Measuring the magnetism of muons

Scientists at Fermilab may have uncovered something in muons beyond our understanding
12 October 2021

Interview with 

Brendan Casey, Fermilab


An experiment is attempting to answer the questions particle physics faces by trying to better quantify a particle discovered way back in the 1930s. This is the muon, a kind of heavy electron, and, earlier this year, researchers announced that in their experiments, muons did not appear to be following the pattern of behaviour we would predict if our theories about them are right. This sort of thing gets physicists very excited, because it means there’s potentially something new to be discovered. Chris Smith spoke with Brendan Casey from Fermilab’s Muon g-2 experiment, where these anomalies were uncovered, to find out more...

Chris - What's the experiment you've been doing?

Brendan - Our experiment is called the g-2 experiment, and what we're mainly trying to do is measure the magnetic strength of muons. This is an experiment people do every day around the world. Every time you pull a compass out of your pocket and look at which way the needle points, the needle points north. So, that's the same experiment we're doing, but we're doing it with muons. When we do this experiment with electrons, the compass points north, but when we do the experiment with muons, for some reason the needle is not pointing north. And so it's a huge mystery. It means there's something big out there which is turning the needle of muons, and it seems like it's just happening to muons. It's very strange. We have absolutely no idea what's going on, but we can reproduce it. We know it's happening.

Chris - And the point is, if something is not fitting the mould, there has to be a missing piece to this puzzle. And that missing piece could be what? A new force, a new particle? Something is making those muons buck the trend.

Brendan - Yes. I mean, it could be Bigfoot for all we know.

Chris - Hopefully not.

Brendan - With our experiments we measure muons and we could tell that something is bothering our muons. We can't tell what it is. We could tell it's something big. It could be something like dark matter - that's always our first guest, because we also don't know what dark matter is, but we really have no idea. There's no comprehensive model right now that can put together the things that we see at the LHC and the things that we're seeing at Fermilab with muons. So it's a ginormous mystery at this point.

Chris - Yes, we should mention that the LHC have also been studying muons in a bit of a different way, haven't they? Because when they do collisions, like Sarah Williams was just telling us, and they're looking at the products of those collisions, they know how many electrons they should get, they know how many muons they should get, and they don't get enough muons - something's going wrong. So there's something odd happening there. Do you think that's the same phenomenon that you're seeing, but from a different perspective, or do you think there's two new bits of physics going on?

Brendan - It could easily be the same thing. It could be that there's something big out there and it likes to knock muons around it Fermilab and it likes to gobble up muons at the LHC. When you see these two things, it's natural to try and put them together. There's no easy way to make them fit together right now, but it is absolutely tantalising that it's something happening in the muons. There's something out there and it's not happy with muons.

Chris - Theoretical physicist Ben Allanach told this programme earlier in the year that he's working on the possibility there might be a new fundamental force, a fifth force as it were, that could account for this, but equally he acknowledges dark matter could be responsible. If dark matter is doing this, Sarah Williams was telling us it's a struggle to detect dark matter, does this mean you've potentially got a way to find dark matter then? You could use muons as your torch in the darkness?

Brendan - Well, what we know is that the thing that we're seeing in muons is big, so it is a very important tool that we have for any model that people come up with. If they think it's dark matter, if they think it's Supersymmetry, if they think it's extra dimensions, all those things would have to make a prediction for how big of an effect would you see in the muons. And because we see such a large effect, it makes it this kind of bright candle that's sitting out there in the wilderness, that we have to be able to explain if we're going to be able to explain all these other things as well.

Chris - There may be people listening to this who are thinking, well, that's really interesting, but why does that matter to me as the average man or woman in the street. If you solve this - when you solve this - let's be optimistic, what's this going to change about our understanding of physics?

Brendan - It could revolutionise our understanding of physics. What you learn about physics in school and what you learn once you leave school, about the amount of the Universe that we can explain, is very large, right? So there's something revolutionary out there. And it's hard to say what it's going to look like when we're on the other side of that. We can't predict how it's going to change everyday lives. What we know is that once we understand it, we will never look at the Universe in the same way again.


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