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arXiv 2007.14908hep-phnucl-exnucl-thphysics.atom-ph
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无中性双电子俘获

Neutrinoless Double-Electron Capture

K. Blaum, S. Eliseev, F. A. Danevich, V. I. Tretyak, Sergey Kovalenko, M. I. Krivoruchenko, Yu. N. Novikov, J. Suhonen

备注：
68 pages, 14 tables, 19 figures, accepted for publication in Reviews of Modern Physics
发表信息：
Rev. Mod. Phys. 92, 045007 (2020)
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摘要：

双β过程在探索中微子和弱相互作用性质，以及寻找标准模型之外的效应方面起着关键作用。在过去的半个世纪里，人们进行了许多尝试来寻找发射两个电子的双β衰变，特别是它的无中微子模式（$0\nu2β^-$），后者仍然没有被观察到。双电子俘获（2EC）由于其一般较低的跃迁几率，迄今为止还没有得到关注。然而，无中微子双电子捕获（$0\nu2$ EC）的速率可以经历共振增强的情况下，初始和最终状态是能量简并的许多数量级。在共振的情况下，$0\nu2$ EC过程的灵敏度可以接近$0\nu2β^-$衰变的灵敏度，用于寻找中微子的马约拉纳质量、右手电流和其他新物理。我们提出了一个概述的主要实验和理论结果，在过去的十年中，在这一领域。实验部分概述了2EC过程的搜索结果和可能的共振$0ν$ 2EC跃迁的衰变能量的测量。一个前所未有的精确度在测定衰变能量与潘宁陷阱，使一个完善的退化参数的值为所有以前已知的近共振衰变，并减少了相当大的不确定性估计的$0\nu2$ EC半衰期。理论部分包括电子壳层效应的最新分析和原子核结构模型的概述，其中计算了$0\nu2$ EC衰变的核矩阵元。可以得出结论，$0ν$ 2EC的衰变概率在几种核素中可以经历显著增强。

英文摘要：

Double-beta processes play a key role in the exploration of neutrino and weak interaction properties, and in the searches for effects beyond the Standard Model. During the last half century many attempts were undertaken to search for double-beta decay with emission of two electrons, especially for its neutrinoless mode ($0\nu2β^-$), the latter being still not observed. Double-electron capture (2EC) was not in focus so far because of its in general lower transition probability. However, the rate of neutrinoless double-electron capture ($0\nu2$EC) can experience a resonance enhancement by many orders of magnitude in case the initial and final states are energetically degenerate. In the resonant case, the sensitivity of the $0\nu2$EC process can approach the sensitivity of the $0\nu2β^-$ decay in the search for the Majorana mass of neutrinos, right-handed currents, and other new physics. We present an overview of the main experimental and theoretical results obtained during the last decade in this field. The experimental part outlines search results of 2EC processes and measurements of the decay energies for possible resonant $0ν$2EC transitions. An unprecedented precision in the determination of decay energies with Penning traps has allowed one to refine the values of the degeneracy parameter for all previously known near-resonant decays and has reduced the rather large uncertainties in the estimate of the $0\nu2$EC half-lives. The theoretical part contains an updated analysis of the electron shell effects and an overview of the nuclear structure models, in which the nuclear matrix elements of the $0\nu2$EC decays are calculated. One can conclude that the decay probability of $0ν$2EC can experience a significant enhancement in several nuclides.

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