LEGEND: Neutrinoless Double-Beta Decay and Germanium Detector Technology

Lead Research Organisation: University College London
Department Name: Physics and Astronomy

Abstract

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.

Planned Impact

An Impact Summary will be provided by the lead institution (University of Liverpool)

Publications

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