Reactor on-off antineutrino measurement with KamLAND

The recent long-term shutdown of Japanese nuclear reactors has resulted in a significantly reduced reactor ν¯_e flux at KamLAND. This running condition provides a unique opportunity to confirm and constrain backgrounds for the reactor ν¯_e oscillation analysis. The data set also has improved sensitivity for other ν¯_e signals, in particular ν¯_e’s produced in β-decays from U238 and Th232 within the Earth’s interior, whose energy spectrum overlaps with that of reactor ν¯_e’s. Including constraints on θ₁₃ 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⁡²θ₁₂=0.436_-0.025^+0.029, Δm₂₁²=7.53_-0.18^+0.18×10^-5eV², and sin⁡²θ₁₃=0.023_-0.002^+0.002. Assuming a chondritic Th/U mass ratio, we obtain 116_-27⁺²⁸ ν¯_e events from U238 and Th232, corresponding to a geo ν¯_e flux of 3.4_-0.8^+0.8×10⁶cm^-2s^-1 at the KamLAND location. We evaluate various bulk silicate Earth composition models using the observed geo ν¯_e rate.