SUPER KAMIOKANDE DETECTOR resides in an active zinc mine inside Mount Ikenoyama. Its stainless-steel tank contains 50,000 tons of ultrapure water so transparent that light can pass through 70 meters of it before losing half its intensity (for a swimming pool that figure is a few meters). The water is monitored by 11,000 photomultiplier tubes that cover the walls, floor and ceiling. Each tube is a hand blown, evacuated glass bulb half a meter in diameter. The tubes register conical flashes of Cherenkov light, each of which signals a rare collision of a high-energy neutrino and an atomic nucleus in the water. Technicians in inflatable rafts clean the bulbs while the tank is filled [right). one man's trash is another man's treasure. For a physicist, the trash is background--some unwanted reaction, probably from a mundane and well-understood process. The treasure is signal--a reaction that we hope will reveal new knowledge about the way the universe works. Case in point: over the past two decades, several groups have been hunting for the radioactive decay of the proton, an exceedingly rare signal (if it occurs at all) buried in a background of reactions caused by elusive particles called neutrinos. The proton, one of the main constituents of the atom, seems to be immortal. Its decay would be a strong indication of processes described by the Grand Unified Theories that many believe lie beyond the extremely successful Standard Model of particle physics. Huge proton-decay detectors were placed deep underground, in mines or tunnels around the world, to escape the constant rain of particles called cosmic rays. But no matter how deep they went, these devices were still exposed to penetrating neutrinos produced by the cosmic rays.
The first generation of proton-decay detectors, operating from 1980 to 1995, saw no signal, no signs of proton decay--but along the way the researchers found that j the supposedly mundane CONES OF CHERENKOV LIGHT are emitted when high-energy neutrinos hit a nucleus and produce a charged particle. A muon-neutrino [top] creates a muon, which travels perhaps one meter and projects a sharp ring of light onto the detectors. An electron, produced by an electron-neutrino [bottom], generates a small shower of electrons and positrons, each with its own Cherenkov cone, resulting in a fuzzy ring of light. Green dots indicate light detected in the same narrow time interval. neutrino background was not so easy to understand.