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.

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,

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The Macromolecular Femtosecond Crystallography (MFX) instrument at the Linac Coherent Light Source (LCLS) is the seventh and newest instrument at the world's first hard X-ray free-electron laser. It was designed with a primary focus on structural biology, employing the ultrafast pulses of X-rays from LCLS at atmospheric conditions to overcome radiation damage limitations in biological measurements. It is also capable of performing various time-resolved measurements. The MFX design consists of a versatile base system capable of supporting multiple methods, techniques and experimental endstations. The primary techniques supported are forward scattering and crystallography, with capabilities for various spectroscopic methods and time-resolved measurements. The location of the MFX instrument allows for utilization of multiplexing methods, increasing user access to LCLS by running multiple experiments simultaneously.