First long-distance neutrinos in Japan
30 June 1999
On June 19, 1999, the K2K (KEK to Kamioka) Long Baseline Neutrino Oscillation Experiment in Japan observed its first neutrino from the KEK neutrino beam in the Super-Kamiokande detector, 250km from the beam's origin at the KEK laboratory. Super-Kamiokande is a huge cylindrical detector filled with ultrapure water. Some 9000 phototubes on the walls of the detector pick up any light produced as Cherenkov radiation when charged particles pass through the water. The radiation forms rings of light on the walls of the detector, which is displayed above as if the cylinder has been cut open, and its surfaces laid flat. The neutrino (of muon type) has interacted with an atomic nucleus in the water to produce two charged particles, including a muon (like an electron, but heavier), which decays to an electron. The rings due to the two charged particles and the decay electron are marked in the more detailed image shown here. Both the direction and the time of the event are in the range expected for neutrinos in the beam from KEK.
Credit: K2K/SuperKamiokande
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