Six years ago, John Simpson at the University of Guelph in Ontario reported a rather disquieting discovery. In the beta decay of tritium, he asserted, a small fraction of decay events appear to produce a neutrino weighing 17 keV. That's 130, the mass of the electron—very much larger than anyone was looking for in the many attempts to find evidence for a nonvanishing neutrino mass in the end point of the electron energy spectrum from tritium decay.

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