LEGEND: Neutrinoless Double-Beta Decay and Germanium Detector Technology

标准模型是目前对基本粒子及其相互作用的最佳描述，寻找标准模型之外的物理是高能粒子加速器的首要任务。但研究人员也在通过一种被称为无中微子双β衰变的过程，在原子核的“低能”环境中寻找新的物理学。这种假设的衰变将表明中微子是它们自己的反粒子，自然界中的一个基本定律（轻子数守恒）被违反了。它可以帮助解释为什么中微子如此轻，以及为什么宇宙中的物质比反物质多。中微子的一个显著特征是它们的质量极小。这些粒子存在于三种可能的质量状态中，比第二轻的费米子--电子--轻约100万倍，这种巨大的差异表明中微子质量的起源不同于所有其他费米子，涉及的物理学与标准模型中的希格斯机制截然不同。大多数标准模型的扩展假设中微子是马约拉纳粒子，这意味着它们是自己的反粒子。这些理论将轻中微子的质量解释为与大质量成反比，而大质量与所有自然力在非常高的能量下的大统一有关。这些马约拉纳中微子可以介导无中微子双β衰变，但无论确切的机制是什么，对这种罕见的核衰变过程的观察将表明新物理的存在。该领域的领先实验将无中微子双β衰变的半衰期限制在1 E25至1 E26年的水平（即10的25或26次方年），比宇宙年龄长数十亿倍。目前使用同位素Ge-76的实验，即GERDA和MAJORANA实验，已经在超低背景和精致的能量分辨率方面处于领先地位。最近成立的LEGEND合作旨在通过分阶段的方法将其灵敏度提高两个数量级，从200 kg级实验开始，该实验将于2021年开始获取数据，并在几年后进行吨级实验。LEGEND将是整个领域中最好的实验之一，并可能在参数空间的一个非常好的动机区域发现马约拉纳中微子。英国在锗探测器技术和低本底物理学方面拥有世界知名的专业知识，这在很大程度上是基于英国资助机构之前的投资。我们希望利用这一专业知识，确保英国能够在无中微子双β衰变领域最重要的未来实验中发挥主导作用。该项目还使我们有机会进一步开发锗探测器技术，用于环境和辐射监测等各种应用。

The search for physics beyond the standard model, the current best description of fundamental particles and the interactions between them, is a top priority at high-energy particle accelerators. But researchers are also searching for new physics in the "low-energy" environment of the nucleus through a process known as neutrinoless double-beta decay. This hypothetical decay would show that neutrinos are their own antiparticles and that a fundamental law (the conservation of lepton number) is violated in nature. It could help to explain why neutrinos are so light, and why there is an excess of matter over anti-matter in the universe. A striking feature of neutrinos is their extremely small mass. The particles, which exist in three possible mass states, are about a million times lighter than the next lightest fermion, the electron. This vast discrepancy suggests that the origin of neutrino mass is different from that of all other fermions, involving physics quite different from the Higgs mechanism of the standard model. Most such extensions of the standard model assume that the neutrinos are Majorana particles, meaning they are their own antiparticles. These theories explain the light neutrino masses as being inversely proportional to a large mass scale associated with the grand-unification of all the forces of nature at very high energy. These Majorana neutrinos can mediate neutrinoless double-beta decay but, whatever the exact mechanism, the observation of this rare nuclear decay process would indicate the presence of new physics.The leading experiments in the field are setting limits on the half-life for neutrinoless double-beta decay at the level of 1E25 to 1E26 years (that is, 10 to the power 25 or 26 years), billions of times longer than the age of the universe. The current generation of experiments using the isotope Ge-76, the GERDA and MAJORANA experiments, already lead the way in terms of ultra-low backgrounds and exquisite energy resolution. The recently formed LEGEND collaboration aims to extend their sensitivity by two orders of magnitude in a staged approach, starting with a 200kg class experiment which will start taking data in 2021 and moving on to a tonne-class experiment several years later. LEGEND will be one of the very best experiments in the entire field, and could discover a Majorana neutrino in a very well motivated region of parameter space.The UK has world-renowned expertise in germanium detector technology and low-background physics, based in large part on previous investment from UK funding agencies. We want to use this expertise to ensure the UK can play a leading role in what will be one of the most important future experiments in the field of neutrinoless double-beta decay. The project also gives us the opportunity to further develop germanium detector technology for diverse applications including environmental and radiological monitoring.