For several years now, detectors at three experiments—Daya Bay in China, RENO in South Korea, and Double Chooz in France—have been measuring the flux of electron antineutrinos emanating from nearby nuclear reactors. The experiments’ main purpose has been to help quantify the oscillations of neutrinos from one flavor to another, but along the way their precise measurements have sharpened a mystery that has troubled particle physicists and reactor physicists alike: The overall antineutrino flux is about 6% less than theory predicts. One explanation involves a “sterile” neutrino with a mass of about 1 eV that, unlike the three standard neutrinos, is not subject to the weak force that governs nuclear decay.
Now the Daya Bay group has analyzed 1230 days of data and identified a new factor that influences the antineutrino flux: the evolving isotopic composition of the nuclear fuel in the cores of the reactors at the Daya Bay...
