The MOLLER Experiment

Executive Summary

We present the conceptual design with emphasis on technaical feasibility of the MOLLER experiment, in which we propose to measure the parity-violating asymmetry A_PV in polarized electron-electron (Møller) scattering. In the Standard Model, A_PV is due to the interference between the electromagnetic amplitude and the weak neutral current amplitude, the latter being mediated by the Z⁰ boson. A_PV is predicted to be ≈33 parts per billion (ppb) at our kinematics. Our goal is to measure A_PV to a precision of 0.7 ppb. The result would yield a measurement of the weak charge of the electron Q_W^e to a fractional accuracy of 2.4% at an average Q² of 0.0056 GeV².

The measurement is sensitive to the interference of the electromagnetic amplitude with new neutral current amplitudes as weak as ~10^-3\cdot G_F from as yet undiscovered dynamics beyond the Standard Model. Such discovery reach is unmatched by any proposed experiment measuring a flavor- and CP-conserving process over the next decade, and results in a unique window to new physics at MeV and multi-TeV scales, complementary to direct searches at high energy colliders. Within the Standard Model, the extracted Q_W^e measurement yields a determination of the weak mixing angle sin²θ_W with both precision and accuracy that are unmatched by any conceivable method at Q² < M_Z in the foreseeable future, and matches the uncertainty from the single best such determination from high energy colliders.

The measurement would be carried out in Hall A at Jefferson Laboratory, where a 11 GeV longitudinally polarized electron beam would be incident on a 1.5 m liquid hydrogen target. Mø ller electrons (beam electrons scattering off target electrons) in the full range of the azimuth and spanning the polar angular range 5 mrad < θ_lab < 19 mrad, would be separated from background and brought to a ring focus ≈30 m downstream of the target by a spectrometer system consisting of a pair of toroidal magnet assemblies and precision collimators. The Mø ller ring would be intercepted by a system of fused silica detectors; the resulting Cherenkov light would provide a relative measure of the scattered flux.

Longitudinally polarized electrons are generated via photoemission on a GaAs photocathode by circularly polarized laser light, enabling rapid polarization (helicity) reversal and suppression of spurious systematic effects. A_PV would be extracted from the fractional difference in the integrated Cherenkov light response between helicity reversals. Additional systematic suppression to the sub-ppb level would be accomplished by periodically reversing the sign of the physics asymmetry by three independent methods.

Simultaneously with data collection, the fluctuations in the electron beam energy and trajectory and its potential systematic effects on A_PV would be precisely monitored, active feedback loops would minimize beam helicity correlations, and detector response to beam fluctuations would be continuously calibrated. Background fractions and their helicity-correlated asymmetries would be measured by dedicated auxiliary detectors. The absolute value of Q² would be calibrated periodically using tracking detectors. The longitudinal electron beam polarization would be measured continuously by two independent polarimeter systems.

A strong collaboration with extensive experience in similar experiments is committed to the design, construction and deployment of the apparatus and to data collection and analysis. It is envisioned that construction and assembly will take three years, to be followed by three data collection periods with progressively improved statistical errors and systematic control over a subsequent three to four year period.