LEGEND: Neutrinoless Double-Beta Decay and Germanium Detector Technology

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.

The impact of the LEGEND proposal will derive from the signal-processing and technological advances delivered to the processing of the germanium detector. This impact will be enabled through the partnerships with industrial collaborators, outreach projects, and will impact on other scientific disciplines outside nuclear and particle physics. The innovative gamma-ray imaging technology developed through the LEGEND project will potentially to lead to many collaborative projects for nuclear and particle physics in the medical, security, decommissioning and environmental monitoring areas. The research groups have a strong track record of industrial collaboration with partners including AWE, Kromek, the Defence Academy of the United Kingdom, Mirion Technologies (Canberra), Ametek (Ortec), John Caunt Scientific, National Nuclear Laboratory and Rapiscan. There have been many collaborative projects (for example NuPNET, DEPICT, GRi+, ProSPECTus, PorGamRays, PGRIS and GammaKeV), a number of which directly derive from earlier work pioneered with position sensitive semiconductor detectors. STFC has helped to leverage funds from these partners with support provided through the STFC CLASP, STFC mini-IPS, STFC follow-on, STFC IPS and STFC CASE schemes.

Beyond satisfying human curiosity around the workings of nature, research in nuclear and particle physics has also tremendous societal impact. Our groups have an excellent track record in public engagement and outreach in a subject that has a natural fascination for the public. Indeed, it fulfils the important role of educating the public in nuclear radiation and its wider aspects, both positive and negative and is important to drive interest in the study of STEM subjects. The groups engage widely with school pupils and teachers via several events, such as the year-12 nuclear-physics and particle-physics masterclasses, which have been hosted already by institutions on this project. Events such as these will be used to highlight the innovation and scientific exploitation of the LEGEND programme. The postdoctoral staff employed through the programme, PhD and undergraduate project students will benefit from skills development in nuclear instrumentation, processing of big data sets and signal processing. The skills shortage for the nuclear industry is highlighted in the government's Industrial Strategy as a major challenge to be addressed. The groups have a strong track record of collaborating with industrial partners for research and skills training. The data processing techniques that will be developed in the project will have the potential to improve the performance of detector systems used for gamma-ray spectroscopy and imaging in our existing portfolio of industrial partnerships, which would accelerate the pathway to impact of STFC funded research.