arXiv subjects

V. dos S. Ferreira

Publications and source records attributed to V. dos S. Ferreira.

3 recordsLinked to original sources

arXiv2025

Neutrinoless $ββ$-Decay in DCEQTDA

We have recently developed a nuclear model, which is a natural extension of the $pn$-QRPA model, specially designed to describe double charge exchange (DCE) processes generated by two-body DCE transition operators. It is based on the Quasiparticle Tamm-Dancoff Approximation (QTDA) for $pn$ and $2p2n$ excitations in intermediate and final nuclei, respectively, and will be called DCEQTDA. As such, this model, having the same number of free parameters as the $pn$-QRPA, also brings into play the excitations of four quasiparticles to build up the final nuclear states, which are then used to evaluate the nuclear matrix elements (NMEs) for all $0^+$ and $2^+$ final states, including resonances, and not just for the ground state as in $pn$-QRPA. In addition, it allows us to evaluate: (a) the values of $Q_{ββ}$, (b) the excitation energies in final nuclei, and (c) the DCE sum rules, which are fulfilled in the DCEQTDA. So far, this model has been used mainly to calculate double beta decays with the emission of two neutrinos ($2νββ$-decay). Here, we extend it to the study of these processes when no neutrinos are emitted ($0νββ$-decay), evaluating them in a series of nuclei, but paying special attention to (i) $^{76}$Se, which have been measured recently in the GERDA and MAJORANA experiments, and (ii) $^{124}$Te, for which the first direct observation of the double electron capture $2ν$ has been performed with the XENON1T dark matter detector. We obtain good agreement with the data for both the ground state and the excited states. The validity of the DCEQTDA model is checked by comparing the calculation with the experimental data for the $2νββ$ NMEs, and for the $Q_{ββ}$, in a series of nuclei.

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arXiv2019

A Nuclear Structure Model for Double Charge-Exchange Processes

A new model, based on the BCS approach, is specially designed to describe nuclear phenomena $(A,Z)\rightarrow (A,Z\pm 2)$ of double-charge exchange (DCE). After being proposed, and applied in the particle-hole limit, by one of the authors (F. Krmpotić [1]), so far it was never been applied within the BCS mean-field framework, nor has its ability to describe DCE processes been thoroughly explored. It is a natural extension of the pn-QRPA model, developed by Halbleib and Sorensen [2] to describe the single $β$-decays $(A,Z)\rightarrow (A,Z\pm 1)$, to the DCE processes. As such, it exhibits several advantages over the pn-QRPA model when is used in the evaluation of the double beta decay (DBD) rates. For instance, i) the extreme sensitivity of the nuclear matrix elements (NMEs) on the model parametrization does not occur, ii) it allows to study NMEs, not only for the fundamental state in daughter nuclei, as the pn-QRPA model does, but also for all final $0^+$ and $2^+$ states, accounting at the same time their excitation energies and the corresponding DBD Q-values, iii) together with the DBD-NMEs it provides also the energy spectra of Fermi and Gamow-Teller DCE transition strengths, as well as the locations of the corresponding resonances and their sum rules, iv) the latter are relevant for both the DBD and the DCE reactions, since the involved nuclear structure is the same; this correlation does not exist within the pn-QRPA model. As an example, detailed numerical calculations are presented for the $(A,Z)\rightarrow (A,Z+ 2)$ process in $^{48}$Ca $\rightarrow ^{48}$Ti and the $(A,Z)\rightarrow (A,Z- 2)$ process in $^{96}$Ru $\rightarrow ^{96}$Mo, involving all final $0^+$ states and $2^+$ states.

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arXiv2017

Partial restoration of spin-isospin SU(4) Symmetry and the One-QRPA method in Double Beta Decay

The one-QRPA method is used to describe simultaneously both double decay beta modes, giving special attention to the partial restoration of spin-isospin SU(4) symmetry. To implement this restoration and to fix the model parameters, we resort to the energetics of Gamow-Teller resonances and to the minima of the single $β^+$-decay strengths. This makes the theory predictive regarding the $ββ_{2ν}$-decay, producing the $2ν$ moments in $^{48}$Ca, $^{76}$Ge, $^{82}$Se, $^{96}$Zr, $^{100}$Mo, $^{128,130}$Te, and $^{150}$Nd, that are of the same order of magnitude as the experimental ones; however, the agreement with $ββ_{2ν}$ data is only modest. To include contributions coming from induced nuclear weak currents, we extend the $ββ_{0ν}$-decay formalism employed previously in C. Barbero et. al, Nuc. Phys. A628, 170 (1998). The numerical results for the $ββ_{0ν}$ moments in the above mentioned nuclei are similar to those obtained in other theoretical studies although smaller on averag by $\sim 40\%$. We attribute this difference basically to the one-QRPA-method, employed here for the first time, instead of the currently used two-QRPA-method. The difference is partially due to the way of carrying out the restoration of the spin-isospin symmetry. It is hard to say which is the best way to make the restoration, since the $ββ_{0ν}$ moments are not experimentally measurable. The numerical uncertainties in the $ββ$ moments, related with i) their strong dependence on the residual interaction in the p-p channel when evaluated within the QRPA, and ii) lack of proper knowledge of single-particle energies, have been quantified. It is concluded that the partial restoration of the SU (4) symmetry is crucial in the description of the $ββ$-decays, regardless of the nuclear model used.

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