The matrix element of a bound electron interacting with the nucleus through exchange of a Z boson is studied for the gauge invariant case of $2s_{ 1/2}-2p_{ 1/2}$ transitions in hydrogenic ions. The QED radiative correction to the matrix element, which is $-\alpha/2\pi$ in lowest order, is calculated to all orders in $Z\alpha$ using exact propagators. Previous calculation of the first-order binding correction are confirmed both analytically and by fitting the exact function at low $Z$. Consequences for the interpretation of parity nonconservation in cesium are discussed.
Categories: hep-ph