An Improved Search for Neutrinoless Double Beta Decay with KamLAND-Zen

中微子是一种亚原子粒子，不带电荷，质量很小。中微子是宇宙中最丰富的粒子之一，但由于它们与物质的相互作用非常小，因此很难被探测到。在过去的20年里，在理解中微子的基本性质方面取得了显着的进展，包括测量表明中微子必须具有非零质量。 这引出了一些非常重要的问题。 中微子的绝对质量是多少？ 为什么中微子的质量与其他基本粒子相比如此之小？ 中微子是如何获得质量的？ 这些问题的答案可能与中微子是它自己的反粒子密切相关，也被称为马约拉纳粒子，并可能解释宇宙中物质相对于反物质的普遍性。一种罕见的核过程，称为无中微子双β衰变，是一种强大的工具，可以用来研究中微子的马约拉纳性质。 这个过程，即两个中子同时转化为两个质子，发射两个电子而没有中微子，是非常罕见的，并且由于实验探测器内可能发生大量类似的过程而难以观察。 竞争性的实验研究需要大量的双β衰变同位素和对背景的精确抑制。 该奖项将支持波士顿大学小组在位于日本地下深处的KamLAND-Zen实验中改进无中微子双β衰变搜索的活动。 该奖项将通过波士顿大学本科生研究经验项目吸引来自不同背景的本科生。 与此同时，它将建立一个学生和博士后谁是在学术界，工业界和国家实验室高度抢手的先进技术技能培训的管道。KamLAND-Zen是一个千吨级的液体闪烁体为基础的探测器。最近，该实验开始了一个新的数据采集阶段，使用探测器内大约750公斤的136克仪器。 该奖项将支持KamLAND-Zen的以下增强功能：使用最先进的深度学习分类算法改进背景抑制，独立的贝叶斯分析，以及为触发系统实施在线事件标记软件。 这些增强所提供的背景的大幅降低将使KamLAND-Zen的无中微子双β衰变半衰期灵敏度一直达到5×10^26年。 这几乎是以前KamLAND-Zen测量的五倍改进。 该奖项下开发的增强功能将为一项名为KamLAND 2-Zen的重大探测器升级奠定基础，该升级旨在实现半衰期灵敏度超过2×10^27年。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

A neutrino is a subatomic particle that has no electrical charge and a very small mass. Neutrinos are one of the most abundant particles in the universe, but because they have very little interaction with matter, they are difficult to detect. In the last 20 years, remarkable progress has been made in understanding the fundamental nature of the neutrino, including measurements which have indicated that neutrinos must have a non-zero mass. This leads to some very important questions. What is the absolute mass of the neutrino? Why are neutrino masses so small compared to other elementary particles? How did the neutrino obtain a mass? The answers to these questions could be intimately related to the neutrino being its own antiparticle, also known as a Majorana particle, and may explain the prevalence of matter over anti-matter in the Universe.A rare nuclear process, called neutrinoless double beta decay, is a powerful tool that can be used to investigate the Majorana nature of neutrinos. This process, whereby two neutrons simultaneously transform into two protons with the emission of two electrons and no neutrinos, is extremely rare and is difficult to observe due to the vast number of similar processes that could occur inside an experimental detector. A competitive experimental search requires a large quantity of the double beta decay isotope and exquisite suppression of backgrounds. This award will support the Boston University group’s activities in an improved search for neutrinoless double beta decay in the KamLAND-Zen experiment located deep underground in Japan. This award will engage undergraduates from diverse backgrounds through the Research Experience for Undergraduates program at Boston University. At the same time, it will establish a pipeline of students and postdocs who are trained in advanced technical skills that are highly sought-after in academia, industry, and national laboratories.KamLAND-Zen is a kiloton scale liquid scintillator based detector. Recently, the experiment started a new phase of data-taking using roughly 750 kg of 136-Xe instrumented inside the detector. This award will support the following enhancements to KamLAND-Zen: improved background suppression with state-of-the-art deep learning classification algorithms, an independent Bayesian analysis, and the implementation of online event tagging software for the trigger system. The substantial reduction of backgrounds offered by these enhancements will push the neutrinoless double beta decay half-life sensitivity of KamLAND-Zen all the way to 5×10^26 years. This is almost a five-fold improvement over previous KamLAND-Zen measurements. The enhancements developed under this award will set the stage for a major detector upgrade, called KamLAND2-Zen, that will aim for a half-life sensitivity greater than 2×10^27 years.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.