$0^+$ to $2^+$ neutrinoless double-$\beta$ decay of $^{76}$Ge, $^{82}$Se, $^{130}$Te and $^{136}$Xe in the microscopic interacting boson model}

TL;DR

This study calculates nuclear matrix elements and phase-space factors for neutrinoless double-beta decay to the 2+ excited states in several isotopes using the IBM-2 model, providing insights into beyond-Standard Model physics.

Contribution

It introduces detailed IBM-2 calculations of NMEs and PSFs for 0+ to 2+ decay modes, extending the understanding of neutrinoless double-beta decay beyond the ground state.

Findings

Calculated NMEs for multiple isotopes using IBM-2.

Derived phase-space factors considering finite nuclear size.

Set limits on L-R model couplings from experimental data.

Abstract

Here, we study the neutrinoless double-$\beta$ ($0\nu\beta\beta$) decay between the ground state and the first $2^+$ state of $^{76}\mbox{Ge} \rightarrow {}^{76}\mbox{Se}$, $^{82}\mbox{Se} \rightarrow{}^{82}\mbox{Kr}$, $^{130}\mbox{Te} \rightarrow {}^{130}\mbox{Xe}$ and $^{136}\mbox{Xe} \rightarrow {}^{136}\mbox{Ba}$ systems. The relevant nuclear matrix elements (NMEs) involved in the process are calculated within the formalism of the microscopic interacting boson model (IBM-2). The IBM-2 has been widely used to obtain predictions for nuclear observables, such as the spectrum, but also to explore the possible emergence of beyond-the-Standard Model effects in the weak interactions of nuclei. Our calculations are carried out by considering the exchange of a Majorana neutrino between two nucleons ($2N$-mechanism). In addition to NMEs, we calculate the associated leptonic phase-space factors (PSFs) using electron radial wave functions, which are obtained by solving numerically the Dirac equation of a screened Coulomb potential that takes into account finite nuclear size. By combining our IBM-2 results for the NMEs with those for the PSFs along with experimental half-life limits, we can set limits on the $\langle \lambda \rangle$ and $\langle \eta \rangle$ couplings of left-right (L-R) models.