Speaker: Hans V. Klapdor-Kleingrothaus (Max Planck Institute for Nuclear Physics, Heidelberg, Germany)

EVIDENCE FOR NEUTRINOLESS DOUBLE BETA DECAY FROM THE HEIDELBERG-MOSCOW EXPERIMENT 1990 - 2003 - AND IMPLICATIONS

Double beta decay is one of the rarest nuclear decay modes and is under investigation already for more than sixty years. The neutrinoless mode is of particular interest since it would violate lepton number and its occurrence would determine the neutrino to be a Majorana particle. Double beta decay experiments are indispensable to solve the structure of the neutrino mass matrix. They further probe, complementary to high energy colliders, other fields of beyond standard model physics. The present experimental status is reviewed. Emphasis is given to the final analysis of the Heidelberg-Moscow experiment for the period August 1990 to May 2003. This experiment, operated with 11 kg of enriched 76Ge in the Gran Sasso Underground Laboratory, is since 10 years the most sensitive double beta experiment worldwide. The results give evidence for neutrinoless double beta decay on a 4.2 sigma confidence level. Implications and future perspectives are discussed.

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