A Precision Measurement of the Weak Mixing Angle in Møller Scattering at Low Q₂

Author: Jones, Gary Mark

Year: 2004

Degree: Dissertation (Ph.D.)

Advisor: Hughes, Emlyn Willard

Committee Members: Hughes, Emlyn Willard; Hitlin, David G.; Kamionkowski, Marc P.; Golwala, Sunil

Option: Physics

Abstract

The electroweak theory has been probed to a high level of precision at the mass scale of the Z^o through the joint contributions of LEP at CERN and the SLC at SLAC. The E158 experiment at SLAC complements these results by measuring the weak mixing angle at a Q² of 0.026 (GeV/c)², far below the weak scale.

The experiment utilizes a 48 GeV longitudinally polarized electron beam on unpolarized atomic electrons in a target of liquid hydrogen to measure the parity-violating asymmetry A^PV in Moller scattering.

The tree-level prediction for A^PV is proportional to 1-4sin²θ_W. Since sin²θ_W is approximately 0.25, the effect of radiative corrections is enhanced, allowing the E158 experiment to probe for physics effects beyond the Standard Model at the TeV scale.

This work presents the results from the first two physics runs of the experiment, covering data collected in the year 2002. The parity-violating asymmetry A^PV was measured to be

A^PV = -158 ppb +/- 21 ppb (stat) +/- 17 ppb (sys). The result represents the first demonstration of parity violation in Moller scattering. The observed value of A^PV corresponds to a measurement of the weak mixing angle of sin²θ_W^eff = 0.2380 +/- 0.0016 (stat) +/- 0.0013 (sys), which is in good agreement with the theoretical prediction of sin²θ_W^eff = 0.2385 _+/- 0.0006 (theory).

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