<!--intro-->The next time a distant supernova glitters in the night sky, scientists may be able to solve a mystery about subatomic particles here on Earth. An Ohio State University astrophysicist and his colleagues have devised a way to use the speed of material streaming outward from a supernova to measure the mass of an elusive subatomic particle known as the neutrino.<!--/intro--> Knowing the mass of this particle may help scientists better understand nuclear reactions inside stars, as well as the so-called missing dark matter of the universe, said Richard Boyd, professor of physics and astronomy at Ohio State. Scientists currently believe that three types of neutrinos exist, each with a different mass some ten thousand times less than the mass of an electron, Boyd explained. If so, then heavier neutrinos ejected from a supernova will take longer to reach Earth than lighter neutrinos, Boyd and his coauthors wrote in a recent issue of Physical Review Letters. Boyd's collaborators include John Beacom, formerly a postdoctoral researcher at the California Institute of Technology and now a research fellow at Fermi National Accelerator Laboratory; and Anthony Mezzacappa, head of the Supernova Theory Group at Oak Ridge National Laboratory. Boyd described neutrinos as the key to scientists' understanding of the nuclear reactions that take place in stars. According to current theories, millions of neutrinos should be radiating out of our sun, or other stars, every second. "If we don't know the mass of neutrinos, then we can't use that information to test our theories," Boyd said. "These extremely small masses are hard to measure here on Earth, but if we could measure the differences in flight time of neutrinos from a supernova, we could improve our measurements a million times over." Neutrinos are the most penetrating subatomic particles known, Boyd said. They pass easily through stars, the Earth -- and very often through solid lead. Scientists have built giant underground water tanks to catch neutrinos that pass through. While many neutrino-induced events have been observed, no one has succeeded in measuring the masses of neutrinos, despite decades of effort. At the same time, scientists have been working to explain certain gravitational effects that indicate much of the mass of the universe may be made of unseen, or "dark," matter. If neutrinos exist in the numbers scientists expect, even with a tiny mass, then they make an ideal candidate for dark matter, because they are both abundant and nearly invisible. The researchers' new technique for measuring neutrino mass hinges on the idea that about half of the supernovas that occur in the future -- at least, the ones we can observe from Earth -- will spawn black holes. Only a small portion of stars end their lives in supernovas -- cataclysmic explosions so bright that the star may temporarily outshine its home galaxy. While only a handful of supernovas have been recorded in the Milky Way Galaxy since the early 17th century, all have occurred close to Earth. This suggests that most galactic supernovas are hidden from astronomers' view. But they would not be at all hidden from the supernova neutrino detectors, Boyd said. Boyd explained that as an exploding star collapsed to form a black hole, the star could release 99 percent of its final energy in the form of neutrinos. The very last neutrinos released would all have to leave the star at the same time -- just before the black hole formed. Like the crack of a starting pistol before a race, the instant when a black hole forms could give researchers a definite starting point for timing a neutrino's journey from a supernova to Earth. As the neutrinos raced to Earth, heavier neutrinos should fall behind lighter neutrinos, if only by a second or two in tens of thousands of years of travel, Boyd said. "It's a very small time shift, but one we can measure," he added. "And it would allow us the most precise way we've ever had of detecting the masses of neutrinos." Boyd estimates that Earth will witness at least a few supernovas in the next hundred years. Ohio State is one of a team of institutions collaborating on the design of a new detector, the Observatory for Multiflavor Neutrinos from Supernovae (OMNIS). The word "multiflavor" refers to scientists' dubbing of the three different types of neutrinos as different "flavors" of the particle. If OMNIS secures the funding it plans to request from the National Science Foundation (NSF) and the Department of Energy, the member institutions will construct a lead and iron detector in a salt mine in New Mexico. Today's detectors can only detect one type of neutrino, but the OMNIS detector will be able to detect the other two, Boyd said. Scientists expect all three types of neutrinos to be emitted from a supernova. The challenge is to determine which type has been detected, Boyd said. OMNIS will be able to detect the two types other detectors would see only very faintly. Boyd's part of the collaboration was funded by the NSF. Mezzacappa received support from Oak Ridge National Laboratory, managed by the non-profit company UT-Battelle, LLC, for the U.S. Department of Energy. Beacom's work was funded by the California Institute of Technology.