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 tan2θ12=0.436+0.029−0.025, Δm221=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+28−27 ν¯e events from U238 and Th232, corresponding to a geo ν¯e 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 ν¯e rate.