Parity Violation in Elastic Scattering of Polarized Electons from Carbon

This thesis describes a measurement of the parity -violating asymmetry in elastic scattering of polarized electrons from ^{12}C. The asymmetry is due to the isoscalar vector hadronic piece of the weak neutral current and is obtained by measuring the interference between the weak and electromagnetic interactions in the scattering of longitudinally polarized electrons from a spinless, isoscalar target. It is predicted by the Standard Model to be dependent only on the weak mixing (or Weinberg) angle. This experiment was performed at the MIT-Bates Linear Accelerator Center at an energy of 250 MeV and a momentum transfer of 150 MeV/c. The scattered electrons were detected at an angle of 35^circ to either side of the beam axis. Longitudinally polarized electrons were provided by a high-intensity GaAs photocathode source and their polarization was measured with a Moller polarimeter. Electron helicity was determined on a pulse-to-pulse basis and the asymmetry was obtained by comparing the scattering cross sections for right- and left-handed electrons. We achieved excellent control of the systematic effects, particularly those relating to helicity-correlated intensity fluctuations in the the beam and to crosstalk in the electronics. In addition, we measured the response of the scattering cross section to the fluctuations in the beam energy, position and angle, and corrected for these effects in our analysis. Overall, we demonstrated the ability to measure the parity-violating asymmetry at the 10^{-7}<=vel and to control the systematic errors at the level of 3%. The techniques developed to do this are described in detail. The measured parity-violating asymmetry is (6.0 +/- 1.4 +/- 0.2) times 10^{-7 } where the first error is purely statistical and the second is purely systematic. From this we extract a value for the isoscalar vector hadronic coupling constant, ~gamma = 0.136 +/- 0.032 +/- 0.009. Based on the predictions of the Standard Model, this leads to a value for the weak mixing angle, given by sin ^2 theta_{W} = 0.203 +/- 0.048 +/- 0.014. This is in agreement with a value of sin^2 theta_ {W} = 0.2327 +/- 0.0018 +/- 0.0007 obtained from the latest measurements of the masses of the W^ +/- and Z^0 intermediate vector bosons. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617 -253-5668; Fax 617-253-1690).