Helicity-correlated systematics for SLAC Experiment E158

Experiment E158 at the Stanford Linear Accelerator Center (SLAC) will make the first measurement of parity violation in Moller scattering. The left-right cross-section asymmetry in the elastic scattering of a 45-GeV polarized electron beam with unpolarized electrons in a liquid hydrogen target will be measured to an accuracy of better than 10^-8, with the expected Standard Model asymmetry being approximately 10^-7. Because helicity-correlated (left-right) charge and position asymmetries in the electron beam can give rise to systematic errors in the measurement, great care must be given to beam monitoring and control. We have developed beam current monitors that measure the charge per pulse at the 3 × 10^-5 level and RF cavity beam position monitors that measure the position per pulse to 1 μm, which should allow precisions of 1 ppb and 1 nm for the final integrated charge and position asymmetries, respectively. In addition, since most helicity-correlated systematics in the electron beam can be traced back to the laser that drives the photoemission from the GaAs source cathode, we first use careful control of laser beam polarization, point-to-point imaging, and other techniques to minimize systematics. We also provide the capability of modulating in a helicity-correlated way the laser beam´s intensity and position as it strikes the photocathode, allowing the implementation of active feedbacks to ensure that the average charge and position asymmetries integrate close to zero over the course of the experiment. We present this system of precision beam monitoring and control and report on its performance during a recent commissioning run, T-437 at SLAC, which demonstrated charge and position asymmetry precisions of 12 ppb and 2 nm, respectively.