Number 417 (Story #2), March 8, 1999 by Phillip F. Schewe and Ben Stein|
PARITY NONCONSERVATION (PNC) IN ATOMS is an area of fundamental physics--- testing parity conservation, or the proposition that interactions are the same even if you view them in a mirror ---carried out not at an immense particle accelerator but on a tabletop. Atoms are chiefly governed by the electromagnetic force, an ally of parity conservation but, according to current theory, also feel a very faint tug from the weak nuclear force, a notorious abuser of parity. Evidence for this has been the observation of rare "forbidden" transitions between particular atomic levels. For example, Carl Wieman of the University of Colorado (303-492-6963, email@example.com) sees such transitions in cesium by detecting the fluorescence from 6S atoms boosted into the 7S state (see Physics Today, April 1997). In chemistry class, one learns of S and P states, in which configurations of the atoms' electrons is such as to give the atom spherical and dumbbell shapes, respectively. (Other states, corresponding to even higher angular momentum levels, also exist.) A photon, with an angular momentum unit of one, cannot link to S states, but a Z boson (one harbinger of the weak force) can. Now new precision in theoretical calculations of the transitions have progressed to a point where the theory of the electroweak force can be put to the test. The Colorado comparison reveals a small but intriguing discrepancy between theory and observation. Added impetus for these studies is the fact that atomic PNC is sensitive to electron-quark interactions different from those explored in high energy experiments. (S.C. Bennett and C.E. Wieman, Physical Review Letters, 22 March 1999; see figure at Physics News Graphics).