Bound-state double-$\beta$ decay

TL;DR

This paper investigates novel double-beta decay modes involving one electron in a bound state, calculating phase-space factors and spectra, and assesses their observability in future experiments.

Contribution

It introduces and analyzes bound-state double-beta decay modes, providing phase-space calculations and spectral predictions for these rare processes.

Findings

Bound-state neutrinoless double-beta decay is highly suppressed.

Two-neutrino bound-state decay modes could be observed in upcoming experiments.

Spectral characteristics of bound-state decay electrons are characterized.

Abstract

We consider new modes of two-neutrino and neutrinoless double-$\beta$ decays in which one $\beta$ electron goes over to a continuous spectrum and the other occupies a vacant bound level of the daughter ion. We calculate the corresponding phase-space factors of the final states, estimate the partial decay rates, and derive the one- and two-electron energy spectra using relativistic many-electron wave functions of atoms provided by the multiconfiguration Dirac-Hartree-Fock package GRASP2K. While the bound-state neutrinoless double-$\beta$ decays are strongly suppressed, their two-neutrino counterparts can be observed in the next-generation double-$\beta$-decay experiments, most notably SuperNEMO.