be a powerful constaint on the symmetry energy $S_{\nu}(n)$ of neutron-rich nuclear matter, including neutron stars. The PREX collaboration has completed a first run, measuring $R_{n}$ to a precision of $\sim2.5$\%. The measurement of $A_{PV}$ in electron-proton scattering is sensitive to vector form-factors associated with an intrinsic strange quark content of the nucleon. While at one time such contributions were considered to be potentially large, a world-wide program of parity-violation measurements has constrained these form-factors to be smaller than a few percent of the electric and magnetic form-factors of the proton at low momentum-transfer. HAPPEX-III has recently completed a measurement to improve the precision of this constaint at $Q^2\sim0.6\mbox{GeV}^2$, a region in which previous experiments had indicated the possibility of intriguingly large strange contributions. Results from each experiment, and prospects for more precise $R_n$ measurements, will be discussed.