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Abstract

The data of the Heidelberg-Moscow double beta decay experiment for the measuring period August 1990-May 2000 (54.9813 kg y or 723.44 molyears), published recently, axe analyzed using the potential of the Bayesian method for low counting rates. First evidence for neutrinoless double beta decay is observed giving first evidence for lepton number violation. The evidence for this decay mode is 97% (2.2sigma) with the Bayesian method, and 99.8% c.l. (3.1sigma) with the method recommended by the Particle Data Group. The half-life of the process is found with the Bayesian method to be T-1/2(0nu) = (0.8-18.3) x 10(25) y (95% c.l.) with a best value of 1.5 x 10(25) y. The deduced value of the effective neutrino mass is, with the nuclear matrix elements from Ref. 1, (m) = (0.11-0.56) eV (95% c.l.), with a best value of 0.39 eV. Uncertainties in the nuclear matrix elements may widen the range given for the effective neutrino mass by at most a factor 2. Our observation which at the same time means evidence that the neutrino is a Majorana particle, will be of fundamental importance for neutrino physics.