Reactor on-off antineutrino measurement with KamLAND

The recent long-term shutdown of Japanese nuclear reactors has resulted in a significantly reduced reactor νₑ flux at KamLAND. This running condition provides a unique opportunity to confirm and constrain backgrounds for the reactor νₑ oscillation analysis. The data set also has improved sensitivity for other νₑ signals, in particular νₑ's produced in β-decays from ²³⁸U and ²³²Th within the Earth's interior, whose energy spectrum overlaps with that of reactor νₑ'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 tan²θ₁₂=0.436_-0.025+0.029, Δm₂₁²=7.53_-0.18+0.18×10^-5 eV², and sin²θ₁₃=0.023_-0.002+0.002. Assuming a chondritic Th/U mass ratio, we obtain 116_-27⁺²⁸ νₑ events from ²³⁸U and ²³²Th, corresponding to a geo νₑ flux of 3.4_-0.8+0.8×10⁶ cm^-2 s^-1 at the KamLAND location. We evaluate various bulk silicate Earth composition models using the observed geo νₑ rate.