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.

The Heidelberg-Moscow experiment gives the most stringent limit on the Majorana neutrino mass. After 24 kg yr of data with pulse shape measurements, we set a lower limit on the half-life of the 0νββ-decay in 76 Ge of T 0ν 1/2 ≥ 5.7 × 10 25 yr at 90% C.L., thus excluding an effective Majorana neutrino mass greater than 0.2 eV. This allows to set strong constraints on degenerate neutrino mass models.Neutrinoless double beta (0νββ) decay is an extremely sensitive tool to probe theories beyond the standard model (see [1]). While the standard model exactly conserves B-L, 0νββ-decay violates lepton number, and B-L, by two units. The simplest mechanism which can induce 0νββ-decay is the exchange of a Majorana neutrino between the decaying neutrons. Alternatively, any theory that contains lepton number violating interactions can lead to the process. Independently of the underlying mechanism, an observation of the 0νββ-decay would be an evidence for a nonzero Majorana neutrino mass [2]. There are several indications for nonzero neutrino masses, the most stringent ones come from solar and atmospheric neutrino experiments. In particular, the confirmation by Super Kamiokande of the atmospheric neutrino deficit [3], provides strong evidence for neutrino oscillations, although also other solutions are possible [4]. If a neutrino as a hot dark matter (HDM) component is taken into account, then fitting the atmospheric, solar and HDM scales with three neutrinos is only possible in the degenerate mass scenario, where all neutrinos have nearly the same mass, in the order of O(eV) [5]. This would lead to an amplitude for 0νββ-decay mediated by the neutrino mass which is accessible by the present sensitivity of the Heidelberg-Moscow experiment.The Heidelberg-Moscow experiment operates five p-type HPGe detectors in the Gran Sasso Underground Laboratory. The Ge crystals were grown out of 19.2 kg of 86% enriched 76 Ge material. The total active mass of the detectors is 10.96 kg, corresponding to 125.5 mol of 76 Ge, the presently largest source strength of all double beta experiments. Four detectors are placed in a common 30 cm thick lead shielding in a radon free nitrogen atmosphere, surrounded by 10 cm of boron-loaded polyethylene and with two layers of 1 cm thick scintillators on top. The remaining detector is situated in a separate box with 27 cm electrolytical copper and 20 cm lead shielding, flushed with gaseous nitrogen and with 10 cm of boron-loaded polyethylene below the box. A detailed description of the experiment and its background is given in [6]. For a further reduction of the already very low background of the experiment, a pulse shape analysis (PSA) method was developed [7]. The analysis distinguishes between multiple scattered interaction in the Ge crystal, so called multiple site events (MSE) and pointlike interactions, i.e. single site events (SSE). Since double beta decay events belong to the SSE category, the method allows to effectively reduce the background of multiple Compton scattered photons. The probability of...