We present results of a study of neutrino oscillation based on a 766 ton-year
exposure of KamLAND to reactor anti-neutrinos. We observe 258 \nuebar\
candidate events with energies above 3.4 MeV compared to 365.2 events expected
in the absence of neutrino oscillation. Accounting for 17.8 expected background
events, the statistical significance for reactor \nuebar 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 \nuebar oscillation effects. A two-neutrino
oscillation analysis of the KamLAND data gives \DeltaMSq =
7.9−0.5+0.6×10−5 eV2. A global analysis of data from KamLAND
and solar neutrino experiments yields \DeltaMSq =
7.9−0.5+0.6×10−5 eV2 and \ThetaParam =
0.40−0.07+0.10, the most precise determination to date.Comment: 5 pages, 4 figures; submitted to Phys.Rev.Letter