Double beta decay and neutrino mass. The Heidelberg-Moscow experiment

Double beta (ββ) 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 ββ 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. For the neutrinoless decay with majoron emission a half life of 1.7 x 1022 y can be excluded leading to an upper limit for the neutrino-majoron coupling of <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 ∼ 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 ββ-technology has also found application in high resolution balloon and satellite gamma-ray astronomy. Concerning the matrix elements for ββ decay a major step beyond the frequently used QPRA model has been made by applying the Operator Expansion Method (OEM). (Author)