Measurement of Neutrino Oscillation with KamLAND: Evidence of Spectral Distortion

We present results of a study of neutrino oscillation based on a 766 ton/year exposure of KamLAND to reactor antineutrinos. We observe 258 ${\overline{\nu }}_e$ candidate events with energies above 3.4 MeV compared to $365.2\pm 23.7$ events expected in the absence of neutrino oscillation. Accounting for $17.8\pm 7.3$ expected background events, the statistical significance for reactor ${\overline{\nu }}_e$ 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 ${\overline{\nu }}_e$ oscillation effects. A two-neutrino oscillation analysis of the KamLAND data gives $\Delta m^2={7.9}_{-0.5}^{+0.6}\times {10}^{-5}{\mathrm{e}\mathrm{V}}^2$. A global analysis of data from KamLAND and solar-neutrino experiments yields $\Delta m^2={7.9}_{-0.5}^{+0.6}\times {10}^{-5}{\mathrm{e}\mathrm{V}}^2$ and ${tan}^2\theta ={0.40}_{-0.07}^{+0.10}$, the most precise determination to date.

Figure 1

(a) Estimated time variation of the reactor ${\overline{\nu }}_e$ flux at KamLAND assuming no antineutrino 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 $E_{\mathrm{d}\mathrm{e}\mathrm{l}\mathrm{a}\mathrm{y}\mathrm{e}\mathrm{d}}\sim 5\mathrm{M}\mathrm{e}\mathrm{V}$ 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. The first bin in the accidentals histogram contains $\sim 113$ events.

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 $L_0=180\mathrm{k}\mathrm{m}$, as if all antineutrinos detected in KamLAND were due to a single reactor at this distance.

Figure 4

(a) Neutrino oscillation parameter allowed region from KamLAND antineutrino data (shaded regions) and solar-neutrino experiments (lines) [13]. (b) Result of a combined two-neutrino oscillation analysis of KamLAND and observed solar-neutrino fluxes under the assumption of CPT invariance. The fit gives $\Delta m^2={7.9}_{-0.5}^{+0.6}\times {10}^{-5}{\mathrm{e}\mathrm{V}}^2$ and ${tan}^2\theta ={0.40}_{-0.07}^{+0.10}$ including the allowed 1-sigma parameter range.