Reactor on-off antineutrino measurement with KamLAND

Abstract

The recent long-term shutdown of Japanese nuclear reactors has resulted in a significantly reduced reactor ${\overline{\ensuremath{\nu}}}_{e}$ flux at KamLAND. This running condition provides a unique opportunity to confirm and constrain backgrounds for the reactor ${\overline{\ensuremath{\nu}}}_{e}$ oscillation analysis. The data set also has improved sensitivity for other ${\overline{\ensuremath{\nu}}}_{e}$ signals, in particular ${\overline{\ensuremath{\nu}}}_{e}$'s produced in $\ensuremath{\beta}$-decays from $^{238}\mathrm{U}$ and $^{232}\mathrm{Th}$ within the Earth's interior, whose energy spectrum overlaps with that of reactor ${\overline{\ensuremath{\nu}}}_{e}$'s. Including constraints on ${\ensuremath{\theta}}_{13}$ from accelerator and short-baseline reactor neutrino experiments, a combined three-flavor analysis of solar and KamLAND data gives fit values for the oscillation parameters of ${tan}^{2}{\ensuremath{\theta}}_{12}={0.436}_{\ensuremath{-}0.025}^{+0.029}$, $\ensuremath{\Delta}{m}_{21}^{2}={7.53}_{\ensuremath{-}0.18}^{+0.18}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\text{ }\text{ }{\mathrm{eV}}^{2}$, and ${sin}^{2}{\ensuremath{\theta}}_{13}={0.023}_{\ensuremath{-}0.002}^{+0.002}$. Assuming a chondritic Th/U mass ratio, we obtain ${116}_{\ensuremath{-}27}^{+28}$ ${\overline{\ensuremath{\nu}}}_{e}$ events from $^{238}\mathrm{U}$ and $^{232}\mathrm{Th}$, corresponding to a geo ${\overline{\ensuremath{\nu}}}_{e}$ flux of ${3.4}_{\ensuremath{-}0.8}^{+0.8}\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ at the KamLAND location. We evaluate various bulk silicate Earth composition models using the observed geo ${\overline{\ensuremath{\nu}}}_{e}$ rate.