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强大的工具。丰富的氙气天文台(EXO)计划调查中微子的基本性质，中微子是一种有趣的基本粒子，长期以来被认为没有质量，不与许多东西相互作用，可能掌握着宇宙中为什么有更多物质而不是反物质的关键。EXO计划通过寻找一种称为无中微子双β衰变的核衰变的存在来进行对中微子的研究。贝塔衰变涉及从原子核发射电子，并且总是伴随着反中微子的发射。无中微子的双β衰变将是原子核(在这种情况下是氙气-136)通过发射两个电子(没有其他电子)而发生转变的过程。根据粒子物理的标准模型，只有在中微子和反中微子完全相同的情况下，才会发生这种情况。物质和反物质之间的这种直接重叠可能被证明是回答科学中最基本的问题之一的关键：为什么我们的宇宙完全由物质组成？该计划的第一个实验EXO-200使用200公斤富含136同位素的液化氙作来源，自2010年以来一直在新墨西哥州的地下废物分离试验厂运行。Exo-200将再运行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能够探索是否存在无中微子的双贝塔衰变，如果这个过程发生的话，这个过程的特征半衰期是宇宙当前年龄的40亿倍。EXO-200中使用的探测器技术开创了使用具有超低残留放射性水平的选定材料的先河，以便为探测罕见事件创造最安静的无线电环境。其中一些清洁度协议和要求现在正在到达电子和制药行业。此外，大型稀有液体探测器，如EXO-200中使用的探测器，正在粒子物理学之外得到越来越多的应用，特别是在核反应堆燃料循环和成分监测以及医学成像(如PET扫描机)中。EXO-200项目被证明是研究生成长为拥有数据分析和硬件专业知识的研究人员的理想平台。马萨诸塞州大学的团队有成功地让不同人群的本科生参与该项目核心领域的记录。新成立的阿默斯特基础相互作用中心为实验者和理论家之间的广泛科学交流提供了一个刺激和富有成效的环境。在充分利用来自EXO-200实验的数据方面，UMasss小组发挥了核心作用。所选的氙气时间投影室(TPC)技术使该项目能够利用高Z氙气的辐射自屏蔽特性，在具有出色的事件跟踪能力和能量分辨率的均匀布局中。这项工作包括对具有空间拓扑的过程的全面研究，例如136Xe到136Ba、137Xe本底的激发态的BB衰变，以及来自宇宙µ子与探测器组件相互作用的高能伽马射线。该提案的一个组成部分是UMassas的一个实验室计划，该计划将测试液体氙气中各种探测器材料和表面的光学性质，并将这些测量结果用于EXO-200的光线跟踪光学模拟。连同对氙气产生的真空紫外线(VUV)闪烁光(特别是硅光倍增器)的新型探测器的聚焦测试，这些测试是正在进行的吨级、下一代无中微子双β衰变实验(Nexo)研究的一部分。

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.