We present a new contribution of the R-parity violating (Rp / ) supersymmetry (SUSY) to neutrinoless double beta decay (0νββ) via the pion exchange between decaying neutrons.The pion coupling to the final state electrons is induced by the Rp / 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 π-meson.The constraints on the certain Rp / SUSY parameters are extracted from the current experimental 0νββ-decay half-life limit. These constraints are significantly stronger than those previously known or expected from the ongoing accelerator experiments.12.60. Jv, 11.30.Er, 23.40.Bw Neutrinoless double beta decay (0νββ) has long been recognized as a sensitive probe of the new physics beyond the standard model (SM) (see [1]-[2]). Various mechanisms of 0νββ decay were proposed and studied in the last two decades. The conventional mechanism is based on the exchange of a massive Majorana neutrino between the two decaying neutrons. A new mechanism was found within supersymmetric (SUSY) models with R-parity violation (3B+L+2S where S, B and L are spin, baryon and lepton numbers.) It was later studied in more details in [4]. A complete analysis of this mechanism within the minimal supersymmetric standard model (MSSM) was carried out in [5].The nuclear 0νββ-decay is triggered by the 0νββ quark transition d + d → u + u + 2e − which is induced by certain fundamental interactions. It was a common practice to put the initial d-quarks separately inside the two initial neutrons of a 0νββ-decaying nucleus. This is the so called two-nucleon mode of the 0νββ-decay [see Fig. 1(a)]. If the above 0νββ quark transition proceeds at short distances, as in the case of R p / SUSY interactions, then the basic nucleon transition amplitude n + n → p + p + 2e − is strongly suppressed for relative distances larger than the mean nucleon radius.In this letter we propose a new pion-exchange SUSY mechanism which is based on the double-pion exchange between the decaying neutrons [ Fig. 1(b)]. At the quark level this mechanism implies the same short-distance R p / MSSM interactions as in [5]. However, it essentially differs from the previous consideration of the SUSY contribution to the 0νββ-decay at the stage of the hadronization. We assume that the R p / MSSM quark interactions induce ππ → 2e transition at the middle point of the diagram in Fig. 1(b). The importance of the pion-exchange currents in 0νββ-decay was first been pointed out by B. Pontecorvo [6]. Latter, this idea was quantitatively realized in [7]-[8] for the case of the heavy Majorana neutrino exchange. It was shown that the pion-exchange contribution can not be neglected in this case. We will show that in the case of the R p / MSSM induced quark transition the pion-exchange contribution absolutely dominates over the conventional two-nucleon mode.The R p -violating part of the superpotential breaki...
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 of 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.Comment: Accepted for publication in Nucl. Phys. A, 22 pages, including 5 figures, LaTeX (using REVTeX and epsfig-style
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 M o, 128 T e and 130 T e. 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.
The incoherent matrix elements of the exotic ( Ϫ ,e Ϫ ) conversion in nuclei process are studied in detail for a set of nuclei throughout the periodic table in the context of the quasiparticle random phase approximation ͑RPA͒. The contaminations, usually inserted in the 1 Ϫ RPA excitation modes ͑the most important incoherent →e conversion channel͒, are removed by explicitly constructing the purely spurious center-of-mass state. We found that mostly the lowest-lying 1 1 Ϫ state is affected by the use of non-self-consistent single-particle energies and a truncated model space in the RPA. To a good approximation we can regard this state as fully spurious and treat the other states as the physical ones. The elimination of the spuriousness requires a different renormalization of the interaction. This allows us to reproduce the excitation spectrum, needed to calculate reliably the incoherent matrix elements of the →e process, with realistic forces ͑Bonn potential͒ which cannot be achieved with the contaminated wave functions. We focus on the investigation of the incoherent rate of 48 Ti, from which the best upper limit for the flavor number violation has been extracted, and 208 Pb, which is currently used in the SINDRUM II experiment at PSI. ͓S0556-2813͑97͒03311-6͔
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