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 ${\overline{\nu }}_e$ candidate events with energies above 3.4 MeV compared to $365.2\pm 23.7$ events expected in the absence of neutrino oscillation. Accounting for $17.8\pm 7.3$ expected background events, the statistical significance for reactor ${\overline{\nu }}_e$ 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 ${\overline{\nu }}_e$ oscillation effects. A two-neutrino oscillation analysis of the KamLAND data gives $\Delta m^2={7.9}_{-0.5}^{+0.6}\times {10}^{-5}{\mathrm{e}\mathrm{V}}^2$. A global analysis of data from KamLAND and solar-neutrino experiments yields $\Delta m^2={7.9}_{-0.5}^{+0.6}\times {10}^{-5}{\mathrm{e}\mathrm{V}}^2$ and ${tan}^2\theta ={0.40}_{-0.07}^{+0.10}$, the most precise determination to date.