Observation of Parity Nonconservation in Møller Scattering

We report a measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: $A_{\mathrm{PV}}=[-175\pm 30(\mathrm{stat})\pm 20(\mathrm{syst})]\times {10}^{-9}$. This first direct observation of parity nonconservation in Møller scattering leads to a measurement of the electron’s weak charge at low energy $Q_W^e=-0.053\pm 0.011$. This is consistent with the standard model expectation at the current level of precision: ${sin}^2{\theta }_{\mathrm{W}}(M_Z{)}_{\overline{\mathrm{MS}}}=0.2293\pm 0.0024(\mathrm{stat})\pm 0.0016(\mathrm{syst})\pm 0.0006(\mathrm{theory})$.

Figure 1

Schematic plan view of the experimental configuration in SLAC End Station A.

Figure 2

The charged particle radial profile at the calorimeter. The points are the data scan, and the open histogram is the Monte Carlo simulation. Møller (shaded) and $ep$ (hatched) contributions are also shown. Region I and III PMTs were used to measure Møller and $ep$ asymmetries, respectively.

Figure 3

$A_{\mathrm{phys}}$ for each of 24 data samples. Data collected with half-wave plate inserted (removed) at a beam energy of 45 (48) GeV are shown as solid (open) circles (triangles). The solid line represents the grand average, with the expected modulation of the asymmetry sign for each beam energy and half-wave plate state. Only statistical uncertainties are shown.