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A precision low-energy measurement of the weak mixing angle in Moller scattering

Citation

Mastromarino, Peter A. (2004) A precision low-energy measurement of the weak mixing angle in Moller scattering. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-05262004-173512

Abstract

The E-158 experiment at the Stanford Linear Accelerator Center (SLAC) measures the parity-violating cross-section asymmetry in electron-electron (Moller) scattering at low Q^2. This asymmetry, whose Standard Model prediction is roughly -150 parts per billion (ppb), is directly proportional to (1 - 4sin^2q_W, where q_W is the weak mixing angle. Measuring this asymmetry to within 10% provides an important test of the Standard Model at the quantum loop level and probes for new physics at the TeV scale.

The experiment employs the SLAC 50 GeV electron beam, scattering it off a liquid hydrogen target. A system of magnets and collimators is used to isolate and focus the Moller scattering events into an integrating calorimeter. The electron beam is generated at the source using a strained, gradient-doped GaAs photocathode, which produces roughly 5 x 10^11 electrons/pulse (at a beam rate of 120 Hz) with ~80% longitudinal polarization. The helicity of the beam can be rapidly switched, eliminating problems associated with slow drifts. Helicity correlations in the beam parameters (charge, position, angle, and energy) are minimized at the source and corrected for using precision beam monitoring devices.

The parity-violating cross-section asymmetry A_PV in Moller scattering is measured to be A_PV = -160 +/- 21 (stat) +/- 16 (syst) ppb, at an average Q^2 of 0.026 GeV^2. This represents the first observation of parity violation in Moller scattering, and corresponds to the following low-energy determination of the weak mixing angle:

sin^2q_W(Q^2 = 0.026 GeV^2) = 0.2381 +/- 0.0015 (stat) +/ 0.0014 (syst).

This agrees with the Standard Model prediction of 0.2385 +/- 0.0006. Roughly half of the experiment's total data set is represented here. This thesis provides a full description of the experimental method and analysis procedure used to obtain the above result. It also discusses the result's physical implications in terms of possible extensions to the Standard Model.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:asymmetry; E158; electron scattering; parity violation
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Hughes, Emlyn Willard
Thesis Committee:
  • Hughes, Emlyn Willard (chair)
  • Weinstein, Alan Jay
  • Porter, Frank C.
  • Wise, Mark B.
Defense Date:10 May 2004
Author Email:peterm (AT) slac.stanford.edu
Record Number:CaltechETD:etd-05262004-173512
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-05262004-173512
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2083
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:27 May 2004
Last Modified:26 Dec 2012 02:46

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