Neutrinos are the most abundant matter particles in the universe, but no one knows how much they weigh. This matters because the neutrino mass is the last unknown mass in the Standard Model of particle physics—the theory that describes all known fundamental particles and forces. Neutrinos carry 99% of the energy released in supernova explosions and shaped the early universe, yet their mass remains a gap in our understanding of how the cosmos works. The researchers cannot weigh a neutrino directly. Instead, they will measure the tiny energy carried by electrons emitted when tritium decays, using Einstein’s equation to infer the neutrino’s mass. This project is fundamental science with no immediate practical application. It aims to prove that the necessary technologies—magnetic traps for large numbers of tritium atoms and quantum sensors sensitive enough to detect vanishingly low-power microwave signals from trapped electrons—can work in principle. If successful, it would pave the way for a much larger experiment, potentially hosted at the UK’s Culham Centre for Fusion Energy, that could measure the neutrino mass for the first time. Past fundamental research on neutrinos has led to Nobel prizes and unexpected insights into why matter exists at all.
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The neutrino is the most abundant matter particle in the universe, and yet we do not know how much it weighs. We know that this particle, which carries 99% of the energy released in supernova explosions and has played an important role in the evolution of the early universe, has an anomalously small mass but we also know that it cannot weigh nothing. It is therefore imperative that we measure this, the last unknown mass in the Standard Model of particle physics. We cannot measure the neutrino mass directly in the laboratory. Rather, we try to constrain as precisely as possible the energy that has gone into creating the neutrino in processes such as nuclear beta-decay. Einstein's famous equation then tells us how to calculate the neutrino mass. Since the neutrino escapes undetected, the experimental task involved in measuring the minimum neutrino energy is actually to measure the maximum energy carried by all of the other particles. The most promising system to use is tritium, in which the proton inside a normal hydrogen nucleus is accompanied by two neutrons. Tritium beta-decays with a half-life of 12.3 years and a very small decay energy of 18.6 kilo-electron-volts; the fact that this decay energy is so small makes it uniquely sensitive to the tiny neutrino mass. We will need to develop techniques for trapping very large populations of tritium and measuring with exquisite sensitivity the energy of beta-decay electrons. As a first step we will use deuterium, which is much easier to handle than radioactive tritium. We will magnetically decelerate beams of deuterium into very well characterised magnetic traps. Electrons generated inside the trap will undergo circular motion and in so doing will emit microwave radiation. We will develop the quantum sensors that are capable of detecting the vanishingly low-power signals that are generated in this way. The ultimate aim of this project is to show that we have, in principle, the technologies required for a much larger experiment that would have sensitivity to all possible values of the neutrino mass. Such an experiment could perhaps be hosted in the UK where, at the Culham Centre for Fusion Energy, world-leading facilities for handling large tritium inventories exist and are being further developed.
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