Abstract

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+0.18×10−5 eV2, 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+0.8×106 cm−2 s−1 at the KamLAND location. We evaluate various bulk silicate Earth composition models using the observed geo ν¯e rate.1 MoreReceived 19 March 2013DOI:https://doi.org/10.1103/PhysRevD.88.033001© 2013 American Physical Society