The Hall A Collaboration at Jefferson Lab is reporting new precision measurements of the cross section for elastic electron-proton scattering at large values of the momentum transfer Q² and low values of the virtual photon polarization ε. These new data significantly improve the precision of our knowledge of the proton’s magnetic form factor at large Q² values. When combined with existing high-ε data at similar Q², the new Hall A data allow us to nearly double the Q² range for which it is possible to directly separate the contributions of longitudinally (L) and transversely (T) polarized virtual photons to the cross section, in a procedure known as Rosenbluth separation or L/T separation. In the one-photon-exchange approximation, this procedure allows us to separate the electric (G_E) and magnetic (G_M) contributions to the scattering. The new data are consistent with approximate form factor scaling; i.e., μ_p G_E/G_M = 1 (here μ_p = 2.79284734462(82) (according to PDG), is the proton’s magnetic dipole moment in units of the nuclear magneton). This result contradicts the expectations for this ratio based on precise measurements of the proton’s electric/magnetic form factor ratio using the polarization transfer technique. The new data significantly increase the range of Q² for which significant two-photon-exchange contributions to the elastic electron-proton scattering cross section are  conclusively established.

A manuscript reporting the new results has been submitted to Physical Review Letters. The preprint of the manuscript can be found here.

The PREX/PREX-II collaboration is reporting new measurements of the parity-violating asymmetry in elastic electron scattering from the Lead-208 nucleus. This asymmetry, which is generated by the interference between the electromagnetic and weak neutral current interactions between the electron and the nucleus, is highly sensitive to the difference in size between the proton and neutron distributions in the Lead-208 nucleus.

The new measurement improves on the statistical precision of the PREX-I measurement by more than a factor of 3, and has significant implications for the size and composition of neutron stars.

• Preprint of the manuscript reporting the PREX-II result, prepared for submission to Physical Review Letters
• The PREX-I result, published in Physical Review Letters in 2012

The SeaQuest/E906 Collaboration at Fermilab has published new data on measurements of the ratio of anti-down over anti-up quarks in the proton, obtained by studying the Drell-Yan process in collisions of a 120 GeV proton beam from the Fermilab main injector with protons and neutrons in liquid hydrogen and deuterium targets.

In the Drell-Yan process, quark-antiquark annihilation events are clearly identified by detecting high-energy muon-antimuon pairs resulting from the annihilation of a quark (antiquark) in the beam proton with an antiquark (quark) in the target proton or neutron.

By studying Drell-Yan events in kinematics dominated by beam quarks and target antiquarks with large momentum, and comparing measurements on hydrogen and deuterium under the assumption of charge symmetry (same number of down/anti-down quarks in the neutron as up/anti-up quarks in the proton), the collaboration was able to establish an excess of anti-down quarks over anti-up quarks in the proton. This important and highly anticipated result has no clearly agreed-upon theoretical explanation, and contradicts naive expectations of quark “flavor” symmetry in the antiquark distributions.

The paper can be read here.