DOUBLE-BETA DECAY AND NEUTRINO MASS - THE HEIDELBERG-MOSCOW EXPERIMENT

Double beta (beta beta) decay probes beyond standard model physics and at present is the most sensitive tool to probe the electron neutrino mass. The experimental and theoretical status of beta beta research is briefly reviewed. The main emphasis is put on the HEIDELBERG-MOSCOW experiment, which is undertaken in the GRAN SASSO laboratory and which at present has the largest sensitivity and gives the sharpest limits. These are from 2924 kg d of data taking with 6 kg of enriched (86%) Ge-76 detectors: T-1/2(Ov)(0(+) --> 0(+))>1.9 x 10(24) y and T-1/2 (Ov)(0(+) --> 2(+))> 8.0 x 10(23)y with 90% c.l. They correspond to an effective neutrino mass limit of < m(v) > >< 1.1 eV. For the neutrinoless decay with majoron emission a half life of 1.7 x 10(22) y can be excluded leading to an upper limit for the neutrino-majoron coupling of <g(vy)> < 1.8 10(-4) (90% c.l.). The background around 2 MeV is b = 0.2 counts/kg y keV for the total array. The experiment opens new perspectives for the investigation of exotic processes like electron decay, solar axions and dark matter. It was, e.g., possible to improve the existing cross section limits for WIMP masses above similar to 150 GeV and to exclude Dirac neutrinos in the range 26 GeV to 4.7 TeV as the dominant component of the dark halo. The new beta beta-technology has found also application in high resolution balloon and satellite gamma-ray astronomy. Concerning the matrix elements for pp decay a major step beyond the frequently used QRPA model has been made by applying the Operator Expansion Method (OEM).