Results from the Palo Verde neutrino oscillation experiment

The ν̄_e flux and spectrum have been measured at a distance of about 800 m from the reactors of the Palo Verde Nuclear Generating Station using a segmented Gd-loaded liquid scintillator detector. Correlated positron-neutron events from the reaction ν̄_ep-→e⁺n were recorded for a period of 200 d including 55 d with one of the three reactors off for refueling. Backgrounds were accounted for by making use of the reactor-on and reactor-off cycles, and also with a novel technique based on the difference between signal and background under reversal of the e⁺ and n portions of the events. A detailed description of the detector calibration, background subtraction, and data analysis is presented here. Results from the experiment show no evidence for neutrino oscillations. ν̄_e→ν̄_x oscillations were excluded at 90% C.L. for Δm²>1.12×10^-3 eV² for full mixing and sin²2θ>0.21 for large Δm². These results support the conclusion that the observed atmospheric neutrino oscillations do not involve ν_e.