Q&A WITH A PARTICLE PHYSICIST
Neutrinos – almost massless, neutral particles – could help explain why the Universe didn’t just disappear in a flash of light after the Big Bang
Why is the study of neutrinos important? Neutrinos are truly fundamental particles. They are some of the elementary building blocks of our Universe. They come in three different types: tau, muon and electron neutrinos. In the 1960s, theoretical physicists wrote the rule book on particle interactions – known as the ‘standard model’ – which has been very solid for more than 50 years now. Yet, neutrinos break the rules.
How does neutrino behaviour break the rules? They were assumed to be massless, and yet we have experimental proof that they do carry a tiny mass, thanks to the observation of ‘neutrino oscillations’, a phenomenon that makes neutrinos change flavour, so to speak, during their propagation. For example, most neutrinos from the Sun are electron neutrinos, but about two-thirds turn into one of the other two types by the time they get to Earth. Their behaviour could help explain why the Universe did not simply disappear in a flash of light just after the Big Bang.
What is it about neutrino behaviour that could tell us about the early Universe?
Neutrinos could help answer the matter–antimatter asymmetry problem. We know that antimatter exists, but we live in a Universe that’s overwhelmingly made of matter. This is strange because there’s nothing that makes matter special compared to antimatter, in terms of fundamental interactions. They should have been created in equal parts in the early Universe. It must be that there’s a mechanism where for every billion particles of antimatter, a billion plus one particles of matter were created – a violation of the symmetry between matter and antimatter.
We know the fundamental components of protons, quarks, are partly responsible, but not enough to explain the overwhelming difference between matter and antimatter we see. By studying the oscillation patterns of neutrinos, we can understand how neutrinos contribute to this violation.
How can studying neutrino oscillation patterns unlock this secret? Experiments such as the Short Baseline Neutrino programme will tell us if a fourth kind of ‘hidden’ neutrino exists. Future experiments – such as the next international flagship experiment, Fermilab’s DUNE (Deep Underground Neutrino Experiment) that I’m working on – will be able to shed light on the matter–antimatter asymmetry in the Universe. These experiments are based on accelerator neutrinos. At Fermilab, we produce beams of neutrinos and build detectors along their path to record interactions at different distances from the origin point. By counting the number of interactions at different distances, we measure the oscillation pattern.
That sounds challenging.
Yes, counting neutrino interactions is quite tricky! Neutrinos are neutral, which means we can study them only if they interact. While they are the most abundant massive particle in the Universe, their probability of interaction is extremely small. So we need to build huge detectors to record a meaningful number of interactions.
You’re developing new technology for DUNE. What will it do and what new findings could it unlock?
I’m developing a Liquid Argon Time Projection Chamber (LArTPC) with a powerful light-collection system. If selected, this technology will help us reach DUNE’s goals faster, but mostly we expect it to enhance our understanding at low energies. This will unlock DUNE’s potential in seeing neutrinos from the Sun and supernovae, as well as efficiently recognising proton decay events – an observation long coveted but never observed.
How is that process going?
It’s a collaborative effort. We’re now working on proof-of-principle designs to ensure the viability of LArTPC’s new sensors and characterise their performance. We’ll then move to medium-scale prototypes where we’ll record real neutrino interactions. This will allow us to put our technology to the test in a real physics environment.