Constraints onθ13from a three-flavor oscillation analysis of reactor antineutrinos at KamLAND

Abstract

We present new constraints on the neutrino oscillation parameters $\Delta m_{21}^2$, ${\theta }_{12}$, and ${\theta }_{13}$ from a three-flavor analysis of solar and KamLAND data. The KamLAND data set includes data acquired following a radiopurity upgrade and amounts to a total exposure of $3.49\times {10}^{32}$ target-proton-year. Under the assumption of $CPT$ invariance, a two-flavor analysis (${\theta }_{13}=0$) of the KamLAND and solar data yields the best-fit values ${tan}^2{\theta }_{12}={0.444}_{-0.030}^{+0.036}$ and $\Delta m_{21}^2={7.50}_{-0.20}^{+0.19}\times {10}^{-5}{\mathrm{eV}}^2$; a three-flavor analysis with ${\theta }_{13}$ as a free parameter yields the best-fit values ${tan}^2{\theta }_{12}={0.452}_{-0.033}^{+0.035}$, $\Delta m_{21}^2={7.50}_{-0.20}^{+0.19}\times {10}^{-5}{\mathrm{eV}}^2$, and ${sin}^2{\theta }_{13}={0.020}_{-0.016}^{+0.016}$. This ${\theta }_{13}$ interval is consistent with other recent work combining the CHOOZ, atmospheric and long-baseline accelerator experiments. We also present a new global ${\theta }_{13}$ analysis, incorporating the CHOOZ, atmospheric, and accelerator data, which indicates ${sin}^2{\theta }_{13}={0.009}_{-0.007}^{+0.013}$. A nonzero value is suggested, but only at the 79% C.L.