Results from KamLAND Reactor Neutrino Detection

Abstract

Data corresponding to a KamLAND detector exposure of 766.3 ton-years to reactor neutrinos was used to search for neutrino oscillations. Based on this data-sample, KamLAND demonstrates the reactor ν̄e disappearance at the 99.998% significance and finds the reactor ν̄e spectrum distortion at the 99.6% significance. The reactor neutrino anomaly defined as the flux disappearance and spectrum shape distortion is confirmed at the 99.999995% significance. A two-flavor neutrino oscillation analysis including the rate and energy spectrum yields a best fit at Δm2 = 7.9 × 10-5 eV2 and sin2 2θ = 0.86. A global analysis of data from KamLAND and solar neutrino experiments yields Δm2 = 7.9+0.6-0.5 × 10-5 eV2 and tan2 θ = 0.40+0.10-0.07. The present result gives the most precise determination of Δm2 to date. To test the goodness of the oscillation hypothesis, the ratio of observed anti-neutrino spectrum to the expected for no-oscillation as a function of L0/E is compared with the predictions which give neutrino-flavor changes. The neutrino oscillation is much favored with more than 99% C.L., while the decay and decoherence are excluded at the 95% and 94% C.L.