A. Balysh

Latest results from the HEIDELBERG-MOSCOW double beta decay experiment

Abstract:New results for the double beta decay of 76Ge are presented. They are extracted from data obtained with the HEIDELBERG-MOSCOW experiment, which operates five enriched 76Ge detectors in an extreme low-level environment in the Gran Sasso underground laboratory. The two-neutrino-accompanied double beta decay is evaluated for the first time for all five detectors with a statistical significance of 47.7 kg y resulting in a half-life of T1/22ν = [1.55±0.01(stat)+0.19-0.15(syst)]×1021 y. The lower limit on the half-life of the 0νββ decay obtained with pulse shape analysis is T1/20ν > 1.9×1025(3.1×1025) y with 90% C.L. (68% C.L.) (with 35.5 kg y). This results in an upper limit of the effective Majorana-neutrino mass of 0.35 eV (0.27 eV) using the matrix elements of A. Staudt et al.s work (Europhys. Lett. 13, 31 (1990)). This is the most stringent limit at present from double beta decay. No evidence for a majoron-emitting decay mode is observed.

LIMITS ON THE MAJORANA NEUTRINO MASS IN THE 0.1 EV RANGE

The Heidelberg-Moscow experiment gives the most stringent limit on the Majorana neutrino mass. After 24 kg yr of data with pulse shape measurements, we set a lower limit on the half-life of the neutrinoless double beta decay in 76Ge of T_1/2 > 5.7 * 10^{25} yr at 90% C.L., thus excluding an effective Majorana neutrino mass greater than 0.2 eV. This allows to set strong constraints on degenerate neutrino mass models.

Searching for dark matter with the enriched Ge detectors of the Heidelberg-Moscow ββ experiment

Abstract For the first time a search for dark matter with isotopically enriched material is done, by using the Ge detectors of the Heidelberg-Moscow experiment. A measuring time of 165.6 kg·d is used to set limits on the spin-independent cross section of weakly interacting massive particles (WIMPs). A background level of 0.102±0.005 events/(kg·d·keV) was achieved (average value between 11 keV and 30 keV). It was possible to extend the exclusion range for Dirac neutrino masses up to 4.7 TeV.

The Heidelberg-Moscow experiment: improved sensitivity for 76Ge neutrinoless double beta decay

Abstract The Heidelberg-Moscow experiment operates five enriched 76Ge detectors with 10.96 kg active mass in the Gran Sasso underground laboratory. After 28.7 kg y of measurement with a counting rate between 2000 keV and 2080 keV of 0.20 ± 0.01 cts/kg y keV we receive a lower limit on the neutrinoless double beta decay of 76Ge of T 1 2 (0 + → 0 + ) > 1.1 × 10 25 y (90% C.L.). This restricts the Majorana neutrino mass to 0.46 eV. Recently we achieved a reduction in the background counting rate by a factor of 3–5 within the energy region of the neutrinoless double beta decay by recording the differentiated preamplifier pulse shapes and doing an off-line analysis to distinguish between pointlike and multiple scattered interactions.

Double Beta Decay of 48 Ca

{sup 48}Ca, the lightest experimentally accessible double beta decay candidate, is the only one simple enough to be treated exactly in the nuclear shell model. Thus the {beta}{beta}{sub 2{nu}} half-life measurement, reported here, provides a unique test of the nuclear physics involved in the {beta}{beta} matrix element calculation. Enriched {sup 48}Ca sources of two different thicknesses have been exposed in a time projection chamber. We observe a half-life of T{sub 1/2}{sup 2{nu}}=(4.3{sub {minus}1.1}{sup +2.4}[stat]{plus_minus}1.4[syst]){times}10{sup 19} yr, consistent with shell model calculations. {copyright} {ital 1996 The American Physical Society.}

Sub-eV limit for the neutrino mass from 76Ge double beta decay by the HEIDELBERG-MOSCOW experiment

Abstract The HEIDELBERG-MOSCOW double beta decay experiment using HP germanium semiconductor detectors enriched in 76Ge is the first ββ experiment to penetrate into the sub-eV range. After a measuring time of 10.2 kg·a (117.0 mol·a) the half-life limit for 76Ge 0νββ decay is T 1 2 ≥ 5.6×10 24 a (90% C. L.) or T 1 2 ≥9.4×10 24 a (68% C.L.) which corresponds to an upper limit for the effective Majorana neutrino mass 〈mν〉 ≤ 0.65 eV (90% C.L.) or 〈mν〉 ≤ 0.50 eV (68% C. L.). For a superheavy neutrino, as it occurs, e. g., in seesaw models, we extract a limit of 〈mH〉 ≥ 5.1×107 GeV. These numbers are of importance in the context of presently discussed indications of physics beyond the Standard Model.

Background recognition in Ge detectors by pulse shape analysis

Abstract A method of event identification that distinguishes single and multiple-site events by determining the number of interactions in a high purity germanium detector is reported. The selectivity of the method has been experimentally verified.

Measurement of the ββ2ν decay of 76Ge

Abstract From the data taken with one of the enriched detectors of the Heidelberg-Moscow ββ experiment a half-life of T 1 2 2v = (1.42 ± 0.03 ( stat ) ± 0.13 ( syst )) × 10 12 yr for the two-neutrino double-beta ( ββ 2 ν ) decay of 76 Ge is derived. The 76 Ge exposure is 19.3 mol yr. This result represents the first high statistics measurement and probably the first undoubtable evidence of this extremely rare nuclear decay mode. The measured decay rate is in good agreement with the theoretical predictions.

Bounds on new Majoron models from the Heidelberg-Moscow experiment.

In recent years several new Majoron models were invented to avoid the shortcomings of the ordinary models while leading to observable decay rates in double {beta} experiments. We give the first experimental half-life bounds on double {beta} decays with new Majoron emission and derive bounds on the effective neutrino-Majoron couplings from the data of the {sup 76}Ge Heidelberg-Moscow experiment. While stringent half-life limits for all decay modes and the coupling constants of the ordinary models were obtained, small matrix elements and phase space integrals result in much weaker limits on the effective coupling constants of the new Majoron models. {copyright} {ital 1996 The American Physical Society.}

New experimental limits for electron decay and charge conservation

Abstract New experimental limits for the decay e− → γ + νe are reported. The lower limit for the half-life of this decay mode is T e 1 2 > 1.63 × 10 25 yr (68% CL). The data were collected for 3199 h by using one of the enriched germanium detectors of the Heidelberg-Moscow ββ Collaboration. This detector has an active volume of 591 cm3. This value is up to now the most stringent laboratory limit for this decay mode. Also charge nonconservation in nuclei is shortly discussed in the GaGe system using the data of gallium solar neutrino experiments.