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DURING my final year of high school, in 1998, my mother excitedly announced to me one day that neutrinos had mass. I didn’t care, and I couldn’t figure out why she did. At the time, I didn’t realise what a big deal it was. I didn’t even know what a neutrino was and arrogantly thought that because I hadn’t heard about them, they weren’t terribly important. By the time Takaaki Kajita and Arthur B. McDonald shared a 2015 Nobel prize for their role in the discovery, I knew more about these weird particles.

Neutrinos are, in my view, the strangest and most compelling members of the menagerie that we call the Standard Model of particle physics. They constantly pass through us and are hard to see. They fall into the family of particles called leptons. You are probably familiar with one lepton: the electron. The other two charged leptons are called the muon and tau.

Neutrinos are a different branch of the lepton family and like another particle you have probably heard of – the neutron – neutrinos don’t have a charge. They are special in a way that neutrons aren’t, however, as they are fundamental particles that can’t be broken into smaller constituent parts, while neutrons are actually made of quarks.

Despite their differences in charge, the lepton family is bound together in interesting ways. Each charged lepton has a neutrino partner, so there are three kinds of neutrino: electron neutrinos, muon neutrinos and tau neutrinos. These partnerships aren’t just a matter of naming convention, but they have meaning. In fact, the six flavours of lepton – because why say “particle” when you can say “flavour” – are organised into three generations.

Electrons are first generation, muons are second and taus are third, and the particles from higher generations tend to decay into the particles from the lower generations.

“Neutrinos are the strangest members of the menagerie that we call the Standard Model of particle physics”

Curiously, these three particles get their mass through interactions with the famed Higgs boson, but mathematically, interactions with the Higgs leave the neutrino massless on paper. That is why neutrino mass is such an interesting topic.

Kajita and McDonald received their Nobel prize because they played a leading role in the first confirmed observation of something called neutrino oscillations, which occur if the neutrino has mass. What’s fun about neutrino oscillations is that it is a phenomenon where effectively a neutrino is created with one flavour, but when it is measured, it has another flavour. In other words, if you make a muon neutrino, when you measure it, it might be a tau neutrino.

In fact, in 1998, scientists at the Super-Kamiokande (Super-K for short) experiment in Japan, observed this exact muon neutrino to tau neutrino transition. High-energy particles called cosmic rays hit the atmosphere regularly, and when they do, they produce muon neutrinos. These hit Earth at all angles, and because neutrinos pass through everyday matter pretty easily, they fly through the planet. The Super-K experiment observed that the neutrinos coming from the sky were muon neutrinos, but the ones coming up through Earth were tau neutrinos. Neutrino oscillations had occurred.

We didn’t know whether neutrinos had mass for 50 years after they were discovered in the 1950s. Today, we still don’t have a precise measurement, but we have confirmed the maximum value that their mass could be. This means we have been able to write off the hypothesis that dark matter is made of neutrinos. Their maximum mass is just too small.

What we are in need of is a theory to explain why neutrinos have a small, non-zero mass, and when you are a theorist like me, this is good news.