Determination of Absolute Neutrino Mass Using Quantum Technologies

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.

Our aim is to deliver high quality science and to ensure that all our scientific results are published in journals and further disseminated through conference talks and reports. In the course of our research, whenever we find something which looks promising from the point of view of economic and/or wider societal impact, we will take every care to ensure that such potential impact pursues the numerous pathways available to the collaboration.

The long term goal of this effort is to measure the absolute value of the neutrino mass. In layman's terms we are attempting to weigh the lightest thing in the Universe. This is a unique challenge and one which is easy to describe to an audience and engage the public everywhere from primary schools to pubs. The UCL Physics and Astronomy department have a very active outreach and public engagement portfolio, which will help the proponents to inspire the current and future generations.

The quantum technologies used and developed for the neutrino mass measurement campaign have a wide range of exciting possible applications. Many of these technologies have been developed through previous grants from UKRI and other funding agencies, as such their full impact is discussed in detail elsewhere.

The higher microwave frequency amplifiers operating at the quantum limit of sensitivity, developed in WP4, have potential use cases in many areas of applied physics and engineering. Some of the potential growth sectors that could utilise these devices include: quantum information processing, quantum computing, quantum metrology and the communications sector.

The development of the 'geonium atom' quantum microwave sensor detailed in WP5 has been supported by an EPSRC grant and one of the potential outcomes is a revolutionary new mass spectrometry technology.

The project presents a unique opportunity to provide PhD students and early career physicists with truly interdisciplinary training that cuts across such diverse areas as particle, atomic and cold matter physics, quantum sensor technologies, electronics and data intensive science. The advanced novel technologies and results this project sets out to deliver contributes directly to the National Strategy for Quantum Technologies (https://epsrc.ukri.org/newsevents/pubs/quantumtechstrategy) and to growing a skilled UK workforce in this high priority area.