Two‐Neutrino Double β‐Decay Seen; Neutrinoless Decay Sought

Many nuclei with even numbers of protons and neutrons can undergodouble beta decay, emitting two electrons and two neutrinos, but the halflife for this process is so long that it had been deduced until recently only by measuring the abundance of daughter nuclei from double beta decay of elements in geologic materials. Now Steven Elliott, Alan Hahn and Michael Moe of the University of California at Irvine have observed the double beta decay of selenium‐82 in their laboratory and determined its half life to be $1.1−0.3+0.8×1020$ years, a time interval that is orders of magnitude longer than any previously detected in a laboratory. Besides being a feat in its own right, the Irvine measurement is a milestone en route to a more elusive goal—observation of a double beta decay in which two electrons but no neutrinos emerge. This neutrinoless decay, which proceeds by the exchange of a virtual neutrino between the two neutrons, can occur only if neutrinos have mass. It is strictly forbidden by the standard theory of electroweak interactions but is predicted as a manifestation of the small symmetry breaking that arises in some grand unified field theories.