The slippery neutrino

A mysterious particle that could help us understand the past and future of the universe.

by Stefano Bevacqua
12 November 2020
5 min read
by Stefano Bevacqua
12 November 2020
5 min read

The history of the neutrino is probably the best particle physics has to over. This tiny element is involved in every hypothesis about the origin of the universe, as well as its eventual demise. It’s still enough of a conundrum to make explaining it a hard task, but it’s increasingly looking like the key to the many mysteries that still shroud the universe. There are a large and growing number of laboratories around the world putting their research into chasing this shady little particle. There’s the recent experiment T2K (Tokai to Kamioka) in Japan and the work at the Los Alamos National Laboratory (LANL) in New Mexico, in the United States.

Japanese research seems to be telling us that neutrinos can explain why the universe exists, including us humans, tiny agglomerations of molecules blessed with biological life and conscience that we are. American scientists, meanwhile, are trying to shed light on how much more mystery is out there, in the form of dark matter and dark energy. These components, taken all together, would make up 95% of the universe. You could not do without them, not without dismissing every theory so far made and, even if only partly, proved through experiments.

Different flavours

The existence of the neutrino was put forward in the 1930s by the Austrian physicist Wolfgang Pauli. He wanted to account for some inconsistencies observed in the radioactive decay of atomic nuclei. It was christened by Enrico Fermi and finally discovered in 1956 by Cowan and Reines, two American scientists. It is not easy to see neutrinos; they travel at very high speeds and have a tiny mass, 100,000 to 1 million times smaller than that of an electron. By virtue of being almost ephemeral, billions of neutrinos zoom past us every second without us even realising they are there, having been pumped out in vast amounts by the sun and all other stars, the earth itself and even nuclear power stations.

You cannot see them. They are incredibly difficult to make out. All you can do is spot the traces of their movement. Which is how we know that every neutrino can take on different characteristics, or flavours as scientists have light-heartedly called them. These are based on the elementary particle to which they are linked, either an electron, a muon or a tau. So far it seems almost clear. But the observations of the Japanese researchers at the Super-Kamiokande Observatory brought new details to light. It was in 1998 that they first realised that neutrinos change flavour as they travel through space and matter.

The fourth neutrino

But let’s not get ahead of ourselves. A few thousandths of a second after the Big Bang, the universe was still tiny, and was made of as much matter as anti-matter. Every electrically charged particle had a doppelganger with an opposite charge. Now, if an electron, which has a negative charge, encounters an anti-electron, with a positive one, the two destroy each other and vanish. At this point you might well ask how on earth the universe came about. The explanation science has given us so far is that, for some obscure reason, matter was slightly larger than anti-matter. This would have been enough that there was still progress, however small, after the inevitable destruction of both pluses and minuses, enough to develop what we call our universe.

But what accounted for this asymmetry? The theory, which can be advanced thanks to research from the T2K experiment, goes that there is a fourth type of neutrino –a so-called sterile neutrino, thrown up in immense quantities in the aftermath of the Big Bang. These would explain the asymmetry between the numbers and charges of matter and anti-matter particles and the direction they take when they move about in space. In short, a fourth neutrino would miraculously make the sums add up, reassuring us that we really exist and that everything is not just an illusion.

The mystery of the sterile phase

The boffins from Los Alamos are going down much the same path, avidly supporting the idea of a sterile neutrino, albeit one that no one has been yet able to detect traces of. Just like the sun, nuclear reactors churn out lots of neutrinos, the traces of which can be detected with the right sensors. Now, it so happens that the number of neutrinos you can count is always less than the number of neutrinos you would estimate based on the nature of the nuclear reaction that makes them. And that’s a reaction which, triggered and controlled as it is by technicians, leaves no question as to how it unfolds.

There are two possibilities: either all the technicians working on this research in dozens of nuclear reactors around the world are all making the same mistake and calculating wrongly the number of neutrinos produced under certain conditions, or when neutrinos pass through sensors, many of them assume a sterile flavour and leave no recognisable traces. Which brings us to an exciting, if potentially rash, conclusion: what if we simple humans cannot detect dark matter simply because it behaves exactly like a sterile neutrino? This could unravel the mysteries of the universe. But there is a lot more to do before we can get there.