Results from the Palo Verde neutrino oscillation experiment

The ${\overline{\nu }}_e$ 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 ${\overline{\nu }}_e\overrightarrow{p}{e}^{+}n$ 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. ${\overline{\nu }}_e\to {\overline{\nu }}_x$ oscillations were excluded at 90% C.L. for $\Delta m^2>1.12\mathrm{}\times {10}^{-3}$ ${\mathrm{eV}}^2$ for full mixing and ${\sin }^{2}2\theta >0.21\mathrm{}$ for large $\Delta m^2.$ These results support the conclusion that the observed atmospheric neutrino oscillations do not involve ${\nu }_e.$