K2k (kek to Kamioka) Neutrino-oscillation Experiment at Kek-ps * )

In recent years, several underground neutrino observatories 1) 3) have reported that the atmospheric neutrino ratio, R ≡ (νμ + ν̄μ)/(νe + ν̄e), is significantly smaller than the theoretical expectations. This “atmospheric neutrino anomaly” can be explained by a neutrino oscillation hypothesis with an oscillation parameter region of ∆m2 ∼ 10−2eV2 and sin 2θ ∼ 1. The atmospheric neutrino anomaly has also been confirmed by a preliminary result from the Super-Kamiokande experiment, 4), which shows a rather smaller ∆m2 than does Kamiokande. The K2K experiment 5) (formerly called the KEK-PS E362 experiment) is the first long-baseline neutrino-oscillation experiment using an artificial neutrino beam. The almost pure νμ beam from π + decays is generated in the KEK 12-GeV Proton Synchrotron, and is directed toward the Super-Kamiokande detector, which is about 250km away from KEK. The neutrino events observed in the Super-Kamiokande detector are compared with neutrino events in the front detector constructed at the KEK site. The nominal sensitive region on the neutrino-oscillation parameter is ∆m2 = 10−2eV2 ∼ 10−3eV2, which covers the parameter region suggested by the atmospheric neutrino anomaly.