Question: What interesting information have you gathered from your work with T2K in Japan?

Susan Cartwright answered on 27 May 2020:

To answer this question, I need to explain a little bit about neutrinos. First, there are three types of neutrino. We can distinguish these three types by the charged particle that they pair up with: the electron has a matching electron-neutrino, the muon has a muon-neutrino, and the still more massive tau has a tau-neutrino. We can also distinguish them by mass: we unimaginatively call these mass 1, mass 2 and mass 3. The key point is that distinguishing by type (or “flavour” as particle physicists call it) is NOT the same as distinguishing by mass: an electron-neutrino can have any of the three masses, and a mass 1 neutrino could have any of the three flavours. Because of quantum mechanics, it is not the case that some mass 1 neutrinos are electron-neutrinos, some are muon-neutrinos, and some are tau-neutrinos: instead, ALL mass 1 neutrinos are a MIXTURE of the three flavours, and when we make a measurement there is some probability (say 50%) of getting electron-type, some other probability (say 30%) of getting muon-type, and some probability (say 20%) of getting tau-type (and, conversely, a given flavour is a mixture of three masses).

Now, in T2K we use an accelerator to make a beam that starts out as muon-type neutrinos, so a well-defined mixture of masses 1, 2 and 3. We can measure this beam soon after it was made (280 m away from the start of the neutrino-making process) and check that it is (nearly) all muon-neutrinos. However, as the neutrinos travel away from where they are made, the different masses get out of phase with each other (as they are quantum particles, we can think of them as waves, and the different masses have slightly different wavelengths), so once they have travelled 295 km across Japan to Super-Kamiokande, the mixture of masses is not the same as it was when they started out. This means that the mixture of masses at 295 km no longer corresponds to a pure muon-neutrino, and when you make a measurement there is some probability of observing an electron-neutrino or a tau-neutrino (except that in T2K it’s actually very hard to identigy tau-neutrinos, because our beam energy is not high enough to produce taus; remember E = mc^2).

So: there are three possible mixtures of three states a, b and c (ab, bc and ca). When we built T2K, we knew that two of those three mixtures really happened in nature, but we did not know if the third one happened (if the third mixture did not happen, this would correspond to some neutrinos being mixtures of only to types instad of all three). The first useful thing T2K did was show that all three mixtures happen. We JUST managed to collect enough data to do this before the big Japanese earthquake of a few years ago shut us down for a year!

A feature of having three non-zero mixings is that it allows neutrinos to mix differently from antineutrinos. This is not obvious, and you’ll have to take my word for it because the maths that you need to prove it is university level, but it is true. This is important, because we see around us that the Universe contains far more matter than antimatter, and in order to produce this asymmetry there have to be some particles that behave differently from their antiparticles. Neutrinos are one of the very few particles where this is theoretically possible. The second interesting thing that T2K has done, earlier this year, is to present very strong (if not quite conclusive) evidence that in fact neutrinos do behace differently from antineutrinos. So this might be poiting the way to solving one of the great mysteries of cosmology.

T2K has done lots of other things, such as measuring the ways neutrinos interact with different nuclei, but these are the two that are most important to people who are not neutrino physicists!