Boston University physicist Chris Walter turns to Makoto Mura, sitting at a workstation in the underground operations center here at the Kamioka Observatory, and asks, "How many events do we see a day?" He's referring to the telltale signatures from neutrinos encountering the 50,000 tons of water sitting in a tank nearby.
"Maybe 10 or 12," replies Dr. Mura, a staff member with the University of Tokyo's Institute for Cosmic Ray Research, which runs the observatory.
But physicists must look at thousands of such events to draw confident conclusions about neutrinos' properties and sources. A neutrino's interaction with other particles in the tank is extremely rare. Fortunately, nature provides countless neutrinos, which flit throughout the universe at the speed of light. And humans supply particle-rich targets like Super Kamiokande's tank of water. Some 20 trillion neutrinos from cosmic-ray collisions with the atmosphere, for example, pass through Super K's tank each day. On average eight leave their mark
When a neutrino does interact with a nucleus in the water, it might produce another neutrino, which continues to flit undetected. Or it might vanish, giving its energy to an electron, or to one of an electron's heavier cousins, a muon or a tau. Like an electron, these cousins carry a negative electrical charge. Initially, these freshly minted charged particles travel at the neutrino's speed-of-light pace. But the speed of light in water is slower than in a vacuum. So as the new particle travels, it produces a shock wave in the water that shows up as an expanding cone of light. This light, dubbed Cherenkov radiation, continues to travel until it hits the photomultiplier tubes, which cover the detector's wall, floor, and ceiling. The tubes turn the light into electronic signals and amplify them. When displayed on a screen, the signals from the array of tubes provide a mosaic physicists can analyze to reconstruct the neutrino's direction of travel, source, energy level, and type: electron, muon, or tau.