New observation of parity nonconservation in atomic thallium

Refined observations of parity nonconservation in the 6{sup 2}P{sub 1/2} -7{sup 2}P{sub 1/2} transition in {sub 81}Tl {sup 203,205} are reported. Absorption of circularly polarized 293 nm photons by 6{sup 2 }P{sub 1/2} atoms in an E field results in polarization of the 7{sup 2}P{sub 1/2} state, through interference of the Stark E1 amplitude with M1 and parity-nonconserving E1 amplitudes. Detection of this polarization yields the circular dichroism δ=+(2.8±{sup 1.0}{sub 0.9})×10{sup -3}, which agrees with theoretical estimates based on the Weinberg-Salam model, for sin{sup 2} θ{sub W} =0.23 . The present experiment is an improved version of an earlier one in which the result δ = +(5.2 ± 2.4) x 10{sup -3} was obtained.