Kengo Nakamura: Kamland-Zen
Kamland-Zen aims to measure the neutrinoless double beta decay using 136Xe in liquid scintillator. It was funded in 2009, and installation in KamLand occurred in the summer of 2011. They published results for 2-nu double beta decay and neutrinoless decays in 2012, and now they have a new result for the neutrinoless decay.
A mini-balloon of 136Xe-loaded liquid scintillator is placed in the middle of the KamLAND vessel. It weighs 320 kg.
Energy calibration was performed using a 208Ti source. Radioactivity was inserted in the Xe LS. 214Bi was used for this additional calibration.
The datasets are divided in two: the first one was used for the earlier results; for the new result they use the full dataset.
The speaker showed the visible energy distribution , which at energies of up to 2 MeV is dominated by the two-neutrino decay.The interesting region for the neutirnoless mode sees some discrepancy with the model. The peak they observe may be due to radioactive impurities in the scintillator. This is consistent with the fallout from the Fukushima reactor incident. This is observed in the ratio of 134Cs/137Cs.
There are several candidates for the observed excesses in the data. They fit the energy region from 2.2 to 3 MeV with these four isotopes and get a good fit, while the neutrinoless double-beta decay would provide a very bad fit probability. They believe that the Silver 110 metastable isotope is the most likely background.
They then made a filtration of the liquid scintillator in February 2012. After this, the event rate decreased only slightly, but the effect is still present with a peak in the energy spectrum at about 2.7 MeV, close to the location of the expected peak for the much smaller expected component from neutrinoless double beta decay candidates. Their upper limit on the rare decay is thus worse than expected. The half-life is measured to be larger than 1.9E25 years at 90%CL. If they combine this with EXO-200 results, the lower limit goes up to 3.4E25y. This corresponds to a 120-250 meV for the majorana neutrino mass. From this combined result one sees that there is inconsistency with the earlier claim of observing a signal in KK.
The strategy for silver contamination removal is now to extract the xenon, to see if the silver remains in the liquid xenon, and replace the balloon with new purified liquid scintillator. They believe they can reduce the background by afactor of 100. They also plan to increase the amount of xenon to 400kg.
In summary, KamLand-Zen observed the two-nu double beta decay very precisely, measuring the half life as 2.3+-0.02+-0.12×10^21 years. They also obtained lower limits on the zero-neutrino mode, which combined with EXO-200 gives an inconsistency with the KKDC claim at more than 97.5% confidence level. To improve their preformance, they are re-purifying the xenon gas and the liquid scintillator. In the future we will completely refurbish the KamLand-Zen experiment.