Breakthrough into the Sub-eV Neutrino Mass Range: Status of the HEIDELBERG-MOSCOW Double Beta Decay Experiment with enriched $^{76}$Ge

Abstract

Recent results of the Heidelberg-Moscow double beta decay experiment are presented. After 8.6 kg$\cdot$a of measuring time no signal is seen for the neutrinoless decay mode. A half-life limit of $T_{1/2}^{0\nu\beta\beta} > 5.1\cdot 10^{24}$~a is deduced which converts into a neutrino mass limit of $\langle m_{\nu_e}\rangle < 0.68 $ eV (90\% CL). The experiment thus is the first one penetrating into the sub-eV range for the neutrino mass. For the $2\nu$ mode a half life of $T_{1/2}^{2\nu\beta\beta} = (1.53\pm0.04 _{stat}\pm0.13_{sys})\cdot 10^{24}$ a is derived. More than 10000 $2\nu$ double beta events are observed. This is the first high statistics observation of this nuclear decay mode. Limits on more exotic decay modes are also presented. Concerning dark matter the experiment now gives the sharpest limits for the observation of WIMPs.