Breit correction to the parity-nonconservation amplitude in cesium

Parity nonconservation ~PNC! in atoms, described in the standard model of the electroweak interaction by exchange of Z bosons between bound electrons and nuclear quarks, leads to nonvanishing electric-dipole matrix elements between atomic states with the same parity. The dominant part of PNC matrix elements ~arising from the vector nucleon current! is proportional to a conserved weak charge QW , which is sensitive to physics beyond the standard model such as the existence of additional neutral Z8 bosons. Measurements of the 6s-7s PNC amplitude in Cs, following the procedure described by Bouchiat and Bouchiat @1#, were carried out at the 2% level of accuracy by Gilbert and Wieman @2#. When combined with calculations of the PNC amplitudes, which were estimated to be accurate to 1% @3–5#, the measurements led to an experimental value for the weak charge,