First Results from KamLAND: Evidence for Reactor Antineutrino Disappearance

KamLAND has measured the flux of ν ¯_e’s from distant nuclear reactors. We find fewer ν ¯_e events than expected from standard assumptions about ν ¯_e propagation at the 99.95% C.L. In a 162 ton·yr exposure the ratio of the observed inverse β-decay events to the expected number without ν ¯_e disappearance is 0.611±0.085(stat)±0.041(syst) for ν ¯_e energies >3.4 MeV. In the context of two-flavor neutrino oscillations with CPT invariance, all solutions to the solar neutrino problem except for the “large mixing angle” region are excluded.