Measurement of Neutrino Oscillation with KamLAND: Evidence of Spectral Distortion

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.

Figures (4)

• Figure 1: (a) Estimated time variation of the reactor $\overline{\nu}_{e}$ flux at KamLAND assuming no anti-neutrino oscillation. (b) Observed $\overline{\nu}_{e}$ event rate versus no-oscillation reactor $\overline{\nu}_{e}$ flux. Data points correspond to intervals of approximately equal $\overline{\nu}_{e}$ flux. The dashed line is a fit, the 90% C.L. is shown in gray. The solid line is a fit constrained to the expected background. The reactor distance distribution for $\overline{\nu}_{e}$ events in the absence of oscillations is shown in the inset.
• Figure 2: (a) The correlation between the prompt and delayed event energies after cuts. The three events with $E_{\text{delayed}}\sim5$ MeV are consistent with neutron capture on carbon. (b) Prompt event energy spectrum of $\overline{\nu}_{e}$ candidate events with associated background spectra. The shaded band indicates the systematic error in the best-fit reactor spectrum above 2.6 MeV.
• Figure 3: Ratio of the observed $\overline{\nu}_{e}$ spectrum to the expectation for no-oscillation versus L$_{0}$/E. The curves show the expectation for the best-fit oscillation, best-fit decay and best-fit decoherence models taking into account the individual time-dependent flux variations of all reactors and detector effects. The data points and models are plotted with L$_{0}$=180 km, as if all anti-neutrinos detected in KamLAND were due to a single reactor at this distance.
• Figure 4: (a) Neutrino oscillation parameter allowed region from KamLAND anti-neutrino data (shaded regions) and solar neutrino experiments (lines) snosalt. (b) Result of a combined two-neutrino oscillation analysis of KamLAND and the observed solar neutrino fluxes under the assumption of CPT invariance. The fit gives $\Delta m^{2}$ = 7.9$^{+0.6}_{-0.5}\times$10$^{-5}$ eV$^{2}$ and $\tan^2 \theta$ = 0.40$^{+0.10}_{-0.07}$ including the allowed 1-sigma parameter range.