SLAC's polarized electron source laser system and minimization of electron beam helicity correlations for the E-158 parity violation experiment

Abstract

SLAC E-158 is an experiment designed to make the first measurement of parity violation in Møller scattering. E-158 will measure the right-left cross-section asymmetry, A M øller LR , in the elastic scattering of a 45-GeV polarized electron beam off unpolarized electrons in a liquid hydrogen target. E-158 plans to measure the expected Standard Model asymmetry of ∼ 10 −7 to an accuracy of better than 10 −8 . To make this measurement, the polarized electron source requires for operation an intense circularly polarized laser beam and the ability to quickly switch between right-and left-helicity polarization states with minimal right-left helicity-correlated asymmetries in the resulting beam parameters (intensity, position, angle, spot size, and energy), beam A LR 's. This laser beam is produced by a unique SLAC-designed flashlamp-pumped Ti:Sapphire laser and is propagated through a carefully designed set of polarization optics. We analyze the transport of nearly circularly polarized light through the optical system and identify several mechanisms that generate beam A LR 's. We show that the dominant effects depend linearly on particular polarization phase shifts in the optical system. We present the laser system design and a discussion of the suppression and control of beam A LR 's. We also present results on beam performance from engineering and physics runs for E-158.

A Measurement of the Weak Charge of the Proton through Parity Violating Electron Scattering using the Qweak Apparatus: A 21% Result

“…A good reference for origins of charge asymmetry and feedback principle from the SLAC experiments can be found at [86]. The charge feedback system first measures the charge asymmetry in a "2 min."…”

Measurement Of Neutron Radius In Lead By Parity Violating Scattering Flash ADC DAQ

“…Let us assume that x ′ and y ′ are the orthogonal transmission axis of the optical analyzer with T x ′ and T y ′ are their transmission factors respectively and x ′ makes an angle θ with the Pockels cells' fast axis. Then the intensity asymmetry is given by [44]: …”

Active feedback scheme for minimization of helicity-dependent instrumental asymmetries

“…In previous work [2] we have shown that the upper limit of IA with passively stabilized optical spin reversal is ~5 × 10 −4 . Active feedback to correct this problem has been investigated by several groups [3][4][5][6][7][8], but to our knowledge this is the first scheme that does not make use of electro-optic or electromechanical feedback operating at the helicity-reversal frequency to force instrumental asymmetries to zero. The work reported here was necessitated by the requirements of a larger experiment designed to observe electron circular dichroism (ECD) [9,10].…”