Precision Measurement of Neutrino Oscillation Parameters with KamLAND

The KamLAND experiment has determined a precise value for the neutrino oscillation parameter $\Delta m_{21}^2$ and stringent constraints on ${\theta }_{12}$. The exposure to nuclear reactor antineutrinos is increased almost fourfold over previous results to $2.44\times {10}^{32}\mathrm{proton}\mathrm{yr}$ due to longer livetime and an enlarged fiducial volume. An undistorted reactor ${\overline{\nu }}_e$ energy spectrum is now rejected at $>5\sigma$. Analysis of the reactor spectrum above the inverse beta decay energy threshold, and including geoneutrinos, gives a best fit at $\Delta m_{21}^2={7.58}_{-0.13}^{+0.14}(\mathrm{stat}{)}_{-0.15}^{+0.15}(\mathrm{syst})\times {10}^{-5}{\mathrm{eV}}^2$ and ${tan}^2{\theta }_{12}={0.56}_{-0.07}^{+0.10}(\mathrm{stat}{)}_{-0.06}^{+0.10}(\mathrm{syst})$. Local $\Delta {\chi }^2$ minima at higher and lower $\Delta m_{21}^2$ are disfavored at $>4\sigma$. Combining with solar neutrino data, we obtain $\Delta m_{21}^2={7.59}_{-0.21}^{+0.21}\times {10}^{-5}{\mathrm{eV}}^2$ and ${tan}^2{\theta }_{12}={0.47}_{-0.05}^{+0.06}$.