First Results from KamLAND: Evidence for Reactor Anti-Neutrino Disappearance

TL;DR

KamLAND's initial measurements of reactor anti-neutrinos show a significant deficit consistent with neutrino oscillations, providing strong evidence for anti-neutrino disappearance and supporting the Large Mixing Angle solution.

Contribution

First experimental evidence from KamLAND for reactor anti-neutrino disappearance, confirming neutrino oscillation phenomena with high confidence.

Findings

Observed anti-neutrino flux deficit at 99.95% confidence

Excludes all but the Large Mixing Angle oscillation solution

Supports neutrino oscillation theory with reactor anti-neutrinos

Abstract

KamLAND has been used to measure the flux of $\bar{\nu}_e$'s from distant nuclear reactors. In an exposure of 162 ton$\cdot$yr (145.1 days) the ratio of the number of observed inverse $\beta$-decay events to the expected number of events without disappearance is $0.611\pm 0.085 {\rm (stat)} \pm 0.041 {\rm (syst)} $ for $\bar{\nu}_e$ energies $>$ 3.4 MeV. The deficit of events is inconsistent with the expected rate for standard $\bar{\nu}_e$ propagation at the 99.95% confidence level. In the context of two-flavor neutrino oscillations with CPT invariance, these results exclude all oscillation solutions but the `Large Mixing Angle' solution to the solar neutrino problem using reactor $\bar{\nu}_e$ sources.