H.V. Klapdor-Kleingrothaus

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.

Supersymmetry and neutrinoless double beta decay

Neutrinoless double beta decay (0���) induced by superparticle exchange is investigated. Such a supersymmetric (SUSY) mechanism of 0��� decay arises within SUSY theories with R-parity non-conservation (Rp / ). We consider the minimal supersymmetric standard model (MSSM) with explicit Rp / terms in the superpotential (Rp

Data acquisition and analysis of the 76Ge double beta experiment in Gran Sasso 1990-2003

Abstract Data acquisition in a long-running underground experiment has its specific experimental challenges, concerning data acquisition, stability of the experiment and background reduction. These problems are addressed here for the HEIDELBERG–MOSCOW experiment, which collected data in the period August 1990–May 2003. The measurement and the analysis of the data are presented. The duty cycle of the experiment was ∼80%, and the collected statistics is 71.7 kg year. The background achieved in the energy region of the Q value for double beta decay is 0.11 events / kg year keV . The two-neutrino accompanied half-life is determined on the basis of more than 100 000 events. The confidence level for the neutrinoless signal has been improved to a 4 σ level.

Nuclear structure calculation ofβ+β+,β+/EC and EC/EC decay matrix elements

Nuclear matrix elements for double positron emisson (β+β+), positron emission/electron capture (β+/ EC) and double electron capture (EC/EC) in the 2νββ decay mode and forβ+β+ andβ+/EC decay in the 0νββ mode are calculated for the experimentally most promising isotopes58Ni,78Kr,96Ru,106Cd,124Xe,130Ba and136Ce within pn-QRPA. We point out that the matrix element for the 2νβ+/EC decay differs from the 2νβ+β+ matrix element, an effect not considered previously. For the neutrino accompanied decays our calculation predicts for theβ+/EC and the EC/EC mode half lives which are shorter typically by 4–7 orders of magnitude than those for the double positron emission. However, even for the best candidates typical values for 2νβ+/EC (2ν EC/EC) are still in the range of ∼1022 ((some) 1021) years. For 0νββ decay we have calculated all matrix elements relevant for both, the mass mechanism and the right-handed currents for the first time complete. A detailed discussion of the differences between the 0νβ+β+, the 0νβ+/EC and 0νβ−β− decay is given.

Double beta decay in left-right symmetric models

Abstract Left-right symmetric models provide a natural framework for neutrinoless double beta (0νββ) decay. In the analysis of 0νββ decay in left-right symmetric models, however, it is usually assumed that all neutrinos are light. On the other hand, heavy right-handed neutrinos appear quite naturally in left-right symmetric models and should therefore not be neglected. Assuming the existence of at least one right-handed heavy neutrino, absence of 0νββ decay of 76 Ge currently provides the following limits on the mass and mixing angle of right-handed W -bosons: m W R ≥ 1.1 TeV and tan(ζ) ≤ 4.7 × 10 −3 for a particular value of the effective right-handed neutrino mass, 〈 m N ( V ) 〉 = 1 TeV, and in the limit of infinitely massive doubly charged Higgs ( Δ −− ). The effects of the inclusion of the Higgs triplet on 0νββ decay are also discussed.

B-L-violating masses in softly broken supersymmetry

Abstract We prove a general low-energy theorem establishing a generic relation between the neutrino Majorana mass and the superpartner sneutrino B-L-violating “Majorana”-like mass term. The theorem states that if one of these two quantities is non-zero the other one is also non-zero and, vice versa, if one of them vanishes the other vanishes, too. The theorem is a consequence of the underlying supersymmetry (SUSY) and valid for any realistic gauge model with weak scale softly broken SUSY.

Microscopic Predictions of β+/EC-Decay Half-Lives

Beta-decay half-lives for β+/EC (electron capture) decay for neutron-deficient nuclei with atomic numbers Z = 10-108 have been calculated up to the proton drip line. For this purpose a proton-neutron quasiparticle random-phase approximation model (pn-QRPA) has been used. Both particle-hole and particle-particle residual interactions are taken into account. The calculated half-lives are in good agreement with experimental data, indicating the applicability of the model to the prediction of half-lives of unknown isotopes. The results of the present calculation are also compared to earlier theoretical work. It is shown that the pn-QRPA leads to considerable improvement over gross theory calculations.

A superformula for neutrinoless double beta decay II: the short range part

Abstract A general Lorentz-invariant parameterization for the short-range part of the 0 νββ decay rate is derived. Combined with the long range part already published this general parameterization in terms of effective B – L violating couplings allows one to extract the 0 νββ limits on arbitrary lepton number violating theories.

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.