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.

The ratio of the electric and magnetic form factors of the proton $G_{E_p}{/G}_{M_p}$, which is an image of its charge and magnetization distributions, was measured at the Thomas Jefferson National Accelerator Facility (JLab) using the recoil polarization technique. The ratio of the form factors is directly proportional to the ratio of the transverse to longitudinal components of the polarization of the recoil proton in the elastic $\overrightarrow{e}p\to e\overrightarrow{p}$ reaction. The new data presented span the range $3.5<\mathrm{}{Q}^2<5.6\mathrm{}{\mathrm{GeV}}^2$ and are well described by a linear $Q^2$ fit. Also, the ratio $\sqrt{Q^2}{F}_{2_p}{/F}_{1_p}$ reaches a constant value above $Q^2=2\mathrm{}{\mathrm{GeV}}^2$.

The protons and neutrons in a nucleus can form strongly correlated nucleon pairs. Scattering experiments, in which a proton is knocked out of the nucleus with high-momentum transfer and high missing momentum, show that in carbon-12 the neutron-proton pairs are nearly 20 times as prevalent as proton-proton pairs and, by inference, neutron-neutron pairs. This difference between the types of pairs is due to the nature of the strong force and has implications for understanding cold dense nuclear systems such as neutron stars.

We present a search at the Jefferson Laboratory for new forces mediated by sub-GeV vector bosons with weak coupling ${\alpha }^{\prime }$ to electrons. Such a particle $A^{\prime }$ can be produced in electron-nucleus fixed-target scattering and then decay to an $e^{+}{e}^{-}$ pair, producing a narrow resonance in the QED trident spectrum. Using APEX test run data, we searched in the mass range 175–250 MeV, found no evidence for an $A^{\prime }\to e^{+}{e}^{-}$ reaction, and set an upper limit of ${\alpha }^{\prime }/\alpha \simeq {10}^{-6}$. Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.

Scattering electrons off nuclear targets: Atomic nuclei consist of fermions—protons and neutrons—bound together by interactions. Because two identical fermions cannot occupy the same quantum state, both protons and neutrons have a broad range of momenta inside the nucleus. Hen et al. scattered electrons off nuclei of varying sizes to study the distribution of the protons' and neutrons' momenta. Protons formed high-momentum pairs with neutrons much more frequently than with other protons. Thus, surprisingly, the average momentum of a neutron was lower than that of a proton, even in nuclei with a larger number of neutrons than protons. Science , this issue p. 614

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.