Improved measurement of parity nonconserving optical rotation in atomic lead

We have measured parity nonconserving optical rotation near the 63P0 → 63P1, 1.28 μm magnetic dipole transition in lead. Electroweak interactions in the lead atom contribute a small E1 amplitude to this transition. The interference between these two amplitudes leads to a peak optical rotation of order 10-6 radians in our experiment. Our apparatus uses a tunable external cavity InGaAs diode laser and calcite polarizers as a sensitive polarimeter. By alternately placing a tube of heated lead vapor or an empty tube in the optical path, we can effectively subtract spurious background rotations. Measurement of the parity-conserving Faraday rotation is used as a check of the absolute angle calibration. Data have been obtained for a variety of operating conditions including a range of optical depths from 10 to 50. We have measured R, the ratio of the PNC E1 amplitude to the M1 amplitude, to better than 1% statistical accuracy. Possible systematic effects at the 1% level continue to be investigated. Current results will be presented