J. Schwieger

Dominance of pion exchange in R-parity violating supersymmetry contributions to neutrinoless double beta decay

We present a new contribution of the R-parity violating supersymmetry (SUSY) to neutrinoless double beta decay via the pion exchange between decaying neutrons. The pion coupling to the final state electrons is induced by the R-parity violating SUSY interactions. We have found this pion-exchange mechanism to dominate over the conventional two-nucleon one. The latter corresponds to direct interaction between quarks from two decaying neutrons without any light hadronic mediator like pion. The constraints on the certain R-parity violating SUSY parameters are extracted from the current experimental neutrinoless double beta decay half-life limit. These constraints are significantly stronger than those previously known or expected from the ongoing accelerator experiments.

The Pauli principle, QRPA and the two neutrino double beta decay

Abstract We examine the violation of the Pauli exclusion principle in the Quasiparticle Random Phase Approximation (QRPA) calculation of the two-neutrino double beta decay matrix elements, which has its origin in the quasi-boson approximation. For that purpose we propose a new renormalized QRPA with proton-neutron pairing method (full-RQRPA) for nuclear-structure studies, which includes ground-state correlation beyond the QRPA. This is achieved by using the renormalized quasi-boson approximation, in which the Pauli exclusion principle is taken into account more carefully. The full-RQRPA has been applied to two-neutrino double beta decay of 76 Ge, 82 Se, 128 Te and 130 Te. The nuclear matrix elements have been found significantly less sensitive to the increasing strength of particle-particle interaction in the physically interesting region in comparison with QRPA results. The strong differences between the results of both methods indicate that the Pauli exclusion principle plays an important role in the evaluation of the double beta decay. The inclusion of the Pauli principle removes the difficulties with the strong dependence on the particle-particle strength g pp in the QRPA on the two-neutrino double beta decay.

Non-collapsing renormalized QRPA with proton-neutron pairing for neutrinoless double beta decay☆

Abstract Using the renormalized quasiparticle random phase approximation (RQRPA), we calculate the light neutrino mass mediated mode of neutrinoless double beta decay (0νββ-decay) of 76 Ge, 100 Mo, 128 Te and 130 Te. Our results indicate that the simple quasiboson approximation is not good enough to study the 0νββ-decay, because its solutions collapse for physical values of g pp . We find that extension of the Hilbert space and inclusion of the Pauli principle in the QRPA with proton-neutron pairing, allows us to extend our calculations beyond the point of collapse, for physical values of the nuclear force strength. As a consequence one might be able to extract more accurate values on the effective neutrino mass by using the best available experimental limits on the half-life of 0νββ-decay.

The exotic μ−→e− conversion rates by explicit renormalized quasiparticle RPA calculations.

Abstract We investigate the μ−e conversion matrix elements by explicit calculations within the renormalized quasiparticle random phase approximation (RQRPA). In this approach, by promoting the quasi-Boson approximation mostly used in ordinary QRPA, the Pauli-principle is restored to a large extent and the nucleon-nucleon ground state correlations are reliably taken into account. We have pointed out that the usual QRPA includes quite strong ground state correlations and it, therefore, overestimates their effect on the μ−e conversion rates and especially on the coherent mode, which is the most important μ−e conversion channel. The rate of this channel is currently measured in the SINDRUM II experiment at PSI and is expected to be extracted in the designed MECO experiment at Brookhaven. By combining our nuclear calculations with the existing experimental limits on the branching ratio Rμe− for 208 Pb and 48 Ti we put constraints on the flavour violating parameters entering the μ−e conversion effective currents in modern gauge theories.

Double beta decay to excited states in 76Ge within renormalized QRPA

We investigate the relevant nuclear matrix element for the neutrino-accompanied double beta decay of to the first excited -state in . We show, that most of the dependence of the matrix element can be removed by the inclusion of the Pauli principle in the QRPA-calculations. The matrix element remains stable in the physical region of and would thus allow to reduce limits of the decay.

A large Hilbert space QRPA and RQRPA calculation of neutrinoless double beta decay

A large Hilbert space is used for the calculation of the nuclear matrix elements governing the light neutrino mass mediated mode of neutrinoless double beta decay (Ovββ-decay) of76Ge,100Mo,116Cd,128Te, and136Xe within the proton-neutron quasiparticle random phase approximation (pn-QRPA) and the renormalized QRPA with proton-neutron pairing (full-RQRPA) methods. We have found that the nuclear matrix elements obtained with the standard pn-QRPA for several nuclear transitions are extremely sensitive to the renormalization of the particle-particle component of the residual interaction of the nuclear hamiltonian. Therefore the standard pn-QRPA does not guarantee the necessary accuracy to allow us to extract a reliable limit on the effective neutrino mass. This behavior already known from the calculation of the two-neutrino double beta decay matrix elements, manifests itself in the neutrinoless double-beta decay but only if a large model space is used. The full-RQRPA, which takes into account proton-neutron pairing and considers the Pauli principle in an approximate way, offers a stable solution in the physically acceptable region of the particle-particle strength. In this way more accurate values on the effective neutrino mass have been deduced from the experimental lower limits of the half-lifes of neutrinoless double beta decay.

Pauli principle and pion distortion in the p-wave pionic double charge exchange reaction

We propose an approach which takes the Pauli principle and pion distortion into account to investigate the p-wave absorption/emission mechanism of the double charge exchange reaction. The ground state to ground-state processes and are discussed in the new approach. Excitation functions with a maximum energy distribution at have been obtained. The calculated differential cross sections at with and their ratio agree well with experimental data and depend only slightly on the particle-particle interaction parameter . Results indicate that the Pauli principle lowers the cross section and stabilizes its dependence on evidently while pion distortion reduces the cross section in the energy region above the resonance.

RQRPA, nuclear structure and double beta processes

Abstract The influence of the nuclear structure on the extraction of lepton number violating parameters like the light effective Majorana-mass of the neutrino is complex. Normal Quasiparticle RPA (QRPA) calculations are unstable against a variation of the residual interaction strength around the physical value. By including the Pauli-Principle in QRPA (Renormalized QRPA) this dependence can be stabilized in the physical region. For R-parity violating SUSY-extensions of the standard model the pion-exchange is dominant compared to the two nucleon mode. With the RQRPA-method very stringent limits can be deduced.

Rqrpa in ββ- and μ− → e−-processes

We compare the different influences of the improved treatment of groundstate-correlations in RQRPA-calculations for the matrix-elements of double-beta-decay and myon-electron conversion processes. While description of the double-beta-decay elements significantly improves in an RQRPA-approach, in the μ− → e−-conversion the shortcomming of the RQRPA, the failure to exhaust the Ikeda sum rule, can be observed.

Non-collapsing QRPA for neutrinoless double beta decay

The standard quasiparticle random phase approximation(QRPA) is widely used to describe the neutrinoless double beta decay process. Although it has been quite successful in many cases of interest, it has some shortcomings. The most important one is that its solutions collapse for physical values of the particle-particle strength. We shall show that modifications can be done which can extend the validity of this standard QRPA beyond the point of collapse. Such modifications are: The introduction of proton-neutron pairing, the inclusion of the Pauli principle and the extension of the Hilbert space. If all these modifications are introduced into the standard QRPA then the collapse does not occur for physical values of the particle-particle strengths. Thus, one might be able to extract more accurate values on the effective neutrino mass by using the best available experimental limits on the half life of neutrinoless double beta decay.