SLAC Experiment E158:. a Precision Measurement of the Weak Mixing Angle in MØLLER Scattering

An overview of SLAC experiment E158 is presented. E158 makes a precision measurement of the parity-nonconserving left-right asymmetry in the scattering of longitudinally polarized electrons from the atomic electrons in a hydrogen target (Møller scattering). The asymmetry measures the effective pseudo-scalar weak neutral-current contribution to Møller scattering at an average Q² of 0.03 (GeV/c)², and it is proportional to (1/4 - sin² θ_W), where sin² θ_W is the electroweak mixing angle. The asymmetry will be measured to a precision of 7 × 10^-9, which corresponds to δ(sin² θ_W) 0.0007. A comparison of this measurement with precision asymmetry measurements of the Z⁰ resonance provides the first statistically significant measurement of sin ² hat θ _W ( {M_Z² } ; ) - sin ² hat θ _W ( 0 ; ) , testing the electroweak theory at the quantum loop level away from the Z⁰ resonance. The experimental setup suited to the challenges of this precision measurement is described.