Andreas Muller

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 double beta decay experiment with enriched 76Ge. First results

Abstract The status of the Heidelberg-Moscow ββ-experiment using isotopically enriched 76 Ge is reported. The results of 14.8 mol yr (or 1.29 kg yr) of operation are presented. From these data a new half life time for the ββ0v-decay of 76 Ge to the ground state of 76 Se of T 1 2 1.4 (2.5) X 10 24 yr with 90% (68%) CL can be deduced. For a possible neutrinoless decay to the first excited state a half life of 4.3(8.2)X10 23 yr can be excluded with 90% (68%) CL.

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.

Investigation of the ββ decay of 116Cd into excited states of 116Sn

Abstract The double-beta decay of 116Cd into excited states of 116Sn is experimentally and theoretically investigated. From an inclusive experiment, using an external source of isotopically enriched Cd, new most stringent limits for the allowed and non-standard-model decays into excited states are derived. It is further investigated whether the bremsstrahlung emitted by the ββ electrons can be used to derive information on the ground-state decay. For the two-neutrino-decay mode a calculation, using the quasiparticle random-phase approximation, shows that the disadvantage in phase space, in comparison to the ground-state decay, is partially compensated through the nuclear-matrix element. Experimental perspectives for other ββ-unstable nuclides are discussed.

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.

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.

Investigation ofβ + β + andβ +/EC decay of106Cd

A low background scintillation detector with a CdWO4 crystal of 1.046 kg was used to search forβ+β+ andβ+/EC processes in106Cd. For the neutrinoless mode the limits T1/2(0νβ+β+)≥2.2·1019 y and T1/2(0νβ+/EC)≥5.5·1019 y were obtained with 90% C.L. For the possible two neutrino decay limits of T1/2(2νβ+β+)≥9.2·1017 y and T1/2(2νβ+/EC)≥2.6·1017 y have been determined with 99% C.L.

Recent double beta decay results

The status and recent results of second generation [beta][beta]-experiments using isotopically enriched source materials are described. These experiments are at present the most sensitive tools to distinguish Dirac from Majorana neutrinos. The at present most advanced experimental techniques, namely the use of high-resolution calorimetric detectors and of time projection chambers are compared. New limits on the Majorana neutrino mass as well as for the Majoron-neutrino coupling are presented.