First Evidence for Neutrinoless Double Beta Decay - and World Status of Double Beta Experiments

Abstract

The occurrence of the neutrinoless decay 0n-bb 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, 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 given in 2001, by the HEIDELBERG-MOSCOW experiment. The HEIDELBERG-MOSCOW experiment is the by far most sensitive 0-nu-bb experiment since more than 10 years. It operated 11 kg of enriched 76Ge in the Gran Sasso. The collected statistics in the period 1990 - 2003 is 71.7 kgy. The background achieved in the energy region of the Q value for double beta decay is 0.11events/kgykeV. The two-neutrino accompanied half-life is determined on the basis of more than 100000 events to be (1.74. {+0.18}{-0.16})x10^{21}years. The confidence level for the neutrinoless signal is 4.2 sigma level (more than 5 sigma in the pulse-shape-selected spectrum). The half-life is T_{1/2}^{0ν}=(1.19{+0.37} {-0.23})x10^{25}years. The effective neutrino mass deduced is (0.2-0.6)eV (99.73% c.l.), i.e. neutrinos have degenerate masses, and consequently can considerably contribute to hot dark matter in the Universe. The sharp boundaries for other beyond SM physics, mentioned above, are comfortably competitive to corresponding results from high-energy accelerators like TEVATRON, HERA, etc. Some discussion is given on future beta-beta experiments.