Neutrinoless double-beta (0νββ) decay is a promising beyond Standard Model process. Twoneutrino double-beta (2νββ) decay is an associated process that is allowed by the Standard Model, and it was observed in about 10 isotopes, including decays to the excited states of the daughter.124 Sn was the first isotope whose double-beta decay modes were investigated experimentally, and despite few other recent efforts, no signal has been seen so far. Shell model calculations were able to make reliable predictions for 2νββ decay half-lives. Here we use shell model calculations to predict the 2νββ decay half-life of 124 Sn. Our results are quite different from the existing quasiparticle randomphase approximation (QRPA) results, and we envision that they will be useful for guiding future experiments. We also present shell model nuclear matrix elements for two potentially competing mechanisms to the 0νββ decay of 124 Sn.
Most uncertainties regarding the theoretical study of the neutrinoless double-beta decay are related to the accuracy of the nuclear matrix elements that appear in the expressions of the lifetimes. We calculate the nuclear matrix elements for the 0νββ decay of 130 Te in a shell model approach, using a recently proposed effective Hamiltonian. To ensure the reliability of the results, we investigate this Hamiltonian by performing calculations of spectroscopic quantities and comparing them to the latest experimental data available, and we analyze the 2νββ and the 0νββ decay nuclear matrix elements of 136 Xe. Finally, we report new nuclear matrix for the 130 Te considering the light neutrino exchange and heavy neutrino exchange mechanisms, alongside with an overview of some recent values reported in the literature.
Neutrinoless double-beta decay, is a beyond the Standard Model process that would indicate that neutrinos are Majorana fermions and the lepton number is not conserved. It could be interesting to use the neutrinoless double-beta decay observations to distinguish between several beyond Standard Model mechanisms that could contribute to this process. Accurate nuclear structure calculations of the nuclear matrix elements necessary to analyze the decay rates could be helpful to narrow down the list of contributing mechanisms. We investigate the information one can get from the angular and energy distribution of the emitted electrons, and from the half-lives of several isotopes, assuming that the right-handed currents exist. For the analysis of these distributions we calculate the necessary nuclear matrix elements using shell model techniques, and we explicitly consider interference terms.
We analyze the effects that different nuclear structure approximations associated with the short range correlations (SRC), finite nucleon size (FNS), higher order terms in the nucleon currents (HOC) and with some nuclear input parameters, have on the values of the nuclear matrix elements (NMEs) for the neutrinoless double beta (0νββ) decay. The calculations are performed with a new Shell Model(ShM) code which allows a fast computation of the two-body matrix elements of the transition operators. The treatment of SRC, FNS and HOC and the use of quenched or unquenched values for the axial vector coupling constant produces the most important effects on the NMEs values. Equivalent effects of some of these approximations are also possible, which may lead (accidentally) to close final results. We found that the cumulative effect of all these nuclear ingredients on the calculated nuclear matrix elements NMEs is significant. Since the NMEs values are often obtained with different approximations and/or with different input parameters, a convergent view point on their inclusion/neglecting and an uniformization of the calculations are needed, in order to enter in an era of precision concerning the computation of the NMEs for double beta decay.
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