A kilometer underground
Deep inside a former zinc mine, 1,000 meters under the rock of Mount Ikeno, sits an enormous particle detector. This is the Kamioka Observatory, and its main instrument, Super-Kamiokande (Super-K), is a colossal stainless-steel tank containing 50,000 tons of ultrapure water. The tank itself is a cylinder 41.4 meters high and 39.3 meters wide. The mountain above has a specific purpose: it shields the highly sensitive detector from the constant bombardment of cosmic rays and other background radiation that would otherwise obscure the faint signals it is designed to find.
The detector's target is the neutrino, an elementary particle so elusive it is called a "ghost particle." Neutrinos interact so weakly with other matter that trillions of them pass through the Earth every second without a trace. Super-K's method for spotting the rare interaction is to watch for something called Cherenkov radiation. When a neutrino happens to strike a nucleus or electron in the water, it can produce a charged particle moving faster than the speed of light in water. This creates a cone of faint blue light, which is essentially an optical sonic boom. To see this light, the inner walls of the tank are lined with 11,146 photomultiplier tubes, each 50 centimeters in diameter. These incredibly sensitive light detectors capture the Cherenkov light, allowing scientists to reconstruct the neutrino's energy and direction.
Rewriting physics
The observatory began as a smaller experiment in 1983 called the Kamioka Nucleon Decay Experiment (Kamiokande). Its original goal was to find evidence of proton decay, a phenomenon predicted by some theories but never seen. While it never found a decaying proton, it was exceptionally good at detecting neutrinos. On February 23, 1987, the Kamiokande detector captured 11 neutrinos from Supernova 1987A, an exploding star in the Large Magellanic Cloud some 168,000 light-years away. This was the first time neutrinos from a specific source outside our solar system had ever been detected, and it confirmed theories about how stars collapse.
Its successor, the much larger Super-Kamiokande, began operating in 1996. Two years later, in 1998, the Super-K collaboration announced a discovery that changed fundamental physics. By analyzing atmospheric neutrinos—created when cosmic rays strike the upper atmosphere—they found that fewer muon-neutrinos were arriving from below (having traveled through the Earth) than from above. This indicated that the neutrinos were changing from one type, or "flavor," into another during their journey. This process, known as neutrino oscillation, is only possible if neutrinos have mass. This result overturned a long-held assumption of the Standard Model of particle physics and led to the 2015 Nobel Prize in Physics for researcher Takaaki Kajita.