We report the first measurement of the parity-violating asymmetry $A_{\mathrm{PV}}$ in the elastic scattering of polarized electrons from ${}^{208}\mathrm{Pb}$. $A_{\mathrm{PV}}$ is sensitive to the radius of the neutron distribution ($R_n$). The result $A_{\mathrm{PV}}=0.656\pm 0.060(\mathrm{stat})\pm 0.014(\mathrm{syst})\mathrm{ppm}$ corresponds to a difference between the radii of the neutron and proton distributions $R_n-R_p={0.33}_{-0.18}^{+0.16}\mathrm{fm}$ and provides the first electroweak observation of the neutron skin which is expected in a heavy, neutron-rich nucleus.

Among the most fundamental observables of nucleon structure, electromagnetic form factors are a crucial benchmark for modern calculations describing the strong interaction dynamics of the nucleon’s quark constituents; indeed, recent proton data have attracted intense theoretical interest. In this Letter, we report new measurements of the proton electromagnetic form factor ratio using the recoil polarization method, at momentum transfers $Q^2=5.2$, 6.7, and $8.5{\mathrm{GeV}}^2$. By extending the range of $Q^2$ for which $G_E^p$ is accurately determined by more than 50%, these measurements will provide significant constraints on models of nucleon structure in the nonperturbative regime.

We report the first measurement of target single spin asymmetries in the semi-inclusive ${}^3\mathrm{He}(e,e^{\prime }{\pi }^{\pm })X$ reaction on a transversely polarized target. The experiment, conducted at Jefferson Lab using a 5.9 GeV electron beam, covers a range of $0.16<x<0.35$ with $1.4<Q^2<2.7{\mathrm{GeV}}^2$. The Collins and Sivers moments were extracted from the azimuthal angular dependence of the measured asymmetries. The ${\pi }^{\pm }$ Collins moments for ${}^3\mathrm{He}$ are consistent with zero, except for the ${\pi }^{+}$ moment at $x=0.35$, which deviates from zero by $2.3\sigma$. While the ${\pi }^{-}$ Sivers moments are consistent with zero, the ${\pi }^{+}$ Sivers moments favor negative values. The neutron results were extracted using the nucleon effective polarization and measured cross section ratios of proton to ${}^3\mathrm{He}$, and are largely consistent with the predictions of phenomenological fits and quark model calculations.

We report a new, high-precision measurement of the proton elastic form factor ratio μpGE/GM for the four-momentum transfer squared Q2=0.3–0.7(GeV/c)2. The measurement was performed at Jefferson Lab (JLab) in Hall A using recoil polarimetry. With a total uncertainty of approximately 1%, the new data clearly show that the deviation of the ratio μpGE/GM from unity observed in previous polarization measurements at high Q2 continues down to the lowest Q2 value of this measurement. The updated global fit that includes the new results yields an electric (magnetic) form factor roughly 2% smaller (1% larger) than the previous global fit in this Q2 range. We obtain new extractions of the proton electric and magnetic radii, which are 〈rE2〉1/2=0.875±0.010 fm and 〈rM2〉1/2=0.867±0.020 fm. The charge radius is consistent with other recent extractions based on the electron–proton interaction, including the atomic hydrogen Lamb shift measurements, which suggests a missing correction in the comparison of measurements of the proton charge radius using electron probes and the recent extraction from the muonic hydrogen Lamb shift