Precision Measurement of Neutrino Oscillation Parameters with KamLAND

The KamLAND experiment has determined a precise value for the neutrino oscillation parameter Δm₂₁² and stringent constraints on θ₁₂. The exposure to nuclear reactor antineutrinos is increased almost fourfold over previous results to 2.44×10³² proton yr due to longer livetime and an enlarged fiducial volume. An undistorted reactor νₑ energy spectrum is now rejected at >5σ. Analysis of the reactor spectrum above the inverse beta decay energy threshold, and including geoneutrinos, gives a best fit at ${Δ}{m}₂₁²={7.58}_{{-}0.13}+0.14(stat{)}_{{-}0.15}+0.15(syst)×{}{10}^{{-}5} {eV}²$ and ${tan}²{{θ}}₁₂={0.56}_{{-}0.07}+0.10(stat{)}_{{-}0.06}+0.10(syst)$. Local ${Δ}{{χ}}²$ minima at higher and lower ${Δ}{m}₂₁²$ are disfavored at $>4{σ}$. Combining with solar neutrino data, we obtain ${Δ}{m}₂₁²={7.59}_{{-}0.21}+0.21×{}{10}^{{-}5} {eV}²$ and ${tan}²{{θ}}₁₂={0.47}_{{-}0.05}+0.06$.