The world’s largest neutrino detector, a $100 million device in a mine 300 kilometers west of Tokyo, has confirmed a mysterious deficit in the flux of neutrinos from the sun. This 50% shortfall in the number of solar neutrinos predicted by theory is a crucial clue in the quest to find out whether these shadowy particles have mass–a question with profound consequences for physics and cosmology.The data are coming in so fast, says Yoji Totsuka of the University of Tokyo, spokesperson for Japan’s Super-Kamiokande detector, that researchers should soon be able to piece together the neutrinos’ energy spectrum, which may help determine just what causes the shortfall. In just 102 days of running time since April, Super-K “has more events than all previous solar-neutrino experiments have gotten in 30 years,” says John Bahcall of the Institute for Advanced Study in Princeton, New Jersey. “For me, it’s thrilling.”Neutrinos are commonly produced in many kinds of nuclear reactions, including those that power the sun. They come in three “flavors,” one of which–the electron neutrino–is easiest to spot with water-filled detectors like Super-K. And according to physicists’ standard picture of particles and forces, they are massless.The solar shortfall, however, could be a sign of neutrino mass. Bahcall, who is not part of the Super-K team, and others have suggested that neutrino mass could allow the easily observed electron neutrinos to “oscillate,” or transmute, into one of the other two types as they travel from the sun’s core to Earth, eluding detection. Then again, the apparent deficit might just reflect an imperfect understanding of the sun’s inner workings. Totsuka says the energy spectrum of the arriving neutrinos should begin to emerge from the data in a few years–and perhaps reveal which explanation is correct.