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We present measurements of νμ disappearance in K2K, the KEK to Kamioka long-baseline neutrino oscillation experiment. One-hundred and twelve beam-originated neutrino events are observed in the fiducial volume of Super-Kamiokande with an expectation of 158.1-8.6+9.2 events without oscillation. A distortion of the energy spectrum is also seen in 58 single-ring muonlike events with reconstructed energies. The probability that the observations are explained by the expectation for no neutrino oscillation is 0.0015% (4.3σ). In a two-flavor oscillation scenario, the allowed Δm2 region at sin 22θ=1 is between 1.9 and 3.5×10-3eV2 at the 90% C.L. with a best-fit value of 2.8×10-3eV2. © 2006 The American Physical Society.

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Physical Review D - Particles, Fields, Gravitation and Cosmology