用前两年的EXO-200数据寻找马约拉纳中微子
Search for Majorana neutrinos with the first two years of EXO-200 data
摘要：
粒子物理学标准模型的许多扩展都认为中微子应该是马约拉纳型费米子，但这个假设很难证实。无中微子双β衰变（$0νββ$）是一种可能发生在几个候选核中的自发跃迁，对它的观测将验证中微子的马约拉纳性质，并限制中微子质谱的绝对标度。最近用$^{76}$ Ge（GERDA实验）和$^{136}$ Xe（KamLAND-Zen和EXO-200实验）进行的研究已经确定这种衰变的寿命长于$10^{25}$ yr，对应于0.2-0.4 eV的中微子质量极限。在这里，我们报告了EXO-200的新结果，基于100 kg $\cdot$/年的$^{136}$暴露量，比我们早期发表的数据集增加了近四倍。我们改进了探测器在$^{136}$双β衰变Q值处的分辨率为$σ$ /E = 1.53%，并修改了数据分析。获得的半衰期灵敏度为$1.9\cdot10^{25}$ yr，与之前的EXO-200结果相比提高了2.7倍。我们未发现$0νββ$衰减的统计学显著性证据，并在90% CL下设定了$1.1\cdot10^{25}$年的半衰期限值。高灵敏度为EXO-200探测器的进一步运行和未来使用nEXO进行$0νββ$衰变搜索提供了希望。
英文摘要：
Many extensions of the Standard Model of particle physics suggest that neutrinos should be Majorana-type fermions, but this assumption is difficult to confirm. Observation of neutrinoless double-beta 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 (GERDA experiment) and $^{136}$Xe (KamLAND-Zen and EXO-200 experiments) have established the lifetime of this decay to be longer than $10^{25}$ yr, corresponding to a limit on the neutrino mass of 0.2-0.4 eV. Here we report new results from EXO-200 based on 100 kg$\cdot$yr of $^{136}$Xe exposure, representing an almost fourfold increase from our earlier published datasets. We have improved the detector resolution at the $^{136}$Xe double-beta-decay Q-value to $σ$/E = 1.53% and revised the data analysis. The obtained half-life sensitivity is $1.9\cdot10^{25}$ yr, an improvement by a factor of 2.7 compared to previous EXO-200 results. We find no statistically significant evidence for $0νββ$ decay and set a half-life limit of $1.1\cdot10^{25}$ yr at 90% CL. The high sensitivity holds promise for further running of the EXO-200 detector and future $0νββ$ decay searches with nEXO.
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