Perspective: Atomic Parity Violation and the Nuclear Anapole Moment

(February 9, 2008) Until 40 years ago, physicists had assumed that the fundamental forces of nature did not distinguish between left and right. That is, it was believed that the laws of physics in a mirror-symmetric universe would be the same as in ours. Then in 1957, following a suggestion by Lee and Yang [1], experimenters discovered that the weak force, which is responsible for beta decay in nuclei, violated this conservation of parity [2]. Shortly thereafter, the Soviet physicists V. G. Vaks and Ya. B. Zeldovich [3] independently noted that particles could then have parity-violating couplings to the electromagnetic field. Such “anapole moments” would arise, in the more modern language of today’s standard model of electroweak interactions, from very small effects associated with exchange of W or Z bosons between, for example, the quarks within a nucleon or nucleus. In this issue of Science, the first definitive measurement of an anapole moment is reported by the University of Colorado group of C. S. Wood et al [4]. Some nuclear interactions with the electromagnetic field are quite familiar. As a charged object, the nucleus accelerates when an electric field is applied. If the nucleus has a nonzero spin ~ I, it also has an interaction with an applied magnetic field ~ B of the form μ ~ B · ~ I, where μ is the magnetic moment. More exotic interactions can arise when symmetries preserved by electromagnetism are violated by other, weaker forces. Perhaps the best known of these is the electric dipole moment dN , which can be visualized as an asymmetric distribution of charge along a particle’s spin axis. A particle with an intrinsic dipole moment will experience an interaction dN ~ E ·~ I when placed in an electric field ~ E. Electric dipole moments arise only if