Abstract:
We present results of a study of neutrino oscillation based on a 766 ton/year exposure of KamLAND to reactor antineutrinos. We observe 258 v̅ₑ 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 v̅ₑ 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 v̅ₑ 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 tan²θ = 0.40+0.10/−0.07, the most precise determination to date.
