Parity violation in atoms

Optical experiments have demonstrated cases in which mirror symmetry in stable atoms is broken during the absorption or emission of light. Such results, which are in conflict with quantum electrodynamics, support the theory of unification of the electromagnetic and weak interactions. The interpretation of the experimental results is based on exchanges of weak neutral Z0 bosons between the electrons and the nucleus of the atom. A concise review of these phenomena in atomic physics is presented. The role of precise caesium parity-violation experiments, as a source of valuable information about electroweak physics, is illustrated by examples pertaining to experimental conditions which, in some cases, are not accessible to accelerator experiments. We give the basic principles of experiments, some under way and others completed, where a quantitative determination of the nuclear weak charge, QW, which plays for the Z0 exchange the same role as the electric charge for the Coulomb interaction is to be, or has been achieved. In the most recent and most precise experiment the accuracy on QW is limited to 1% by the uncertainty due to atomic physics calculations. Such a result challenges specialists in atomic theory and nuclear structure, since a more accurate determination of QW would mean more stringent constraints upon possible extensions of the standard model. Moreover, clear evidence has recently been obtained for the existence of the nuclear anapole moment, which describes the valence electron interaction with a chiral nuclear-magnetization component induced by the parity-violating nuclear forces. In writing this review, our hope was to make clear that any improvement in atomic parity-violation measurements will allow the exploration of new areas of electroweak physics. § Unité de Recherche Associée au Centre National de la Recherche Scientifique, à l’École Normale Supérieure et à l’Université Pierre et Marie Curie. ‖ Unité Propre du Centre National de la Recherche Scientifique, associée à l’École Normale Supérieure et à l’Université de Paris-Sud. 0034-4885/97/111351+46$59.50 c © 1997 IOP Publishing Ltd 1351 1352 M-A Bouchiat and C Bouchiat