Nature. 2014 Jun 12;510(7504):229-34. doi: 10.1038/nature13432. Epub 2014 Jun 4.
Many extensions of the standard model of particle physics suggest that neutrinos should be Majorana-type fermions-that is, that neutrinos are their own anti-particles-but this assumption is difficult to confirm. Observation of neutrinoless double-β decay (0νββ), a spontaneous transition that may occur in several candidate nuclei, would verify the Majorana nature of the neutrino and constrain the absolute scale of the neutrino mass spectrum. Recent searches carried out with (76)Ge (the GERDA experiment) and (136)Xe (the KamLAND-Zen and EXO (Enriched Xenon Observatory)-200 experiments) have established the lifetime of this decay to be longer than 10(25) years, corresponding to a limit on the neutrino mass of 0.2-0.4 electronvolts. Here we report new results from EXO-200 based on a large (136)Xe exposure that represents an almost fourfold increase from our earlier published data sets. We have improved the detector resolution and revised the data analysis. The half-life sensitivity we obtain is 1.9 × 10(25) years, an improvement by a factor of 2.7 on previous EXO-200 results. We find no statistically significant evidence for 0νββ decay and set a half-life limit of 1.1 × 10(25) years at the 90 per cent confidence level. The high sensitivity holds promise for further running of the EXO-200 detector and future 0νββ decay searches with an improved Xe-based experiment, nEXO.
许多粒子物理学标准模型的扩展表明，中微子应该是马约拉纳型费米子，也就是说，中微子是它们自己的反粒子，但这一假设很难证实。中微子无双β衰变（0νββ）的观测，这是一种可能在几个候选核中发生的自发跃迁，将验证中微子的马约拉纳性质，并约束中微子质量谱的绝对尺度。最近使用（76）Ge（GERDA 实验）和（136）Xe（KamLAND-Zen 和 EXO（富含氙气的观测站）-200 实验）进行的搜索已经确定了这种衰变的寿命长于 10（25）年，对应于中微子质量的限制在 0.2-0.4 电子伏特之间。在这里，我们根据代表我们早期发布的数据集几乎四倍增长的大量（136）Xe 暴露，报告来自 EXO-200 的新结果。我们已经改进了探测器的分辨率并修正了数据分析。我们获得的半衰期灵敏度为 1.9×10（25）年，比之前的 EXO-200 结果提高了 2.7 倍。我们没有发现 0νββ 衰变的统计显著证据，并在 90%置信水平下设定了半衰期限制为 1.1×10（25）年。高灵敏度有望进一步运行 EXO-200 探测器，并在未来使用改进的基于氙气的实验 nEXO 进行 0νββ 衰变搜索。
