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Hiroko Watanabe: KamLand results

March 12, 2013

ImageKamLand (see picture, right) is a 1000T tank of liquid scintillator. Since 2000 they proved the disappearance of neutrinos (2003), then produced a precise measurement of oscillations (2008), and did a three-flavor oscillation analysis in 2011. They also performed several additional measurements of solar antineutrinos, geo-neutrino measurements, extraterrestrial anneutrinos, and 8B solar neutrinos; finally, they are also searching for double beta decay.

It is a 6.5m radius tank sensitive to a wide energy range. Kamland-Zen since 2011 has a Xenon-loaded scintillator installed in a mini-balloon in the center of the tank for 2β decay studies. They are continuing the normal operation of the tank outside of the mini-balloon.

Direct measurements of neutrinos from the earth can answer the questions on the radiogenic contribution to the heat flow (44.2 +-1 TW). 20 of these are believed to come from Uranium (8TW), Thorium (8TW) and potassium (4TW). Kamland is sensitive to antineutrinos above 1.8 MeV, so it is not sensitive to the K component, and most of the flow comes from Uranium decays.

In KamLand backgrounds are from reactor neutrinos and non-neutrino sources: 16O, 210Po alphas and neutrons.  The radiogenic flux is consistent with the 20TW model. Fully-radiogenic models of the thermal flux are disfavoured at over 2-sigma level.

For reactor neutrinos, the dataset until 2011 includes 2135 days of operation. The later 2013 dataset includes 2991 days, but the later period includes the KamLand-Zen phase and the reactor downtime in Japan, which seriously affected the overall fluxes. The rate has been constant in the R<6m fiducial volume, after accounting for these losses. They can thus measure precisely two cycles of oscillation. By combining solar results information and results of theta_13 experiments, they get a measurement of dm_21^2 with a 2.3% precision: (7.53+-0.18) x 10^-5 eV^2). Without external constraints from theta_13 the result does not vary much: 7.50 +- 0.20 x 10^-5 eV^2.

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