From nuclear physics to physics beyond the standard model: First evidence for lepton number violation and the Majorana character of neutrinos

Nuclear double beta decay provides an extraordinarily broad potential to search for beyond-standard-model physics. The occurrence of the neutrinoless decay (Onubetabeta) mode has fundamental consequences: first, the total lepton number is not conserved, and second, the neutrino is a Majorana particle. Furthermore, the measured effective mass provides an absolute scale of the neutrino mass spectrum. In addition, double beta experiments yield sharp restrictions for other beyond-standard-model physics. These include SUSY models (R-paxity breaking and conserving), leptoquarks (leptoquark-Higgs coupling), compositeness, left-right symmetric models (right-handed 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 reported by the HEIDELBERG-MOSCOW experiment in 2001. The HEIDELBERG-MOSCOW experiment is by far the most sensitive Onubetabeta experiment since more than 10 years. It is operating 11 kg of enriched Ge-76 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 71.7 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 100000 events to be (1.74.(+0.18)(-0.16)) x 10(21) years. The confidence level for the neutrinoless signal has been improved to a 4.2sigma level. The half-life is T-1/2(Onu) = (1.19(-0.23)(+0.37)) 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 the corresponding results from high-energy accelerators like TEVATRON, HERA, etc.