First Results from KamLAND: Evidence for Reactor Anti-Neutrino Disappearance

Abstract

KamLAND has been used to measure the flux of ${\bar{\nu }}_e$'s from distant nuclear reactors. In an exposure of 162 ton$\cdot$yr (145.1 days) the ratio of the number of observed inverse $\beta$-decay events to the expected number of events without disappearance is $0.611\pm 0.085(\mathrm{stat})\pm 0.041(\mathrm{syst})$ for ${\bar{\nu }}_e$ energies $>$ 3.4 MeV. The deficit of events is inconsistent with the expected rate for standard ${\bar{\nu }}_e$ propagation at the 99.95% confidence level. In the context of two-flavor neutrino oscillations with CPT invariance, these results exclude all oscillation solutions but the `Large Mixing Angle' solution to the solar neutrino problem using reactor ${\bar{\nu }}_e$ sources.

6 pages, 6 figures