Precision Measurement of Neutrino Oscillation Parameters with KamLAND

Abstract

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 anti-neutrinos is increased almost fourfold over previous results to 2.44$\times 10^{32}$ proton-yr due to longer livetime and an enlarged fiducial volume. An undistorted reactor ${\bar{\nu }}_e$ energy spectrum is now rejected at >5$\sigma$. Extending the analysis down to the inverse beta decay energy threshold, and incorporating geo-neutrinos, gives a best-fit at $\Delta m_{21}^2$= $7.58_{-0.13}^{+0.14}(stat{)}_{-0.15}^{+0.15}(syst)\times 10^{-5}$ eV${}^2$ and ${\tan }^2{\theta }_{12}$=$0.56_{0.07}^{+0.10}(stat{)}_{-0.06}^{+0.10}(syst)$. 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 $\Delta m_{21}^2$= $7.59_{-0.21}^{+0.21}\times 10^{-5}$ eV${}^2$ and ${\tan }^2{\theta }_{12}$=$0.47_{-0.05}^{+0.06}$.

Version as published in PRL. Revised Fig. 2 (allowed contours) due to an error in the figure generating code (numbers or conclusions did not change). The full dChi2-map for this analysis is available at http://www.awa.tohoku.ac.jp/KamLAND/chi2map_3rdresult/chi2map.html . Fig. 3 (low E spectrum) was dropped due to space limitations