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

TL;DR

In the context of two-flavor neutrino oscillations with CPT invariance, all solutions to the solar neutrinos problem except for the "large mixing angle" region are excluded.

Abstract

KamLAND has been used to measure the flux of $\barν_e$'s from distant nuclear reactors. In an exposure of 162 ton$\cdot$yr (145.1 days) the ratio of the number of observed inverse $β$-decay events to the expected number of events without disappearance is $0.611\pm 0.085 {\rm (stat)} \pm 0.041 {\rm (syst)} $ for $\barν_e$ energies $>$ 3.4 MeV. The deficit of events is inconsistent with the expected rate for standard $\barν_e$ 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 $\barν_e$ sources.

Figures (6)

• Figure 1: Schematic diagram of the KamLAND detector.
• Figure 2: (a) The fractional difference of the reconstructed average $\gamma$ energies and average source energies. The dashed line shows the systematic error. (b) The $R^3$ vertex distribution of 2.2 MeV neutron capture $\gamma$'s.
• Figure 3: Distribution of $\bar{\nu}_e$ candidates with fiducial volume cut, time, vertex correlation, and spallation cuts applied. The prompt energy corresponds to the positron and the delayed energy to the captured neutron. The events within the horizontal lines bracketing the delayed energy of 2.2 MeV are due to thermal neutron capture on protons. The events with prompt energy below $\sim$0.7 MeV are obtained from the delayed trigger. The one event with delayed energy near 4.95 MeV is consistent with theexpected 0.54% fraction from ${}^{12}$C$(n,\gamma)$.
• Figure 4: The ratio of measured to expected $\bar{\nu}_e$ flux from reactor experiments pdg. The solid dot is the KamLAND point plotted at a flux-weighted average distance (the dot size is indicative of the spread in reactor distances). The shaded region indicates the range of flux predictions corresponding to the 95% C.L. LMA region found in a global analysis of the solar neutrino data solar. The dotted curve corresponds to $\sin^2 2 \theta =0.833$ and $\Delta m^2= 5.5\times 10^{-5}$ eV$^2$solar and is representative of recent best-fit LMA predictions while the dashed curve shows the case of small mixing angles (or no oscillation).
• Figure 5: Upper panel: Expected reactor $\bar{\nu}_e$ energy spectrum with contributions of $\bar{\nu}_{geo}$ (model Ia of geonue) and accidental backround. Lower panel: Energy spectrum of the observed prompt events (solid circles with error bars), along with the expected no oscillation spectrum (upper histogram, with $\bar{\nu}_{geo}$ and accidentals shown) and best fit (lower histogram) including neutrino oscillations. The shaded band indicates the systematic error in the best-fit spectrum. The vertical dashed line corresponds to the analysis threshold at 2.6 MeV.