There’s a new player in our drive to understand the very deepest secrets of the universe – although ‘new’ may be a bit of a stretch for something that’s been around for more than 13 billion years. But the humble neutrino, everywhere around us but fiendishly difficult to locate, could unlock a new world of understanding, if recent breakthroughs are anything to go by.

The story begins with the Big Bang. This seismic event should, the scientists tell us, have created matter and antimatter in equal proportion. But that’s not the case. “Virtually everything we can see – you, the magazine or screen in front of you, the stars – is made of matter,” says Dr Patrick Dunne (PhD Physics 2016), who is a leading member of the Imperial neutrino team of 20 people, including two postdoctoral research associates and three PhD students. That leaves only the smallest fraction of one per cent as antimatter, and no one knows why.

That could all be about to change, however. Recent developments in our understanding of antimatter – what it is, where it is and, more importantly perhaps, where it isn’t – have fundamentally changed the face of physics. And ongoing research could provide answers to some of the world’s biggest questions.

“Every time we have a fundamental paradigm shift in the way we understand our universe, we can use that to innovate,” says Dunne. “Quantum mechanics gave us the computer, CERN gave us the World Wide Web, this field has also given us proton therapy for cancer, magnets for MRI scanning and the touchscreen. Now there is the tantalising possibility that we may finally understand some of the universe’s biggest conundrums, such as how black holes form and whether the four forces of nature are actually just facets of a single, unified force.”

Until this point, the problem has been that physicists’ best explanation of how the universe works – a toolbox of elementary particles and fundamental forces called the Standard Model, from which everything can be constructed – failed to explain the prevalence of matter over antimatter. But Dunne and his colleagues at Imperial now think that the explanation for the lack of antimatter in the universe could actually be down to one of the oddest particles in the Standard Model toolkit: the neutrino.

Neutrinos were a latecomer to the particle physics party because they are so hard to detect. When they were first proposed, in 1930, it was believed that because of their composition (no mass, zero electric charge and untroubled by the strong nuclear force), neutrinos would only interact via one of the four forces in the Standard Model – the weak nuclear force. That meant it stayed largely out of the limelight, even though the sun – the biggest nuclear reactor in our solar system – produces vast quantities of them as a result of the fusion reactions happening in its core.

However, despite the huge number of neutrinos coming from the Sun every second, experiments in the 1960s seemed to show far fewer of them were arriving on Earth than could be explained by physics. It was only in 2002 that an explanation was found.

In the Standard Model, the electron has two heavier but similar family members known as the muon and the tau particle. Similarly, there are three flavours of neutrino to complement these: an electron neutrino, a muon neutrino and a tau neutrino. The sun’s nuclear reactions only produce the electron neutrino but not enough were seemingly arriving on Earth. What scientists realised was that neutrinos could oscillate between the three different flavours. “It’s like me saying I am going to throw you a football and then finding it had turned into a basketball when it got to the other end. This was really unexpected,” says Dunne. This, in turn, meant that they had to have mass because, as Einstein pointed out, massless things have to travel at the speed of light, and time stands still – there can be no evolution when time is not passing.

And the result of these revelations? It means the trusty Standard Model needs to be revisited – and we may finally be able to explain the absence of antimatter. “We have found indications that neutrinos are different in their behaviour for matter and antimatter,” says Dunne. “In the past five years, we have started to measure something called delta CP, which is an indication of how matter and antimatter neutrinos oscillate differently.”

Imperial is part of a team from 12 countries that is working on an experiment called T2K. In this, a highly intense beam of muon neutrinos is generated at J-PARC, a research complex in Tokai on the east coast of Japan, and directed to a neutrino detector 300km away at Super-Kamiokande in the west of the country.

“We start off with protons, accelerate them until they get to near the speed of light and then fire them into a target made of carbon or beryllium,” says Edward Atkin (PhD Physics 2022), a Postdoctoral Researcher in the Imperial neutrino group. “This produces lots of charged particles called pions and kaons, and we can focus them into a beam using magnets. These charged particles then decay into a mixture of neutrinos and other particles.”

Because neutrinos are so hard to spot, the detector needs to be rather special. Sitting beneath a mountain range and deep inside a mine, Super-Kamiokande is made up of 50,000 tonnes of ultrapure water surrounded by 13,000 light sensors. When a neutrino hits a water molecule, it creates something called Cherenkov radiation as it slows down. This is rather like a sonic boom and creates a distinctive cone of light that can be detected by the sensors. Different types of cones indicate different flavours of neutrino, so by seeing how the beam changes over the 300km journey, scientists are able to better understand oscillation and the strange world of neutrinos.

The Imperial team are already working on two next-generation neutrino experiments, the upgrade of T2K, called Hyper-Kamiokande, and one called the Deep Underground Neutrino Experiment, or DUNE for short. Based in the US, DUNE will use a much more powerful beam than the T2K experiment to fire neutrinos on a 1,300km journey from the Fermi National Accelerator Laboratory in Chicago to the Sanford Underground Research Facility in South Dakota.

Rather than using water to detect these neutrinos, DUNE will use liquid argon – 70,000 tonnes of it. But it will bring a step-change in precision and the amount of data. “These new experiments are going to change physics,” says Atkin. “The amount of data we will collect at DUNE is hundreds of times greater than T2K.”

Ioannis Xiotidis, a postdoc working on the detector side of the neutrino team, says that the Imperial team do everything from designing the hardware and testing the circuit boards to installing the equipment at T2K and DUNE. “You need to design something with a lifetime of 30 years or more. You need a processing unit that can handle the huge amount of data produced by the experiment. Even though it doesn’t exist now, it might in ten years’ time. Not many people have those sorts of technical challenges in their work,” he says.

Whatever new technologies the study of neutrinos yields, it seems as though the world as we know it is about to undergo a shift, and we may have taken one small step towards understanding why the world is made up of something, not nothing. And that could change everything.

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This story was published originally in Imperial 54/Summer 2023.