Results from the Palo Verde Neutrino Oscillation Experiment

TL;DR

The Palo Verde experiment measured reactor antineutrino flux at 800m, finding no evidence of neutrino oscillations and setting limits that suggest atmospheric neutrino oscillations do not involve electron neutrinos.

Contribution

First detailed measurement of reactor antineutrino flux at this distance with novel background subtraction techniques, providing new constraints on neutrino oscillation parameters.

Findings

No evidence for neutrino oscillations was observed.

Oscillation parameters were constrained, excluding certain regions at 90% CL.

Results support that atmospheric neutrino oscillations do not involve electron neutrinos.

Abstract

The $\nuebar$ 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 $\nuebar$p$\to$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. $\nuebar\to\bar\nu_x$ oscillations were excluded at 90% CL for $\dm>1.12\times10^{-3}$ eV^2 for full mixing, and $\sinq>0.21$ for large $\dm$. These results support the conclusion that the observed atmospheric neutrino oscillations does not involve $\nu_{\rm e}$.