for the K2K collaboration

The K2K experiment is the first long-baseline neutrino-oscillation experiment with hundreds of km distance using an accelerator-based neutrino beam. The nominal sensitive region in the neutrino-oscillation parameters is ∆m > 3×10eV. This covers the parameter region suggested by the atmospheric neutrino anomaly observed by several underground experiments, and confirmed by Super-Kamiokande(SK). An overview of the K2K experiment is as follows. Almost a pure wideband νμ beam from π + decays is generated in the KEK 12-GeV/c Proton Synchrotron (PS) and a neutrino beam-line, and is detected in SK at a distance of 250km. Various beam monitors along the beam line and two different types of front detectors (FDs) are also constructed at the KEK site. The FDs are a 1kt water Cherenkov detector (1KT), which is a miniature of the SK detector, and a so-called fine-grained detector (FGD), which is composed of a scintillating fiber tracker (SFT), trigger counters (TRG), lead glass counters (LG) and a muon range detector (MRD). Since the design and performance of these components as well as the properties of the neutrino beam were already described precisely in previous articles, they are not discussed here. The K2K experiment was successfully started in early 1999, and data were recorded in January to March, and May to June in 2000. The total data-taking period in 1999 and 2000 was 112.2 days. The accumulated beam intensity was 22.9×10 protons on target (p.o.t.), which is about 20% of the goal of the experiment, 10 p.o.t.