Measurement of the scalar Stark shift of the 6 1 S 0 \ 6 3 P 1 transition in Hg

Precision measurements of the Stark shifts in alkali-metal atoms @1,2# have been used to refine the calculations of alkali-metal atomic wave functions, motivated, in part, by precision parity nonconservation measurements in Cs. Hg atoms are used to set one of the most significant limits on a permanent electric dipole moment ~EDM! @3#. The search for an EDM does not require very precise atomic calculations, since any nonzero EDM would imply T and CP violation beyond the Standard Model. Nevertheless, recent calculations of CP-violating effects in Hg @4# indicate that current atomic theory is one of the leading uncertainties in the interpretation of the Hg EDM limit. The EDM of Hg is due to the Schiff moment of the Hg nucleus, which is produced by CP-violating nucleon-nucleon interactions and has been calculated using a nuclear shell model with an estimated accuracy of 50% @5#. Possible enhancements of the Schiff moment due to collective nuclear excitations have been considered recently @7#, although no definite estimates exist. Atomic calculations, relating the Schiff moment to the EDM of Hg atom, have been done using many-body perturbation theory with an estimated accuracy of 50% @6#. Finally, the size of the CP-violating nucleon-nucleon interactions has been calculated in terms of the chromo-EDMs of the quarks with an uncertainty of only 20% @4#. Based on the existing calculations, the limits on fundamental CP-violating effects set by the Hg EDM experiment are comparable to or better than the limits set by the experiments searching for a permanent EDM of an electron @8# or a neutron @9#. Thus, improvements in the atomic and nuclear theory of Hg will better define the range of CP-violating parameters allowed by present and future experimental limits.