Nearly-Degenerate Opposite-Parity Levels in

There exists in atomic dysprosium a pair of nearly-degenerate levels of opposite parity. These levels are of potential interest for the study of parity non-conservation because the small energy separation enhances level mixing due to weak-interaction processes. In this paper, we review the unique properties of these levels which make them attractive for measurements of parity non-conservation, and outline the principle of such an experiment now underway. 2 Introduction The study of parity non-conservation (PNC) in atoms is now a mature field. Measurements of PNC effects have reached a precision of 2% or better in several elements. Measurements with higher precision are of interest for several reasons. First, it should be possible to observe the nuclear anapole moment, which arises from PNC in the nucleus and induces a nuclear spindependent component of atomic PNC. In addition, if PNC can be measured to high precision in a chain of isotopes of the same element, information can be obtained about neutron distributions in the nucleus. Finally, if some other method of obtaining neutron distributions is available, measurement of PNC in a chain of isotopes can serve as a high-precision test of the standard model of weak interactions. Measurement of PNC in the system of nearly-degenerate levels of opposite parity in atomic dysprosium (Z=66) offers the possibility in principle to make such a measurement at a level of precision orders of magnitude better than that achieved so far in other elements. This possibility arises because the small energy splitting enhances the weak-interaction induced mixing between the levels. We describe here the unique properties of this system, and will outline the effort now underway to make a first observation of PNC in dysprosium. The nearly-degenerate levels A (even parity) and B (odd parity) both have angular momentum J=10 and lie 19797.96 cm above the ground level (J=8). (The level structure of atomic Dy is tabulated in Ref. 8.) Dysprosium has both even and odd neutronnumber stable isotopes ranging from A=156 to A=164. The magnitude of the energy splitting between A and B is of the order typically associated with hyperfine and isotope shifts, and thus varies greatly among the various HF and IS components. For the even isotopes (which have nuclear spin I=0, and thus no hyperfine structure) this splitting ranges from 235 MHz for Dy to 4200 MHz for Dy. The two odd isotopes (Dy and Dy) both have nuclear spin I = 5/2. Fig. 1 shows the hyperfine structure of levels A and B in Dy. The hyperfine components with F=10.5 of Dy are the closest pair, with a separation of only 3.1 MHz. This pair of levels is used in the current search for PNC. F=12.5