Precision Measurement of Parity Nonconservation in Atomic Cesium: A Low-Energy Test of the Electroweak Theory

We have made an improved measurement of the parity-nonconserving electric-dipole transition amplitude between the $6S$ and $7S$ states of atomic cesium. We obtain $\frac{\mathrm{Im}\left(E_{\mathrm{PNC}}\right)}{\beta =-1.576\mathrm{}\left(34\right)\mathrm{}}$ mV/cm, which is in good agreement with the predictions of the standard model and earlier less precise measurements. This places more stringent constraints on alternatives to the standard model. We also see the first evidence of a nuclear-spin-dependent contribution to atomic parity nonconservation. The nuclear-spin dependence observed is in agreement with that predicted to arise from a nuclear anapole moment.