In all electroweak measurements involving a polarized electron beam, the magnitude of the polarization enters linearly into the measured quantity. Therefore, it is crucial to measure and monitor the magnitude of the electron beam polarization as accurately as possible.
The primary electron beam polarization measurements are undertaken with the Compton Polarimeter \cite{comprop}. This device makes use of the cross section asymmetry in electron-photon scattering. The Compton scattering process is shown in Figure~\ref{fig:compton}. Circularly polarized photons are backscattered off of the electrons in the center of momentum frame. The scattered electrons are momentum analyzed by means of a bending magnet and then detected in both a \v Cerenkov detector and proportional tube detector. By measuring the Compton asymmetry and the photon polarization, the electron polarization can be extracted. %{/net/bovine/home/kevin/ps/pix/slc/compton_3.ps} The Compton Scattering Process.
The layout of the Compton polarimeter is shown in Figure~\ref{fig:compt}. The electrons are brought into collision with the laser approximately 40 meters after the \ep collision point. The laser fires at a rate of 10.9 Hz, coinciding with every eleventh SLC pulse. Data from every laser-beam crossing are used to measure the electron beam polarization. A statistical error of 1-2\% is achieved in a three minute run. The Compton data is shipped to the SLD data acquisition system and written to tape asynchronously with the triggered data from the other subsystems.
It should be noted that the helicity of each pulse is transmitted by means of redundant lines from the Polarized Electron Source directly to both the SLD acquisition system and the Compton polarimeter on a pulse-by-pulse basis. The magnitude of the electron beam polarization is measured every few minutes \cite{compton}. The beam polarization assigned to each event is a combination of the helicity from that beam crossing along with the polarization magnitude from the nearest polarimeter measurement in time. \begin{figure}[hbt] \hspace*{1.65in} % \epsfysize=1.7in \epsfysize=6.5truein \epsffile[605.5 607.5 1100.5 1192.5] The Compton Polarimeter. Polarized photons are scattered off the extracted electrons. The electrons are momentum analyzed to determine the beam polarization.
In addition to the primary Compton polarimeter, two other polarimeters utilize Moller scattering (\mol) to measure beam polarization. Both detectors make use of a foil target in an external magnetic field. The polarized beam is then incident upon a target of polarized electrons. The Moller asymmetry is small and more difficult to extract than is the Compton asymmetry. The Linac Moller polarimeter is stationed at the end of the linear accelerator. Beam must be diverted from the arc to the Linac Moller target. The Extraction Line Moller has only been used sparingly due to high backgrounds in the extraction line area. Also, the location of the Extraction Line Moller is incompatible with the normal operation of the WISRD.