KamLAND @ LBL

Precision Measurement of Neutrino Oscillation Parameters with KamLAND

Allowed region of neutrino oscillation parameters

Allowed region for neutrino oscillation parameters from KamLAND and solar neutrino experiments. The side-panels show the Δχ²-profiles for KamLAND (dashed) and solar experiments (dotted) individually, as well as the combination of the two (solid). The KamLAND data severely restrict Δm² to one region, with the best-fit point at Δm²=7.58^+0.14_-0.13(stat)^+0.15_-0.15(syst)x 10^-5eV².

Ratio of the background and geo-neutrino subtracted anti-neutrino spectrum to the expectation for no-oscillation as a function of L/E

Ratio of the background and geo-neutrino subtracted anti-neutrino spectrum to the expectation for no-oscillation as a function of L/E. L is the effective baseline taken as a flux-weighted average (L=180km). The histogram and curve show the expectation accounting for the distances to the individual reactors, time-dependent flux variations and efficiencies. The figure shows the behavior expected from neutrino oscillation, where the electron anti-neutrino survival probability is: P_ee=1-sin²2θsin²(Δm²L/E).

Abstract:

The KamLAND experiment has determined a precise value for the neutrino oscillation parameter Δm²₂₁ and stringent constraints on θ₁₂. The exposure to nuclear reactor anti-neutrinos is increased almost fourfold over previous results to 2.44x10³² proton-yr due to longer livetime and an enlarged fiducial volume. An undistorted reactor anti-neutrino energy spectrum is now rejected at >5σ. Extending the analysis down to the inverse beta decay energy threshold, and incorporating geo-neutrinos, gives a best-fit at Δm²₂₁= 7.58^+0.14_-0.13(stat)^+0.15_-0.15(syst)x10^-5eV² and tan² θ₁₂=0.56^+0.10_0.07(stat)^+0.10_-0.06(syst). Local Δ χ²-minima at higher and lower Δm²₂₁ are disfavored at >4σ. Combining with solar neutrino data, we obtain Δm²₂₁= 7.59^+0.21_-0.21x10^-5 eV² and tan² θ₁₂=0.47^+0.06_-0.05.

KamLAND stands for "Kamioka Liquid Scintillator Anti-Neutrino Detector". With 1000 tons of light emitting liquid target viewed by 1879 50-cm diameter light-detecting photomultiplier tubes, KamLAND is the largest scintillation detector ever constructed (brief description).