Nonresonance Shake Mechanism in Neutrinoless Double Electron Capture
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CLEI Theory
Nonresonance Shake Mechanism in Neutrinoless Double Electron Capture F. F. Karpeshin1)* , M. B. Trzhaskovskaya2), and L. F. Vitushkin1) Received November 20, 2019; revised January 4, 2020; accepted January 4, 2020
Abstract—It is generally accepted that neutrinoless double electron capture is a resonance process. The probability for the shake followed by electron-shell ionization occurring in the course of transformation of 152 Gd nuclei is calculated. Among all known nuclei, this nuclide is characterized by the lowest resonance defect and is therefore thought to be one of the main candidates for use in performing searches for the neutrinoless decay mode. As a result, the contribution of the new mechanism turns out to be smaller than that of the traditional resonance mechanism, thereby yielding a correction to the decay probability. Nevertheless, this correction is quite large, amounting to 23% in the case of 152 Gd. However, the correction in question grows fast with increasing resonance defect, so that, for other nuclei, we expect it to exceed the resonance-mechanism contribution and to become a dominant mechanism of neutrinoless double electron capture. It therefore appears that by no means does neutrinoless double electron capture proceed as a resonance process. DOI: 10.1134/S1063778820030126
1. INTRODUCTION Recently, the Xenon Collaboration reported on the first ever direct observation of 2e2ν capture in the 124 Хе nucleus [1]. This event was a step of paramount importance in searches for neutrinoless double electron capture. Its investigation is crucial for testing the Majorana nature of the neutrino. This process is traditionally viewed as a resonance one, since no particle is emitted upon the respective nuclear transformation [2]. Therefore, it cannot proceed on isolated nuclei even if the energy deposition is positive, Q > 0. The energy–momentum conservation law requires the transfer of part of energy and momentum to a third body, and the electron shell of the atom involved plays the role of this third body. The emerging vacancies are filled via fluorescence. The energy-momentum conservation law is restored after the emission of the first photon whose energy includes the excess quantity Q. The probability for neutrinoless capture is maximal in the vicinity of the resonance. At the present time, interest is therefore focused primarily on studying nuclei characterized by a small value of Q. In some cases where Q is large, a decrease in the resonance defect is possible upon electron capture from higher lying shells, such as the L1 and M1 shells, or to excited states of the nucleus or atom, in which case 1)
Mendeleyev Institute for Metrology, St. Petersburg, Russia. Petersburg Nuclear Physics Institute, National Research Center Kurchatov Institute, Gatchina, Russia. * E-mail: [email protected]
2)
the process is more probable than the process of capture to the ground-state level. For example, the decay process 152 Gd → 152 Sm proceeds with a higher probability via the capture of KL1 electrons to t
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