First evidence for lepton number violation and of the Majorana character of neutrinos

Nuclear double beta decay provides an extraordinarly broad potential to search for beyond-standard-model physics. The occurrence of the neutrinoless decay (0νββ) mode has fundamental consequences: first total lepton number is not conserved, and second, the neutrino is a Majorana particle. Further the effective mass measured allows to put an absolute scale of the neutrino mass spectrum. In addition, double beta experiments yield sharp restrictions also for other beyond standard model physics. These include SUSY models (R-parity breaking and conserving), leptoquarks (leptoquark-Higgs coupling), compositeness, left-right symmetric models (right-handeld W boson mass), test of special relativity and of the equivalence principle in the neutrino sector and others. First evidence for neutrinoless double beta decay was given in 2001, by the HEIDELBERG-MOSCOW experiment. The HEIDELBERG-MOSCOW experiment is the by far most sensitive 0νββ experiment since more than 10 years. It was operating 11 kg of enriched ⁷⁶Ge in the GRAN SASSO Underground Laboratory. The analysis of the data taken from 2 August 1990 - 20 May 2003, is presented here. The collected statistics is 71.7kg 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 (1.74 ^+0.18 _-0.16) x 10 ²¹ years. The confidence level for the neutrinoless signal has been improved to a 4.2 σ level. The half-life is T ^0ν _1/2 = (1.19 ^+0.37 _-0.23) x 10 ²⁵ years. The effective neutrino mass deduced is (0.2 - 0.6) eV (99.73% c.l.), with the consequence that neutrinos have degenerate masses. The sharp boundaries for other beyond SM physics, mentioned above, are comfortably competitive to corresponding results from high-energy accelerators like TEVATRON, HERA, etc