Precision Compton polarimetry for the Q _Weak experiment at Jefferson Lab

Abstract

The Q _Weak experiment, scheduled to run in 2010–2012 in Hall C at Jefferson Lab, will measure the parity-violating asymmetry in elastic electron-proton scattering at 1.1 GeV to determine the weak charge of the proton, $Q^p_{\rm Weak} = 1 - 4 \sin^2 \theta_W$. The dominant experimental systematic uncertainty will be the knowledge of the electron beam polarization. With a new Compton polarimeter we aim to measure the beam polarization with a statistical precision of 1% in one hour and a systematic uncertainty of 1%. A low-gain Fabry–Pérot cavity laser system provides the circularly polarized photons. The scattered electrons are detected in radiation-hard diamond strip detectors, and form the basis for a coincidence trigger using distributed logic boards. The photon detector uses a fast, undoped CsI crystal with simultaneous sampling and integrating read-out. Coincident events are used to cross-calibrate the photon and electron detectors.