Precision Measurement of Neutrino Oscillation Parameters with KamLAND
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. 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: Pee=1-sin22θsin2(Δm2L/E).
Published in Phys. Rev. Lett. 100, 221803 (2008) arXiv:0801.4589 [hep-ex].
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+0.100.07(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).
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