Observations of parity nonconservation in atomic thallium

A detailed account is given of observations of parity nonconservation in the $6{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}-7\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ transition in ${}_{81}{}^{203,205}{}_{}{}^{}\mathrm{Tl}$. Absorption of circularly polarized 293-nm photons by $6{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ atoms in an $E$ field results in polarization of the $7{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ state through interference of the Stark $E1$ amplitude with $M1$ and parity-nonconserving $E1$ amplitudes. This polarization is detected by selective excitation of $m_F=\pm 1$ components of the $7{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ state to the $8{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state and observation of the ensuing decay fluorescence at 323 nm. Systematic corrections due to imperfect circular polarization, misaligned $E$ fields, and residual magnetic fields are determined precisely by a series of auxiliary experiments. The result is expressed in terms of the circular dichroism ${\delta }_{\mathrm{expt}}=+\left({2.8}_{-0.9\mathrm{}}^{+1.0\mathrm{}}\right)\times {10}^{-3\mathrm{}}$, to be compared with estimates based on the Weinberg-Salam model for ${{sin}^2\theta }_w=0.23:{\delta }_{\mathrm{theo}}=+(2.1\mathrm{}\pm 0.7)\times {10}^{-3\mathrm{}}$.