Evidence for Muon Neutrino Oscillation in an Accelerator-Based Experiment

We present results for νμ oscillationin the KEK to Kamioka (K2K) long-baseline neutrino oscillationexperiment. K2K uses an accelerator-produced νμ beam with a mean energy of 1.3 GeV directed at the Super-Kamiokandedetector. We observed the energy-dependent disappearance of νμ, which we presume have oscillatedto ντ. The probability that we would observe these results if there is no neutrino oscillationis 0.0050% (4.0σ).