Why can't you mix matter with antimatter

Asymmetric neutrinos and the antimatter

Perhaps you have just read the article in the so-called “Wissenschaftsfeuilleton” (apparently motto: a lot of opinion, little science) and, in addition to all the ranting about the fact that Mr. Losing the urge to think too much about things like neutrinos - well, maybe you have also wondered what is actually behind this article. Before I get further upset about the article (which reminds me a lot of this), I would rather tell you briefly what the new findings on neutrinos are all about - that's actually quite interesting. (You can also find more details here.)

It is actually about one (or actually two) problems of the so-called standard model of elementary particle physics (which, contrary to what is claimed in the said article, are not whitewashed, but are generally known and are often discussed) - namely on the one hand the masses of neutrinos and on the other hand, the existence of matter in general.

Let's start with a turbo overview of the standard model: Then there are different types of elementary particles: On the one hand there are the quarks, which combine to build up particles such as protons, neutrons and so on (which in turn are made up of atomic nuclei). You can read more about quarks and how they stick together in this article. And then there are the leptons - this includes first of all the well-known electron that romps about in the atomic shell and is responsible for chemical bonds, electrical conduction and everything else. The electron has two heavier siblings, the muon and the tauon - they are actually identical to the electron, but they are significantly heavier and decay within a few microseconds. An electron emerges from a muon. In addition, there are other particles, the neutrinos.

For a long time it was thought that neutrinos have no mass and that there are simply three kinds of them, namely one each for the electron, muon and tauon. We now know it's a little more complicated, but that's beyond the standard model.

In addition to all these particles there are antiparticles - they are electrically charged in opposite directions (at least in the case of quarks and electrons / muons / tauons; neutrinos are electrically neutral). When a particle and an antiparticle meet, they destroy each other - the energy released in the process creates new particles (exactly what you do in particle accelerators to create new elementary particles).

There are various interactions between all these elementary particles - the well-known electromagnetic interaction (when the particles are charged, neutrinos do not notice anything), the so-called weak nuclear force (which is responsible, for example, that a muon decays), the strong nuclear force, which only notice the quarks (explained in detail in the article linked above), and then gravity (which is better ignored because nobody knows exactly how gravity works on the elementary particle level ...).

So much for the standard model. Even during its development, it was known that it had a problem: according to the Standard Model, the world is practically completely symmetrical for particles and antiparticles - in pretty much all processes, the same number of particles and antiparticles should always arise. (There are a few exceptions, for example the decay of the K or B meson. That is why CERN also looks for exactly these decays - this is not only there for the Higgs particle, but also does other things.)

And then a second violation of the Standard Model was discovered (for which, by the way, we won the Nobel Prize last year): neutrinos have a (albeit very small) mass and, what is even more amazing, they can transform into one another - if a muon neutrino flies off somewhere, then it has a certain probability of arriving as an electron or tau neutrino. (This is because the state is a real quantum mechanical overlay, you can find a brief explanation here, I've only read it, I'm not sure how good it is.)

According to the Standard Model, this conversion between the neutrinos (and their mass) should not take place either.

The new experiment (thanks to Bjoern for the link) examines the conversion of such muon neutrinos in detail. To do this, you generate muon neutrinos in a particle accelerator (today it's about theoretical physics, so I'm not going to explain exactly how it works) and send them 300 kilometers through the Japanese underground. (This works because neutrinos only interact very rarely with matter. That is why you have to produce huge amounts of them in order to scrape together a few measurement results.) There they end up in the Super-Kamiokande detector. A few fewer of the neutrinos interact with the water in this detector and generate a measurable signal (as I said, I'll save the details today), which you can then evaluate to see what kind of neutrino you have captured.

This experiment has now been done twice - once with launched muon neutrinos, once with anti-muon neutrinos. As expected, roughly the same thing should happen both times - muon neutrinos turn into electron neutrinos at a certain rate, and anti-muon neutrinos into anti-electron neutrinos. The rate should actually be the same in both cases.

But that's not how it turned out - if you start with muon neutrinos, then significantly more electron neutrinos arrive than if you start with antineutrinos. Muon neutrino and muon anti-neutrino therefore differ in their conversion. With the neutrinos we have thus combined both problems of the Standard Model in one particle, so to speak: they violate the matter-antimatter symmetry and - contrary to the prediction of the Standard Model - they have a mass. To be fair, it has to be said that the results are not yet completely statistically reliable and that there is a certain (not too low) probability that they will disappear with further measurements, because the absolute number of events is small: If I am correct in this presentation I understand, then 32 electron neutrinos and 4 electron anti-neutrinos were measured, without a matter-antimatter symmetry one would have expected 23 and 7 (the discrepancy there comes from the different rate at which neutrinos and antineutrinos are generated). A more detailed examination (more data ...) is therefore necessary.

So if we want to go beyond the Standard Model towards a theory that has slightly fewer gaps, then neutrinos are probably the way to get there. This does not necessarily mean (as in the article over there) that the neutrinos themselves are responsible for the asymmetry - but they are the elementary particles whose properties differ the most from the Standard Model. This is precisely why current neutrino research is so exciting, because it may open a door to the physics behind the standard model.