First evidence for neutrinoless double beta decay, with enriched 76Ge in Gran Sasso 1990-2003.

The HEIDELBERG-MOSCOW experiment operating 11 kg of enriched Ge-76 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 similar to 80%. the collected statistics is 71.7kgy. The background achieved in the energy region of the Q value for double beta decay is 0.11 events/kgy keV. The two-nentrino 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 sigma 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.