Enhanced Constraints on $θ_{13}$ from A Three-Flavor Oscillation Analysis of Reactor Antineutrinos at KamLAND

We present new constraints on the neutrino oscillation parameters $\Delta m^{2}_{21}$, $\theta_{12}$, 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 {\it CPT} invariance, a two-flavor analysis ({$\theta_{13} = 0$}) of the KamLAND and solar data yields the best-fit values $\tan^{2} \theta_{12} = 0.444^{+0.036}_{-0.030}$ and $\Delta m^{2}_{21} = 7.50^{+0.19}_{-0.20} \times 10^{-5} ~ {\rm eV}^{2}$; a three-flavor analysis with $\theta_{13}$ as a free parameter yields the best-fit values $\tan^{2} \theta_{12} = 0.452^{+0.035}_{-0.033}$, $\Delta m^{2}_{21} = 7.50^{+0.19}_{-0.20} \times 10^{-5} ~ {\rm eV}^{2}$, 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 $\sin^{2} \theta_{13} = 0.017^{+0.010}_{-0.009}$, a nonzero value at the 93% C.L. This finding will be further tested by upcoming accelerator and reactor experiments.