Weighing the neutrino.

Does the neutrino have a rest mass? When physicists first postulated its existence, the neutrino has to be a neutral particle of 0 rest mass, which was needed to properly balance energy in certain radioactive decays. When it was discovered experimentally, the mass was found to be 0, within the accuracy limits of the experiment. Recent theories give the neutrino an important place in the overall scheme of physics, and part of that importance depends on whether or not it has a tiny rest mass.

A few years ago some experimenters in the Soviet Union reported a small rest mass for the neutrino. Since then one experiment done in the United States has claimed a positive result; several have come up 0. Now J.J. Simpson of the University of Guelph in Ontario claims a positive result of 17,100 electron-volts -- about 4 percent of the electron's mass, the smallest now known for certain. Simpson's experiment observes radioactive decay of tritium (an isotope of hydrogen) embedded in a silicon-lithium detector. Although the neutrino is not directly seen, the energy balance of particles emitted with it can be used to figure its mass. In the April 29 Physical REVIEW LETTERS Simpson invites others to copy his method with theh remark, "The effects of such neutrinos should be seen in all [beta dacay] spectra for which their emission is energetically allowed."