Measurement of Neutrino Oscillation with KamLAND: Evidence of Spectral Distortion

We present results of a study of neutrino oscillation based on a 766 ton/year exposure of KamLAND to reactor antineutrinos. We observe 258 ν¯_e candidate events with energies above 3.4 MeV compared to 365.2±23.7 events expected in the absence of neutrino oscillation. Accounting for 17.8±7.3 expected background events, the statistical significance for reactor ν¯_e disappearance is 99.998%. The observed energy spectrum disagrees with the expected spectral shape in the absence of neutrino oscillation at 99.6% significance and prefers the distortion expected from ν¯_e oscillation effects. A two-neutrino oscillation analysis of the KamLAND data gives Δm²=7.9^+0.6_-0.5×10^-5 eV². A global analysis of data from KamLAND and solar-neutrino experiments yields Δm²=7.9^+0.6_-0.5×10^-5 eV² and tan(²θ=0.40^+0.10_-0.07, the most precise determination to date.