We have measured parity-violating asymmetries in elastic electron-proton scattering over the range of momentum transfers 0.12 < or =Q2 < or =1.0 GeV2. These asymmetries, arising from interference of the electromagnetic and neutral weak interactions, are sensitive to strange-quark contributions to the currents of the proton. The measurements were made at Jefferson Laboratory using a toroidal spectrometer to detect the recoiling protons from a liquid hydrogen target. The results indicate nonzero, Q2 dependent, strange-quark contributions and provide new information beyond that obtained in previous experiments.

International audienceThe Polarized Electrons for Polarized Positrons experiment at the injector of the Continuous ElectronBeam Accelerator Facility has demonstrated for the first time the efficient transfer of polarization fromelectrons to positrons produced by the polarized bremsstrahlung radiation induced by a polarized electronbeam in a high-Z target. Positron polarization up to 82% have been measured for an initial electron beammomentum of 8.19 MeV=c, limited only by the electron beam polarization. This technique extendspolarized positron capabilities from GeV to MeV electron beams, and opens access to polarized positronbeam physics to a wide community

We have measured the beam-normal single-spin asymmetry in elastic scattering of transversely polarized 3 GeV electrons from unpolarized protons at Q 2 0:15, 0:25 GeV=c 2 . The results are inconsistent with calculations solely using the elastic nucleon intermediate state and generally agree with calculations with significant inelastic hadronic intermediate state contributions. A n provides a direct probe of the imaginary component of the 2 exchange amplitude, the complete description of which is important in the interpretation of data from precision electron-scattering experiments. DOI: 10.1103/PhysRevLett.99.092301 PACS numbers: 25.30.Bf, 13.40.ÿf, 14.20.Dh, 24.70.+s Elastic scattering of electrons from nucleons is usually treated in the single-photon exchange (Born) approximation. Higher order processes, such as two-photon exchange, are generally treated as small radiative corrections. However, interest in two-photon exchange was recently renewed when it was argued that contributions from the real part of this amplitude play a role in the discrepancy between the Rosenbluth separation and polarization transfer measurements of the ratio of the elastic form factors G p E =G p M [1][2][3]. In addition, although the twophoton exchange contribution is small, it is comparable to the parity-violating elastic electron-nucleon scattering asymmetry [4], and recent parity-violation measurements have had to consider possible systematic corrections due to this effect. A good understanding of two-photon exchange contributions can be extended to calculations of diagrams PRL 99,

This contribution describes the latest milestones of a multiyear program to build and operate a compact −300 kV dc high voltage photogun with inverted insulator geometry and alkali-antimonide photocathodes. Photocathode thermal emittance measurements and quantum efficiency charge lifetime measurements at average current up to 4.5 mA are presented, as well as an innovative implementation of ion generation and tracking simulations to explain the benefits of a biased anode to repel beam line ions from the anodecathode gap, to dramatically improve the operating lifetime of the photogun and eliminate the occurrence of micro-arc discharges.

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