Abstract
We study the two-neutrino double electron capture on 124Xe based on an effective theory (ET) and large-scale shell model calculations, two modern nuclear structure approaches that have been tested against Gamow-Teller and double-beta decay data. In the ET, the low-energy constants are fit to electron capture and β− transitions around xenon. For the nuclear shell model, we use an interaction in a large configuration space that reproduces the spectroscopy of nuclei in this mass region. For the dominant transition to the 124Te ground state, we find half-lives T1/22νECEC=(1.3−18)×1022y for the ET and T1/22νECEC=(0.43−2.9)×1022y for the shell model. The ET uncertainty leads to a half-life almost entirely consistent with present experimental limits and largely within the reach of ongoing experiments. The shell model half-life range overlaps with the ET, but extends less beyond current limits. Our findings thus suggest that the two-neutrino double electron capture on 124Xe has a good chance to be discovered by ongoing or future experiments. In addition, we present results for the two-neutrino double electron capture to excited states of 124Te.
Highlights
We study the two-neutrino double electron capture on 124Xe based on an effective theory (ET) and large-scale shell model calculations, two modern nuclear structure approaches that have been tested against Gamow-Teller and double-beta decay data
For the dominant transition to the 124Te ground state, we find half-lives T12/ν2ECEC = (1.3 − 18) × 1022 y for the ET and T12/ν2ECEC = (0.43 − 2.9) × 1022 y for the shell model
The black bars show the theoretical predictions from the effective theory (ET) and the nuclear shell model (NSM), as well as most recent quasiparticle random-phase approximation (QRPA) calculations [38, 39], in comparison to the horizontal lines that indicate the experimental lower limits set by the XENON100 [33] and XMASS [32, 34] collaborations as well as Gavrilyuk et al [31]
Summary
The ET 2νECEC matrix element calculation requires the known ground-state energies and the lowest 1+1 excitation energy to calculate the energy denominator, as well as the Gamow-Teller β-decay and EC matrix elements from the 1+1 to the initial and final states of the 2νECEC to fit the low-energy constants. We follow the strategy of previous shell model ββ decay predictions [41, 42] and include the above “quenching” factors phenomenologically to predict the half-life of 124Xe. The low-energy excitation spectra of the three isotopes are well reproduced.
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