Double-beta decay with emission of single free electron

We study a new mode of the neutrinoless and two-neutrino double-beta decays in which one of the electrons is emitted from the atom, while the other is directly produced in one of the available s1/2 or p1/2 subshells of the daughter ion. We calculate the phase-space factors, estimate the half-lives and derive the single-electron spectra for 0+ → 0+ ground-state nuclear transitions of the most relevant double-beta-decay isotopes: 48Ca, 76Ge, 82Se, 100Mo, 136Xe and 150Nd. The relativistic electron wave functions are evaluated at the nuclear radius by means of the multiconfiguration Dirac–Hartree–Fock package Grasp2K. We discuss the prospects for detecting these new modes in the tracking-and-calorimetry experiments NEMO-3 and SuperNEMO.