Strange, flavour-changing particles may solve matter-antimatter mystery

Scott Sutherland

There's been a persistent question that has plagued physicists for years — why are we made of matter and not antimatter? Now, a team of scientists working in Japan believe they may be close to answering that question.

With all the models of the beginning of the universe, it's been shown that there was an equal amount of matter and antimatter created. Our classic idea of matter and antimatter is that they destroy each other, so if they were produced in equal amounts, the baby universe should have just destroyed itself. Yet here we are, and we're made of matter. So, what happened that not only let the infant universe survive, but also produced the current abundance of matter and near-absence of antimatter?

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Well, matter and antimatter particles only annihilate each other — proton to anti-proton, electron to anti-electron (or 'positron'), etc. If you brought a proton together with a positron, there'd be no annihilation. So, if the matter and antimatter that were produced by the Big Bang underwent a change into other types of matter and antimatter, there'd be an imbalance and one might come out on top.

Physicist have seen this kind of switch before. It's called 'flavour changing'.

Flavour is just a term used to describe different particles. For example, there is a group of particles called 'leptons', which all share one specific property — their 'spin'. A lepton's 'flavour' is the name given to it to differentiate it from the other leptons. There are six different flavours of lepton: the electron, the muon, the tau, and the electron neutrino, the muon neutrino and the tau neutrino.

According to scientists working with the T2K-Experiment at the Japan Proton Accelerator Research Complex, which is specifically designed to investigate how neutrinos change between flavours, it's this neutrino flavour changing that may have made the difference at the birth of the universe.

To see the flavour changing of these neutrinos, the scientists produced a beam specifically made up of muon neutrinos, fired it at a target, and observed what happened at the target end.

When the beam got to the target, it wasn't all muon neutrinos. It also contained some electron neutrinos, which they saw because those electron neutrinos reacted with electrons in the water molecules inside the target. Since they verified that there weren't any electron neutrinos in the beam at the source, this shows that some of the muon neutrinos changed to electron neutrinos along the way.

The first indications of this kind of transformation were seen back in 2011, but the results of this experiment firmly establish that it really happens.

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Since the scientists have now recorded muon neutrinos changing to both of the other flavours (tau and electron), they can use their results to study how anti-muon neutrinos change flavour. If anti-muon neutrinos change flavour in the same way as muon neutrinos, they're probably back to the drawing board (at least with this particular experiment). However, if anti-muon neutrinos change flavour differently, that could explain what we now see in the universe.

"Our findings now open the possibility to study this process for neutrinos and their antimatter partners, the anti-neutrinos," said Alfons Weber a physicist at the University of Oxford, in a statement. "A difference in the rate of electron or anti-electron neutrino being produced may lead us to understand why there is so much more matter than antimatter in the universe. The neutrino may be the very reason we are here."

(Image courtesy: T2K Experiment/J-PARC)

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