Sources of Helicity-correlated Electron Beam Asymmetries

The availability of high current, high polarization electron beams from laser‐driven GaAs photocathodes has enabled a broad program studying parity violation in electron scattering. Precision measurements of the tiny (<1 part per million) parity‐violating beam‐helicity asymmetry are used to study the structure of nuclei or to test the Standard Model of electroweak interactions. As these experiments grow ever more precise, asymmetric beam properties between the two beam helicity states threaten to become the leading source of experimental uncertainty.These helicity‐correlated beam asymmetries are predominantly created in the conversion of circularly polarized laser light to a polarized electron beam. In recent experiments at Jefferson Lab, improved techniques for configuring the laser optics of the polarized source have been used to control beam asymmetries to such a level as to be a negligible source of systematic uncertainty. This successful result serves as a promising start for the next generation of p...