Number 154 (Story #1), December 3, 1993 by Phillip F. Schewe and Ben Stein|
THE MOST ACCURATE MEASUREMENT OF PARITY VIOLATION in an atom has been made by researchers at the University of Washington (D.M. Meekhof et al., Physical Review Letters, 22 Nov.1993). The electromagnetic and strong nuclear forces conserve parity; that is, they do not differentiate between left and right. The weak force, in contrast, does not conserve parity, a fact discovered first in the study of nuclear decays. One way for parity violation to occur in atoms is when one of its electrons gets close enough to the nucleus to experience the weak force. The Washington researchers (contact Steve Lamoreaux, 206-543-2540) detected parity violation in atoms by measuring a slight change (10**-7 radians, with an accuracy of 1%) in the polarization angle of light passing through a vapor of lead atoms. However, because of uncertainties in the atomic theory of that atom, the theoretical predictions for parity violation in lead is uncertain to an 8% level, so an exacting test of electroweak theory is not yet possible. Future plans would involve measuring the effects in different lead isotopes, which would cancel out the uncertainties. The group plans to use their techniques to make precision measurements of parity violation in thallium, which has much smaller theoretical uncertainties than lead. In addition, the researchers are hoping to detect parity violation that occurs within the nucleus. Specifically, they are looking to make the first measurements of the "anapole" moment, the internal electromagnetic moment in the nucleus which comes about because of the weak force.