High-precision measurement of parity nonconserving optical rotation in atomic lead

We have completed a new measurement of parity nonconserving (PNC) optical rotation near the 1.279 μm magnetic dipole absorption line in lead, improving earlier results on this line by a factor of 20. We find the ratio of the PNC E1 amplitude to the M1 amplitude to be scrR=(-9.86±0.12)×${10}^{\mathrm{-}8}$, consistent with theoretical predictions. Difficulties in the atomic theory of lead presently limit the extent to which our result tests the standard model of electroweak interactions. The amplitude of the nuclear spin-dependent PNC rotation is found to be less than 2×${10}^{\mathrm{-}2}$ of the nuclear spin-independent rotation.