Precision Measurement of Neutrino Oscillation Parameters with KamLAND

Abstract

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