Search

Abstract: KamLAND-Zen has started a search for neutrino-less double beta decay with 320kg of 90% enriched 136Xe in September 2011. Results from initial 112 day of measurement are presented. It provides so far the most precise two-neutrino-mode half-life of 136Xe and the most stringent upper limit of the coupling constant of the ordinary Majoron-emitting double-beta-decay. The search for neutrino-less mode has achieved top class sensitivity but is struggling with unexpected backgrounds. Origin of the background and prospects for the future improvements are also discussed. [Copyright &y& Elsevier]

KamLAND has accumulated 766.3 ton-year of data with extended period and enlarged fiducial volume. No oscillation predicts 365.2±23.7 reactor neutrino events and 17.8±7.3 background events in the data, while only 258 events were observed. This deficit is consistent with the previous evidence for reactor antineutrino disappearance and strengthened the confidence level to 99.998%. Increased statistics allowed us to examine spectral distortion and the observed spectrum was inconsistent with no-oscillation shape at 99.6% C.L. Combining these deficit and spectral distortion, no-oscillation is excluded at 99.999995% C.L. The observed distortion is fully consistent with the neutrino oscillation hypothesis with the oscillation parameter of the LMA solution for the solar neutrino problem and the distortion disagrees with the other flavor conversion hypothesis such as neutrino decay and decoherence. If one plots the survival probability as a function of where is chosen at typical reactor distance of 180 km, one sees clear oscillatory behavior. This is a stand-alone evidence of neutrino oscillation. This clear signal of oscillation provided precise determination of to be when analyzed in two neutrino oscillation scenario. [Copyright &y& Elsevier]