Measurement of Neutrino Oscillation with KamLAND: Evidence of Spectral Distortion

TL;DR

KamLAND's reactor anti-neutrino study provides strong evidence of spectral distortion due to neutrino oscillations, precisely measuring oscillation parameters and confirming the phenomenon with high statistical significance.

Contribution

This paper presents the first detailed spectral analysis of reactor anti-neutrinos showing spectral distortion consistent with neutrino oscillation, and provides the most precise measurement of oscillation parameters to date.

Findings

Discovered spectral distortion at 99.6% significance

Measured MSq = 7.90.6  10^{-5} eV^2

Determined mixing angle  = 0.400.10

Abstract

We present results of a study of neutrino oscillation based on a 766 ton-year exposure of KamLAND to reactor anti-neutrinos. We observe 258 \nuebar\ candidate events with energies above 3.4 MeV compared to 365.2 events expected in the absence of neutrino oscillation. Accounting for 17.8 expected background events, the statistical significance for reactor \nuebar disappearance is 99.998%. The observed energy spectrum disagrees with the expected spectral shape in the absence of neutrino oscillation at 99.6% significance and prefers the distortion expected from \nuebar oscillation effects. A two-neutrino oscillation analysis of the KamLAND data gives \DeltaMSq = 7.9$^{+0.6}_{-0.5}\times10^{-5}$ eV$^2$. A global analysis of data from KamLAND and solar neutrino experiments yields \DeltaMSq = 7.9$^{+0.6}_{-0.5}\times10^{-5}$ eV$^2$ and \ThetaParam = 0.40$^{+0.10}_{-0.07}$, the most precise determination to date.