A Search for Neutrino-Less Double Beta Decay and Lepton Number Violation with the nEXO Experiment

研究基本粒子之间最罕见的过程是在最基本的层面上理解自然的一个非常有力的工具。富集氙天文台（EXO）项目研究中微子的基本性质，中微子是一种有趣的基本粒子，长期以来被认为没有质量，不与任何东西相互作用，可能是宇宙中物质多于反物质的关键。EXO计划通过寻找一种被称为无中微子双β衰变的核衰变的存在来开展对中微子的研究。衰变包括从原子核发射一个电子，并且总是伴随着反中微子的发射。无中微子的双β衰变将是一个原子核（在这个例子中是氙-136）通过发射两个电子（没有其他）来转变的过程。根据粒子物理学的标准模型，这只有在中微子和反中微子完全相同的情况下才会发生。物质和反物质之间的这种直接重叠可能对回答科学中最基本的问题之一至关重要：为什么我们的宇宙完全由物质组成？该计划的第一个实验项目EXO-200使用200公斤富含136同位素的液化氙作为来源，自2010年以来一直在新墨西哥州的地下废物隔离试验工厂运行。EXO-200计划再运行3年，该奖项将支持PI和他的团队长达3年。积累的数据将使科学家们能够探测不含中微子的双β衰变的存在，如果这一过程发生时，其特征半衰期是目前宇宙年龄的四万亿倍。EXO-200中使用的探测器技术开创性地使用了具有超低残余放射性水平的选定材料，以便为检测罕见事件提供最安静的无线电环境。其中一些清洁协议和要求现在已经达到了电子和制药行业。此外，大型贵重液体探测器，如EXO-200中使用的探测器，在粒子物理以外的应用越来越多，特别是在核反应堆燃料循环和成分监测以及医学成像（例如PET扫描仪）方面。EXO-200项目被证明是研究生成长为具有数据分析和硬件专业知识的研究人员的理想阶段。马萨诸塞大学的小组在项目的核心领域成功地让不同的本科生参与进来。新成立的阿默斯特基础相互作用中心为实验家和理论家之间广泛的科学交流提供了一个刺激和富有成效的环境。马萨诸塞大学小组在充分利用EXO-200实验数据方面发挥着核心作用。所选择的氙气时间投影室（TPC）技术使该项目能够利用高z氙气的辐射自屏蔽特性，在均匀的布局中具有出色的事件跟踪能力和能量分辨率。这项工作包括对空间拓扑过程的全面研究，如136Xe的bb衰变到136Ba的激发态，137Xe背景，以及宇宙介子与探测器组件相互作用产生的高能伽马射线。该提案的一个组成部分是马萨诸塞大学的一个实验室项目，用于测试液体氙中各种探测器材料和表面的光学特性，并将这些测量结果用于EXO-200的射线追踪光学模拟。再加上对氙（特别是硅光倍增器）产生的真空紫外线（VUV）闪烁光的新型探测器的集中测试，它们是正在进行的吨级下一代中微子双β衰变实验（nEXO）的一部分。

Investigating the rarest of processes between elementary particles is a very powerful tool in understanding Nature at its most fundamental level. The Enriched Xenon Observatory (EXO) program investigates the fundamental nature of the neutrino, that intriguing elementary particle that for a long time was thought to have no mass, doesn't interact with much of anything, and may hold the key to why there is more matter than antimatter in the Universe. The EXO program carries out its study of the neutrino by searching for the existence of a nuclear decay called neutrino-less double beta decay. Beta decay involves the emission of an electron from the nucleus, and is always accompanied by the emission of an antineutrino. Neutrino-less double beta decay would be a process in which a nucleus (xenon-136 in this case) transforms by emitting two electrons (and nothing else). According to the Standard Model of particle physics, this can only happen if neutrinos and antineutrinos are exactly the same. Such a direct overlap between matter and antimatter could prove essential in answering one of the most fundamental questions in science: why is our Universe made exclusively of matter? The first experiment of the program, EXO-200, uses 200 kg of liquefied xenon enriched in the 136 isotope as a source and has operated at the underground Waste Isolation Pilot Plant in New Mexico since 2010. EXO-200 is set to run for another 3 years, and this award will support the PI and his group for up to three years. The accumulated data will allow scientists to probe the existence of neutrino-less double beta decay should this process happen with a characteristic half-life of up four million billion times the current age of the universe. The detector technology used in EXO-200 pioneers the use of selected materials with ultra-low levels of residual radioactivity in order to produce the most radio-quiet environment for the detection of rare events. Some of these cleanliness protocols and requirements are now reaching the electronics and pharmaceutical industries. In addition, the large noble liquid detectors, such as the one used in EXO-200, are finding increased applications outside of particle physics, in particular in nuclear reactor fuel cycle and composition monitoring and in medical imaging (e.g. PET scan machines). The EXO-200 project is proving an ideal stage for graduate students to grow into researchers with both data analysis and hardware expertise. The UMass group has a track record of successfully involving a diverse population of undergraduate students in core areas of the project. The newly established Amherst Center for Fundamental Interactions provides a stimulating and productive environment for a broad scientific exchange between experimentalists and theorists. The UMass group plays a central role in fully exploiting the data from the EXO-200 experiment. The chosen xenon Time Projection Chamber (TPC) technology allows the project to take advantage of the radiation self-shielding properties of high-Z xenon, in a homogeneous layout with excellent event tracking capability and energy resolution. The work includes a comprehensive study of processes with spatial topology, such as bb decay of 136Xe to excited states of 136Ba, 137Xe background, and high energy gamma-rays originating from cosmic muon interactions with detector components. An integral part of the proposal is a laboratory program at UMass to test optical properties of various detector materials and surfaces in liquid xenon and to use the results from these measurements in a ray-tracing optical simulation of EXO-200. Together with focused tests of novel detectors for the Vacuum Ultra-Violet (VUV) scintillation light produced in xenon (silicon photo-multipliers in particular), they are part of investigations under way towards a tonne-scale, next generation experiment (nEXO) for neutrinoless double beta decay.