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 ${\overline{\ensuremath{\nu}}}_{e}$ candidate events with energies above 3.4 MeV compared to $365.2\ifmmode\pm\else\textpm\fi{}23.7$ events expected in the absence of neutrino oscillation. Accounting for $17.8\ifmmode\pm\else\textpm\fi{}7.3$ expected background events, the statistical significance for reactor ${\overline{\ensuremath{\nu}}}_{e}$ 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…