Neutrinos are tiny, electrically neutral, sub-atomic particles.
The experiment yields results that are outside the standard theory of particle physics, which describes the fundamental constituents of matter and their interactions. Until now, there has been no firm evidence that neutrinos possess mass. The new evidence is based upon studies of neutrinos which are created when cosmic rays, fast-moving particles from space, bombard the earth's upper atmosphere, producing cascades of secondary particles which rain down upon the earth. Most of these neutrinos pass through the entire earth unscathed. Faint flashes of light given off by the neutrino interactions in the tank are detected by more than 13,000 photomultiplier tubes that were manufactured for the experiment by Hamamatsu Corporation.