A Precision Low-Energy Measurement of the Weak Mixing Angle in Møller Scattering
<p>The E-158 experiment at the Stanford Linear Accelerator Center (SLAC) measures the parity-violating cross-section asymmetry in electron-electron (Møller) scattering at low Q². This asymmetry, whose Standard Model prediction is roughly -150 parts per billion (ppb), is directly proportional t...
| Summary: | <p>The E-158 experiment at the Stanford Linear Accelerator Center (SLAC) measures the parity-violating cross-section asymmetry in electron-electron (Møller) scattering at low Q². This asymmetry, whose Standard Model prediction is roughly -150 parts per billion (ppb), is directly proportional to (1 - 4sin²θ<sub>W</sub>, where θ<sub>W</sub> 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.</p>
<p>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 Møller 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 10¹¹ 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.</p>
<p>The parity-violating cross-section asymmetry A<sub>PV</sub> in Møller scattering is measured to be A<sub>PV</sub> = -160 ± 21 (stat) ± 16 (syst) ppb, at an average Q² of 0.026 GeV². This represents the first observation of parity violation in Møller scattering, and corresponds to the following low-energy determination of the weak mixing angle:</p>
<p>sin²θ<sub>W</sub>(Q² = 0.026 GeV²)<sub>M̅S̅</sub> = 0.2381 +/- 0.0015 (stat) ± 0.0014 (syst).</p>
<p>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.</p> |
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