The present experimental status in the search for neutrinoless double beta decay is reviewed, with emphasis on the first indication for neutrinoless double beta decay found in the HEIDELBERG-MOSCOW experiment, giving first evidence for lepton number violation and a Majorana nature of the neutrinos. Future perspectives of the field are briefly outlined.

The results of the HEIDELBERG-MOSCOW experiment which searches with 11 kg of enriched 76 Ge for double beta decay in the GRAN Sasso underground laboratory are presented for the full running period August 1990 -May 2003. 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 0.11 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 4.2σ.

Abstract. New results for the double beta decay of76 Ge are presented. They are extracted from Data obtained with the Heidelberg-Moscow experiment, which operates five enriched 76 Ge detectors in an extreme low-level environment in the Gran Sasso underground laboratory. The two neutrino accompanied double beta decay is evaluated for the first time for all five detectors with a statistical significance of 47.7 kg y resulting in a half life of T

In this brief review, a description of the observed evidence for neutrinoless double beta 3-5 in the 76 Ge experiment in Gran Sasso (Heidelberg-Moscow experiment) which has been operated with 11 kg enriched 76 Ge detectors in the period 1990-2003, is provided. Two different methods of pulse shape analysis have been used to select potential 0νββ events from the γ background of the measured spectrum -a selection by a neuronal net approach, 3,4,16 and a selection by a new method comparing measured pulses with a library of pulse shapes of point-like events calculated from simulation of the electric field distribution in the detectors (see Refs. 6-8 and 37). The latter method also allows spatial localization of measured events. Both methods lead to selections of events at Q ββ with almost no γ-background. The observed line at Q ββ is identified as a 0νββ signal. It has a confidence level of more than 6σ.

Neutrinoless double beta decay (O&8) induced by superparticle exchange is investigated.Such a supersymmetric (SUSY) mechanism of O@ decay arises within SUSY theories with R-parity nonconservation (p,). We consider the minimal supersymmetric standard model (MSSM) with explicit 4t, terms in the superpotential (4% MSSM). The decay rate for the SUSY mechanism of Ovpp decay is calculated. Numerical values for nuclear matrix elements for the experimentally most interesting isotopes are calculated within the proton-neutron quasiparticle random phase approximation. Constraints on the 4$ MSSM parameter space are extracted from current experimental half-life limits. The most stringent limits are derived from data on "Ge. It is shown that these constraints are more stringent than those from other low-energy processes and are competitive with or even more stringent than constraints expected from accelerator searches.PACS number(s): 12. 6O.Jv, 11.30.Er, 23.40.B~