在PandaX-III实验中寻找Xe-136的无中微子双贝塔衰变

Neutrinoless Double Beta Decay (NDBD) is one of the key topics of physics beyond the Standard Model of particle physics. A discovery of NDBD experimentally would indicate that neutrinos are Majorana particles, i.e., neutrinos are the anti-particles of themselves, and violates the conservation of lepton number. This proposal emphasizes on data analysis to search for possible NDBD signal of Xe-136 and to establish the half-life (or half-life lower limit) with PandaX-III. The PandaX-III experiment searches for NDBD of Xe-136 using high pressure gas Time Projection Chambers (TPC). A prototype TPC with 20kg of xenon will take data in the near future and the full PandaX-III TPC with 200 kg of enriched Xe-136 gas will start data taking at China Jingping Underground Lab in 2019. We will validate the energy resolution goal of a large-scale gas detector and will also utilize TPC’s tracking capability to effectively enhance the signal and suppress the background. For the final spectrum, an unbinned, extended maximum likelihood method will be used to fit the amplitude of possible NDBD signal (or upper limit) and then the effective Majorana mass will be given. We will also evaluate different contributions of systematic errors to better establish the confidence levels of the half-life. This work will also help guide the design of future ton-scale gas TPC detectors.

无中微子双贝塔衰变是超越粒子物理标准模型的新物理研究的重要方向。其发现将证明中微子为马约拉纳性（中微子是其本身的反物质），也将直接证明轻子数不守恒。本项目的主要物理目标为PandaX-III实验的数据分析，搜索Xe-136的无中微子双贝塔衰变，得出其半衰期（或者下限）。PandaX-III实验利用高压气体时间投影室（TPC）来寻找双贝塔衰变事例。20公斤级的原型TPC即将开始取数；包含200公斤Xe-136气体的主探测器预期于2019年在锦屏地下实验室开始取数。我们将验证气体TPC的高能量分辨率，并利用TPC特有的径迹成像功能有效地提取信号、压低本底。利用不分区（unbinned）最大似然法拟合最终的能谱，我们将给出双贝塔衰变信号大小（上限），进而计算中微子的有效质量。我们还将研究实验中各种系统误差，确定半衰期的置信度，同时为下一阶段吨级气体TPC实验提供设计指导。

无中微子双贝塔衰变是超越粒子物理标准模型的新物理研究的重要方向。其发现将证明中 微子为马约拉纳性(中微子是其本身的反物质)，也将直接证明轻子数不守恒。本项目的利用PandaX-II实验400天暗物质数据，对于Xe-136的无中微子双贝塔衰变进行数据分析，得出其半衰期下限。同时我们对PandaX-III实验原型探测器数据和PandaX-III模拟数据进行了深入分析，利用气体探测器记录的粒子径迹拓扑信息提高信号效率。