The neutrinoless double beta decay from a modern perspective
The neutrinoless double beta decay from a modern perspective
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DOI：
10.1016/s0370-1573(01)00068-0
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发表时间：
2002-04
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影响因子：
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通讯作者：
J. Vergados
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文献类型：
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作者：
J. Vergados
从粒子物理学和核物理学的角度来看，无中微子双β衰变都是一个非常重要的过程。从基本粒子的角度来看，它几乎出现在每个模型中，并产生以下机制：（a）传统的贡献，如轻中微子质量机制以及 jL-jR 轻子干涉（λ 和 η 项）。 (b) 奇异的 R 宇称破坏超对称 (SUSY) 贡献。在该方案中，电流仅是左手性的，并且中间粒子通常非常重。然而，夸克能级的 V-A 流和 P-S 流的组合可能会产生轻中微子。这导致与上述 λ 项相同的结构。类似的考虑也适用于在原子核存在的情况下其姊妹轻子和μ子数违反μ子到正电子的转换。无论如何，无论主导机制如何，从实验角度来看，无中微子双贝塔衰变的观察是两者中最重要的，将严重限制现有模型，并表明中微子是巨大的马约拉纳粒子。从核物理的角度来看，这是具有挑战性的，因为：（1）可以经历双β衰变的原子核具有复杂的核结构。 (2) 能量允许的转变被抑制（耗尽全部力量的一小部分）。 (3) 由于在某些机制中中间粒子非常重，因此必须应对转移算子的短距离行为。因此，必须考虑新的效应，例如核子之间飞行的π介子的双β衰变。在 SUSY 模型中，该机制比标准的二核子机制更重要。 (4) 涉及的中间动量相当高(约100MeV/c)。因此，必须考虑核子流可能的动量相关项，例如由于 PCAC、弱磁项等引起的轴向电流的修改。我们发现，对于质量机制，轻中微子核子流的这种修改减少了约 25% 的核基质元素，几乎与核模型无关。在重中微子的情况下，影响要大得多并且取决于模型。考虑到上述效应，所需的核矩阵元素已可用于所有实验上感兴趣的核 A=76、82、96、100、116、128、130、136 和 150。其中一些已在大基壳模型中获得，但大多数在各种版本的 QRPA 中获得。然后，利用目前可用的 0νββ 衰变半衰期的最佳实验限制，我们提取了各种轻子破坏参数的新限制。特别是，我们发现<mν><0.5 eV /c2，并且为了在允许的SUSY参数空间中合理选择SUSY模型的参数，我们对R宇称违反参数λ111′<0.68×10−3给出了严格的限制。
Neutrinoless double beta decay is a very important process both from the particle and nuclear physics point of view. From the elementary particle point of view it pops up in almost every model, giving rise, among others, to the following mechanisms: (a) The traditional contributions like the light neutrino mass mechanism as well as the jL–jRleptonic interference (λ and η terms). (b) The exotic R-parity violating supersymmetric (SUSY) contributions. In this scheme, the currents are only left handed and the intermediate particles normally are very heavy. There exists, however, the possibility of light intermediate neutrinos arising from the combination of V–A and P–S currents at the quark level. This leads to the same structure as the above λ term. Similar considerations apply to its sister lepton and muon number violating muon to positron conversion in the presence of nuclei. Anyway, regardless of the dominant mechanism, the observation of neutrinoless double betas decay, which is the most important of the two from an experimental point of view, will severely constrain the existing models and will signal that the neutrinos are massive Majorana particles. From the nuclear physics point of view it is challenging, because: (1) The nuclei, which can undergo double beta decay, have a complicated nuclear structure. (2) The energetically allowed transitions are suppressed (exhaust a small part of the entire strength). (3) Since in some mechanisms the intermediate particles are very heavy, one must cope with the short distance behavior of the transition operators. Thus novel effects, like the double beta decay of pions in flight between nucleons, have to be considered. In SUSY models this mechanism is more important than the standard two nucleon mechanism. (4) The intermediate momenta involved are quite high (about 100 MeV /c ). Thus, one has to take into account possible momentum-dependent terms of the nucleon current, like the modification of the axial current due to PCAC, weak magnetism terms, etc. We find that, for the mass mechanism, such modifications of the nucleon current for light neutrinos reduce the nuclear matrix elements by about 25%, almost regardless of the nuclear model. In the case of heavy neutrino the effect is much larger and model dependent. Taking the above effects into account the needed nuclear matrix elements have become available for all the experimentally interesting nuclei A=76, 82, 96, 100, 116, 128, 130, 136 and 150. Some of them have been obtained in the large basis shell model but most of them in various versions of QRPA. Then using the best presently available experimental limits on the half-life of the 0νββ-decay, we have extracted new limits on the various lepton violating parameters. In particular we find 〈mν〉<0.5 eV /c2and, for reasonable choices of the parameters of SUSY models in the allowed SUSY parameter space, we get a stringent limit on the R-parity violating parameter λ111′<0.68×10−3.