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Precision Measurement of Neutrino Oscillation Parameters with KamLAND

The KamLAND Collaboration

Abstract

The KamLAND experiment has determined a precise value for the neutrino oscillation parameter $Δm^{2}_{21}$ and stringent constraints on $θ_{12}$. The exposure to nuclear reactor anti-neutrinos is increased almost fourfold over previous results to 2.44$\times10^{32}$ proton-yr due to longer livetime and an enlarged fiducial volume. An undistorted reactor $\barν_{e}$ energy spectrum is now rejected at >5$σ$. Extending the analysis down to the inverse beta decay energy threshold, and incorporating geo-neutrinos, gives a best-fit at $Δm^{2}_{21}$= $7.58^{+0.14}_{-0.13}(stat)^{+0.15}_{-0.15}(syst)\times10^{-5}$ eV$^{2}$ and $\tan^2 θ_{12}$=$0.56^{+0.10}_{0.07}(stat)^{+0.10}_{-0.06}(syst)$. Local $Δχ^2$-minima at higher and lower $Δm^{2}_{21}$ are disfavored at >4$σ$. Combining with solar neutrino data, we obtain $Δm^{2}_{21}$= $7.59^{+0.21}_{-0.21}\times10^{-5}$ eV$^{2}$ and $\tan^2 θ_{12}$=$0.47^{+0.06}_{-0.05}$.

Precision Measurement of Neutrino Oscillation Parameters with KamLAND

Abstract

The KamLAND experiment has determined a precise value for the neutrino oscillation parameter and stringent constraints on . The exposure to nuclear reactor anti-neutrinos is increased almost fourfold over previous results to 2.44 proton-yr due to longer livetime and an enlarged fiducial volume. An undistorted reactor energy spectrum is now rejected at >5. Extending the analysis down to the inverse beta decay energy threshold, and incorporating geo-neutrinos, gives a best-fit at = eV and =. Local -minima at higher and lower are disfavored at >4. Combining with solar neutrino data, we obtain = eV and =.

Paper Structure

This paper contains 3 figures, 2 tables.

Figures (3)

  • Figure 1: Prompt event energy spectrum of $\overline{\nu}_{e}$ candidate events. All histograms corresponding to reactor spectra and expected backgrounds incorporate the energy-dependent selection efficiency (top panel). The shaded background and geo-neutrino histograms are cumulative. Statistical uncertainties are shown for the data; the band on the blue histogram indicates the event rate systematic uncertainty.
  • Figure 2: Allowed region for neutrino oscillation parameters from KamLAND and solar neutrino experiments. The side-panels show the $\Delta \chi^{2}$-profiles for KamLAND (dashed) and solar experiments (dotted) individually, as well as the combination of the two (solid).
  • Figure 3: Ratio of the background and geo-neutrino-subtracted $\overline{\nu}_{e}$ spectrum to the expectation for no-oscillation as a function of $L_{0}/E$. $L_{0}$ is the effective baseline taken as a flux-weighted average ($L_{0}$ = 180 km). The energy bins are equal probability bins of the best-fit including all backgrounds (see Fig. \ref{['fig:E-spectrum']}). The histogram and curve show the expectation accounting for the distances to the individual reactors, time-dependent flux variations and efficiencies. The error bars are statistical only and do not include, for example, correlated systematic uncertainties in the energy scale.