The Heidelberg-Moscow double beta decay experiment with enriched 76Ge: First results

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Double beta decay probes beyond standard model physics. Status and perspectives of the HEIDELBERG-MOSCOW double beta decay experiment which is undertaken in the GRAN SASSO laboratory, are presented. Of the 16.9 kg of enriched (86%) 76Ge in hands of the cooperation, 9.6 kg have been converted into detectors (or crystals). At present 6 kg of detectors are in operation for data taking. The significance of the experiment at present is 1133 kg. d. Present limits for Ovββ decay to the g.s. and first excited state of 76Se, respectively, are: T 1 2 0v (0 + → 0 +) > 1.93 (3.21) × 10 24 y, and T 1 2 0v (0 + → 2 +) > 8.0 × 10 23 y (1.5 . 10 24 y) with 90% (68%) c.l. These values are the most stringent directly measured 0vββ half life limits known up to now. They correspond to an effective neutrino mass limit of <m v > < 1.1 (0.9) eV. For the neutrinoless decay with majoron emission a half life of 3.89 × 10 22 y can be excluded leading to an upper limit for the neutrino-majoron coupling of < g rx > < 1.10. 10 4 (90% c.l.). The background around 2 MeV is b=0.4 counts/kg y KeV for the total array, and 0.23 counts/kg y keV for the 2.9 kg detector. The experiment opens new perspectives for the investigation of exotic processes like electron decay, solar axions and dark matter. The new ββ-technology has found also application in high resolution balloon and satellite gamma-ray astronomy. Concerning the matrix elements for ββ decay a major step beyond the frequently used QRPA model has been made by applying the Operator Expansion Method (OEM).