Detecting Massive Neutrinos; August 1999; Scientific American Magazine; by Kearns, Kajita, Totsuka; 8 Page(s)

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 reveals 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 atoms, seems to be immortal. Its decay would be a strong indication of processes described by 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 the supposedly mundane neutrino background was not so easy to understand. One such experiment, Kamiokande, was located in Kamioka, Japan, a mining town about 250 kilometers (155 miles) from Tokyo (as the neutrino flies). The name stood for "Kamioka Nucleon Decay Experiment." Scientists there and at the IMB experiment, located in a salt mine near Cleveland, Ohio, used sensitive detectors to peer into ultrapure water, waiting for the telltale flash of a proton decaying.