アイテム詳細

要旨

The HEIDELBERG-MOSCOW experiment operating 11 kg of enriched ⁷⁶Ge in the GRAN SASSO Underground Laboratory, is one of the long-running underground experiments. It is the most sensitive double beta decay experiment since more than ten years. The measurement, and the analysis of the data taken from 2 August 1990-20 May 2003, which has been performed in the second half of 2003, is presented here. The duty cycle of the experiment was ~ 80 %, the collected statistics is 71.7 kg y. The background achieved in the energy region of the Q value for double beta decay is O.11 events/ kg y keV. The two-neutrino accompanied half-life is determined on the basis of more than 100 000 events. The confidence level for the neutrinoless signal has been improved to a 4.2 σ level. Fundamental consequences of this first evidence for neutrinoless double beta decay is that total lepton number is not conserved, and further, that the neutrino is a Majorana particle, and further that only a degenerate neutrino mass model is allowed. The effective neutrino mass deduced is (0.2 - 0.6) eV.