Not unlike beer, it seems Neutrinos come in all manner of flavours that beg sampling…

I just got back from the bi-annual high-energy physics conference of the European Physical Society in Ghent, Belgium. At the end of the meeting I gave a “highlights” talk, and there were some serious highlights (beyond the obvious cricket-related ones), so this seems like a good opportunity to share a few. I won’t go into many details, but will hopefully give you a snapshot over the next few posts of where we are in our understanding of the fundamental constituents of matter, and the forces by which they interact – and of where that understanding is currently changing.

The basic particles of matter come in three progressively heavier “generations”. The most common is the first generation, which contains the electron, and the up and down quarks which make up the protons and neutrons inside atomic nuclei.

The other generations are essentially heavier copies, and the label we give to particles to distinguish between the generations is called “flavour” (for no obvious taste-related reason).

According to our best theory (yes, it’s that Standard Model again) some processes are blind to flavour, so it should for example make no difference in those processes whether an electron is produced, or whether its heavy flavour cousin the muon emerges. Some experiments, especially LHCb at CERN, have been observing marginal differences in such processes, which could be a sign of some new heavier particles or new forces, which break the flavour rules and are not present in the Standard Model. Although no single measurement is conclusive, the pattern is suggestive. LHCb has more data in hand and more coming, and the Belle II experiment, which has just started up in Japan, will have something to say. It’s very much a “watch this space” situation.

The LHCb collaboration seen inside the LHCb cavern (Pic: CERN)

The flavour structure of the quarks also breaks the symmetry between matter and antimatter – so-called CP-violation. We do not really understand how the universe came to be composed primarily of matter, since according to the Standard Model, the big bang should have created equal amounts of matter and antimatter. That’s one reason studying CP-violation is interesting. The LHCb experiment has recently observed CP-violation in the decays of hadrons with charm in them for the first time; previously this had only been seen when strange or bottom quarks were involved.

We also had a Belgian Beer lecture and tasting. Some very special flavours there.

Neutrinos are also about flavour really. In the Standard Model as originally conceived, flavour was the only way to distinguish between the three kinds of neutrino, because they were all massless. We now know they have mass, but (as with the quarks) the different masses do not line up neatly with the different flavours; there is a mixing. In fact the mixing is much bigger for neutrinos than it is for quarks.

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A consequence of this is that there is room for more CP violation, similar to that seen in quark mixing. Whether nature has taken this opportunity or not is one of the currently outstanding big questions.

Our measurements of neutrino mixing only measure mass differences, not the absolute mass of the neutrinos. We know there is one small gap, and two bigger gaps, between the masses of the three neutrinos. This means there are two possible ways the neutrino masses can be arranged, a so-called “normal” hierarchy, with two lighter neutrinos close together in mass and one heavier one further above, or an “inverted” hierarchy, with two heavier ones close together, with a lighter one further below. Which of these options is present in nature is another big open question.

A selection of Neutrino flavours…

Within an extended Standard Model, it is possible to fit all the data to try and determine these aspects, along with the other parameters. Combining new data from T2K and Nova experiments (which use neutrinos produced by accelerators) and experiments using neutrinos produced by nuclear power stations, we see that there probably is some CP violation there, perhaps quite a lot. We also see that the normal hierarchy is favoured over the inverted one. Definitive statements and more precise measurements will come with more data, and with the DUNE and HyperK experiments under construction in the US and Japan.

That’s enough for now but I plan to follow up with more highlights, not so flavourful, over the next week or so.

For more details on the above, all the slides, with references, are available from the conference web site  http://eps-hep2019.eu/, and proceedings will appear in due course.

Professor Jon Butterworth is a physics professor at University College London and a researcher on the ATLAS experiment at CERN involved with, amongst other things, the discovery of the Higgs Boson. He is the author of two popular science books Smashing Physics and A Map of the Invisible. Postcards From the Energy Frontier is the successor to Jon’s hugely successful blog for The Guardian, Life and Physics. He is @jonmbutterworth on Twitter.

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