A subterranean observatory
Deep inside Mount Ikeno in Japan, 1,000 meters below the surface in the former Mozumi Mine, sits the Super-Kamiokande (Super-K) Observatory. The center of the facility is a colossal stainless steel tank, 41.4 meters tall and 39.3 meters in diameter, containing 50,220 metric tons of ultrapure water. This subterranean location filters out cosmic ray muons and other background radiation, allowing the detector to focus on its elusive quarry: neutrinos.
The inner surface of this tank is covered in sensors. It is lined with 11,146 inward-facing, 50-centimeter-diameter photomultiplier tubes (PMTs). An additional 1,885 smaller, 20-centimeter PMTs monitor the outer region of the tank to veto events from incoming cosmic rays. When a neutrino—a nearly massless particle that interacts with matter—happens to strike a nucleus or an electron in the water, it can produce a charged particle. If this particle travels faster than the speed of light in water, it emits a cone of faint blue light called Cherenkov radiation. This is the optical equivalent of a sonic boom. The thousands of PMTs detect this light, and the pattern and timing of the detected photons allow scientists to reconstruct the neutrino's energy, direction, and type, or "flavor."
Rewriting particle physics
Since beginning operations in April 1996, Super-K has contributed to physics. Its most famous discovery came in 1998, when the collaboration announced the first definitive evidence of neutrino oscillation. By observing atmospheric neutrinos—produced when cosmic rays hit the Earth's atmosphere—scientists found that fewer muon neutrinos were arriving from the other side of the planet than were arriving from directly overhead. This implied the muon neutrinos were changing flavor into tau neutrinos during their long journey through the Earth. This phenomenon is only possible if neutrinos have mass, a property not included in the Standard Model of particle physics. This discovery earned Takaaki Kajita a share of the 2015 Nobel Prize in Physics.
The observatory also is the far detector for the T2K (Tokai to Kamioka) long-baseline experiment. An intense beam of muon neutrinos is generated at the J-PARC facility, 295 kilometers away, and aimed at the Super-K tank. In 2011, this experiment made the first observation of electron neutrinos appearing in the muon neutrino beam, confirming the third known type of neutrino oscillation. Since 2020, the detector has been upgraded with gadolinium, which increases its ability to detect neutrons and distinguish between neutrinos and antineutrinos, important for observing neutrinos from past supernovae across the universe.