Results from the Palo Verde neutrino oscillation experiment

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