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

通过 nEXO 实验寻找无中微子双贝塔衰变和轻子数违规

基本信息

批准号：
1506051
负责人：
Andrea Pocar
金额：
$ 55.5万
依托单位：
University of Massachusetts Amherst
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2015
资助国家：
美国
起止时间：
2015-08-01 至 2019-07-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1506051&HistoricalAwards=false
关键词：
Search Neutrino Less Double Beta

项目摘要

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.

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。浓缩氙天文台（EXO）计划研究中微子的基本性质，这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，并且可能是为什么宇宙中物质多于反物质的关键。 EXO计划通过寻找称为无中微子双β衰变的核衰变的存在来进行中微子的研究。 β衰变涉及从原子核发射一个电子，并且总是伴随着反中微子的发射。无中微子双β衰变是一个原子核（在这种情况下是氙-136）通过发射两个电子（而不是其他电子）进行转换的过程。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测中微子较少的双β衰变的存在，如果这个过程发生的特征半衰期是宇宙当前年龄的400万亿倍。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提供最安静的无线电环境。这些清洁协议和要求中的一些现在已经达到电子和制药行业。此外，大型惰性液体探测器，如EXO-200中使用的探测器，在粒子物理学之外的应用越来越多，特别是在核反应堆燃料循环和成分监测以及医学成像（如PET扫描机）方面。EXO-200项目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舞台。麻省大学集团有一个成功的记录，涉及在该项目的核心领域的本科生的不同人群。新成立的阿默斯特基本相互作用中心为实验学家和理论家之间的广泛科学交流提供了一个激励和富有成效的环境。麻省大学的研究小组在充分利用EXO-200实验数据方面发挥着核心作用。所选择的氙时间投影室（TPC）技术使该项目能够利用高Z氙的辐射自屏蔽特性，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，例如136 Ba的bb衰变到136 Ba的激发态，137 Ba的背景，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马萨诸塞大学的一个实验室计划，以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的光线跟踪光学模拟。它们与针对氙中产生的真空紫外（VUV）闪烁光的新型探测器（特别是硅光电倍增器）的重点测试一起，是正在进行的吨级下一代实验（nEXO）研究的一部分。无中微子双β衰变。