Observation of Parity Nonconservation in Møller Scattering

Abstract

We report a measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: A PV ÿ175 30stat 20syst 10ÿ9 . This first direct observation of parity nonconservation in Møller scattering leads to a measurement of the electron's weak charge at low energy Q e W ÿ0:053 0:011. This is consistent with the standard model expectation at the current level of precision: sin 2 W M Z MS 0:2293 0:0024stat 0:0016syst 0:0006theory. Precision measurements of weak neutral current (WNC) processes mediated by Z 0 exchange stringently test the standard model of electroweak interactions. While most WNC measurements have been performed at high energy colliders, the comprehensive search for new physics at TeV energies also requires precision measurements at low momentum transfer (Q 2 M 2 Z ). One class of such measurements involves the scattering of longitudinally polarized electrons from unpolarized targets, allowing the determination of a parity-violating asymmetry A PV R ÿ L = R L , where RL is the cross section for incident right(left)-handed electrons. A PV arises from the interference of the weak and electromagnetic amplitudes [1] and is sensitive to WNC coupling constants and thus the weak mixing angle W .The first observation of A PV was made at the Stanford Linear Accelerator Center (SLAC) using a deuteron target [2]. That experiment established the basic experimental technique to determine small asymmetries that typically range from 0.1 to 100 parts per million (ppm). Subsequent measurements yielded improved precision and accuracy [3,4]. However, theoretical uncertainties related to the use of hadronic targets restricted the interpretation of the experimental results at the quantum loop level.In this Letter we report the first observation of A PV in electron-electron (Møller) scattering. This purely leptonic reaction has a large cross section and has little theoretical uncertainty. The development of the 50 GeV electron beam in SLAC End Station A (ESA) made possible a measurement of A PV [5] with a precision that tests electroweak radiative corrections and probes physics beyond the standard model at the TeV scale.At 50 GeVand a center-of-mass scattering angle of 90 , A PV in Møller scattering is predicted to be ' 320 parts per billion (ppb) [6] at tree level. Electroweak radiative corrections [7,8] and the experimental acceptance reduce the measured asymmetry by more than 50%. The principal components of the experimental apparatus, designed to measure A PV to better than 10%, were the polarized electron beam, a liquid hydrogen target, a spectrometer/ collimator system, and detectors. Møller-scattered electrons were directed into a calorimeter by a magnetic spectrometer. The asymmetry was measured by extracting the fractional difference in the integrated calorimeter response for incident right-and left-handed beam pulses.The longitudinally polarized electron beam, with up to 5:5 10 11 electrons in 270

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