A new limit on the permanent electric

Dipole moment (EDM) of mercury atoms has been set. Many atoms behave like tiny dipole magnets, with their own north and south poles separated by some small distance. In contrast, no permanent EDM has ever been measured for atoms. Such a moment would imply that the centroids of the opposite charges within the neutral atom were slightly offset. Physicists at the University of Washington used an ultraviolet laser to monitor the Zeeman precession frequency of ¹⁹⁹ Hg atoms in parallel electric and magnetic fields, and looked for differences between simultaneous measurements made in two cells having oppositely directed electric fields. The new upper limit on any permanent charge separation in Hg is 2 × 10⁻²⁸ e cm, an improvement by a factor of four. To appreciate this limit, consider that if the Hg atom were the size of Earth, then the centers of its positive and negative charge distributions would be offset by less than 0.001 Å. Because a permanent EDM requires that time-reversal symmetry be violated, the result places new constraints on CP violation in particle physics beyond the Standard Model. (M. V. Romalis et al., Phys. Rev. Lett. 86 , 2505, 2001 http://dx.doi.org/10.1103/PhysRevLett.86.2505 .)