A 3 million dollar question: how fast does a muon wobble? - EMF 2026

[applause] Uh yeah, great. Oh, that's very loud. Oh, I guess you can hear me. Okay. Um thanks for coming out. Um it's difficult lunchtime slot, I know, but um I'm going to talk a bit about um so my talk is about particle physics. I'm an experimental particle physicist and I'm going to tell you Oh yeah, there we go. Um, a bit about one experiment that I work on. Um, as you know, particle physics is a fairly esoteric subject in some ways and this is a fairly esoteric experiment on in a fairly esoteric subject, but it's one that got super interesting um because of what we measured. So, these are some of the headlines that we got really around the world um when our results came out. So, I'm going to try and explain a little bit about what it is we were trying to do and what we measured. It was quite fun, I've got to say, seeing people trying to explain our measurement um in the newspaper. But let's see see how I get on. So, let's start big picture. So, what is particle physics? um we're essentially trying to understand um the the the stuff around us, the the world around us, the universe that we live in by looking at it at its smallest scales. So trying to understand what are the fundamental building blocks that everything is made of and those are the particles that we talk about. Um this is our kind of understanding of the universe where particle physics fits in. You know, there was a big bang 13.7 billion years ago. And in the Big Bang or very soon after the Big Bang, all of the particles that we that we know of were created. And then essentially the universe has evolved since then. As a field, it's been a kind of around for 150 years or so, depending on how you count. Um, we've we've learned a lot. our understanding of the universe on this fundamental scale is is really impressive. Now we have this theory called the standard model and this is like the periodic table of the standard model. So these are all of the fundamental particles that we we know about and as far as we can tell this is this is it. There isn't anything else. So it's quite efficient in a lot of ways. Um to break it down a bit, you only need the actually the four on the left to explain everything that you can see in the universe. Um the top two, those are up and down quarks. Those make up protons and neutrons which make up the atomic nucleus. And then you've got the electron which orbits the nucleus. And that's that's um all the atoms in the universe. We've got the one at the bottom is a nutrino. Um so this is slightly weird particle. It's involved in some kinds of radiation but it's really critical in how um fusion happens in the sun. So the ones over here um these are called bzons. These are force carrying particles. So these are how the ones on the left interact with each other. There are some um three different forces represented here. We've got the photon which is electromagnetic. uh glue on strong force and then this weak force. So this is how everything in the universe interacts if you don't count gravity. That's one of the missing features. Um we've got the the Higs Bzon up there which is a slightly different kind of force carrying particle. It's not really a force carrying particle. That's the most recently discovered particle in this map. It was discovered in 2012 and it's why other particles have mass essentially. Then there's all the other ones in the middle and we don't really know why they're there. Um there seems to be you we've got our four on the left which make up everything in the universe and then there's two copies of that set. They're called generations. So there are three generations of particle and they get basically heavier as you go across and the heavy ones all decay and you're just left with the lighter ones. We don't know why. We don't know why any of this is looks the way it does. And that that's one of the questions that we have nowadays is well how does gravity fit into this picture? Why is this the particle content of the universe? And we also know from cosmological observations there's something called dark matter out in the universe and that isn't here at all. So there's clearly other stuff going on. And where we're at um as a research field is that well any experiment we've tried to do in a lab this picture can explain it but we know there must be something else out there. So we're now like trying to do experiments to discover the next thing essentially. Uh I'm going to talk a bit about this particle which is it it is the best particle. It's called the muon. So we'll come back to that. Uh from where it's sitting you can see it's like an electron but heavier. That's so how do we go about discovering something new? We've got two main ways of doing this really. Um one of them is using Einstein's equation= mc^². So we think there must be some other force of nature out there, some other kind of particle, it's probably quite heavy. If you collide things with enough energy, you might be able to make this very heavy new thing that we've never seen before. Um, the other way you can do it is through the uncertainty principle. It's a bit like quantum tunneling. It's a bit tenuous. I know. I just love this picture so much. I try and work it into all my talks. Um, the idea is that if you measure, so quantum mechanics allows the universe to borrow some energy. So something that's extremely heavy can have a very small effect at a very low mass scale. And that's the kind of measurement that I'm going to talk about. But to come back to the first method, this is the kind of forefront of high energy, the energy frontier as it's called. It's the large Hadron Collider. Uh it's just outside Geneva in Switzerland for scale. This white strip is the runway of Geneva airport. It's underground, so it doesn't it's not this giant glowing orange thing. Um, but it's been running for a few years. It's where the Hig bosom was discovered. It's going to run for another 15 or so years. Um, no other obvious discoveries have dropped out. That doesn't mean there's nothing else there. It just means that whatever if there is something to be discovered, it's quite buried in the data somewhere. So, we're also looking in other directions and and this is the experiment I'm going to talk about. Uh this is um it's called Muon G minus 2. For a sense of scale, it's much smaller. You can see there's there's two people standing in the middle and it's really targeted. So with the the LHC, the large hydron collider, you can measure many many many things all at once. With this, we were trying to measure one number really really precisely. So what were we trying to measure? Um if you've done undergraduate quantum mechanics um you will have heard of this thing called spin procession. It's a bit like a gyroscope really. So these particles so electrons, muons, things like that, they all have this property where they appear to be spinning. Uh and they carry electric charge. So a spinning electric charge acts a little bit like a magnet. And if you put that in an external magnetic field, it will process. the spin will process just like a gyroscope. You get this um rotating motion. Um the equation which describes that procession. So you got the omega there which is the procession frequency. It's given by this equation in a simple way. Um, you know, the the speed of this procession is gi determined by the amount of charge a particle carries, how fast it's spinning, how heavy it is, and how strong is the external magnetic field you've put it in. And then there's this other thing in there, G, and that's just a number. It's like a constant of proportionality, and that's the thing we measure. So, a bit of history. Um the first prediction for what G should be was um comes from very early days of quantum mechanics. Um DRA you can derive it from DAC's equation which is given here very this is like the fundamental equation of the whole of particle physics. It's it's very very important equation. It's on DAC's memorial stone in Westminster Abbey if you ever go to have a look. But from this you can work out that G should be exactly two. And the problem was that people went and measured it about 20 years later and they found that well it's almost two but it's not it's a little bit higher than two. And so that um sorry I don't have a pointer but the plus or minus 6 is the uncertainty on the last digit there. So it's it's a very close to two but it's definitely not two. And this was called the electron anomaly. And this was a real puzzle for well about a year uh cuz in the same year so Julian Schwinger who was a theoretical physicist at the time went away and did a more complicated calculation and down here are some finement diagrams. It doesn't really matter if you don't know what these are but they're ways of representing how particles can interact. Um, the one on the left shows an electron interacting with an external magnetic field and the one on the right shows a slightly more complicated way that that can happen because you got an extra photon in the middle. So the DRA equation basically tells you the one on the left and Schwinger calculated the one on one on the right and he got this answer here. So it's two plus a little bit extra and that little bit extra then bought the prediction very close in agreement with the measurement. And again, this was a very important result in the development of quantum field theory. It's on Schwinger's gravestone as well. Um, current state-of-the-art um the theory well the precision has increased very very dramatically as you can see. Um the predictions mostly done by um Kinoshitan up there who amazing guy well into his 80s he was still doing these incredibly complicated calculations and publishing them. he sadly died a couple of years ago. U but you can see the measurement has also increased um in accuracy and that currently the measurement is more precise than the theory and this is according to Wikipedia the most precisely determined quantity in physics and what it tells you is that you know the theory really works. It really describes nature for the electron. Now we come to the best particle the muon. The muon's a bit heavier. uh and because it's a bit heavier, you have to start worrying about other other kinds of interactions that can happen. It's the thing about the uncertainty principle. Um if an interaction is mediated by a heavy particle and you get a bit closer in in your energy scale, you start to feel the influence of that very heavy particle. So for the muon you have to calculate more things and the question is well what if for the muon there are some unknown interactions happening. So then if we did our calculation we would get one number and if we did our measurement it would include this previously unknown interaction and we would get a different number and that would be the sign that we've discovered something. So how do we measure it? Well it comes back to this equation. Um we know the charge of the muon the spin of the muon the mass of the muon. So we just put some muons in a magnetic field. We measure how strong the magnetic field is and then we measure that procession frequency. Um, this is our magnet. So, it's the thing in it's got a white blanket on it here to keep it the temperature level. Um, our muons, they come in at the top. You can see those red uh magnets there. So, that's the mon beam line. The muons come in, they enter this magnet, and when you put a charged particle in a magnetic field, it starts moving in a circle. So, then they just start going round around our magnet. And the the the animation on the left shows you what happens. So at the start um the the spin vector is aligned with the direction the mon is going. But then the spin vector you get this procession. So the spin vector starts rotating, the momentum vector starts rotating but at a slightly different speed. So now you've got something that's a bit like a gyroscope on a carousel. Uh and we have to measure where the red arrow is pointing basically. Um the method for doing that is is really ingenious and it's been developed over many many decades. Uh but we wait for the muon to decay and it decays into an electron. The electron has a bit less momentum so it falls towards the center of our magnet and then we have all of these um boxes around and they count they measure electrons that hit them and they measure the time and the energy that those electrons arrived. So why is that useful? So the point is that um the electron wants to go generally in the direction the spin vector is pointing the direction of the red arrow. So when the red arrow and the blue arrow are aligned the electron is basically being thrown forward and it will have slightly more energy. When it's anti-aligned the electron is wants to be thrown backwards and it will have slightly lower energy. So the plot in the middle shows the energy spectrum that you measure as a function of time and you can see it's going up and down. as the spin and momentum align and then anti a line and then a line and anti-line and so on. So what we do the simplest thing you can do is just count how many electrons you see over some threshold. So electrons above some energy level and just count the number as a function of time and you get this um the technical term is a wiggle plot for this and the frequency that you see on this wiggle plot is then the spin procession frequency. Then we measure the magnetic field. We've got the procession frequency and from that we can extract the G value. It is a little bit more complicated than that. This is why it takes several years to do this and to do it to the accuracy that we want to do. I'm not going to explain this at all. [laughter] Um, but this is really the full equation that we use. There's many other effects that need to be taken into account and understood at the kind of part per billion level. The plot on the left shows you our full wiggle plot and then some of the other effects that again have to be taken into account there. Um, given the time I'll I'll move on to the result. Um so the the the the results are shown up there. Um what we have uh on the on the axis is it's called A. So this is basically G minus 2 which is the name of the experiment. It's it's how far away G is from two essentially. Um the top the blue line is um a previous experiment which is that um result came out about 20 years ago that was done at Brook Haven to BNL Brook Haven National Lab. And then the red points are from our experiment uh which is in in the US. It's at Fermy Lab just outside Chicago. And we've released um three different data sets. So the first one you can see the top line has a large uncertainty bar on it and then we we improved our understanding of the experiment and added more data and and our measurement got more and more precise um until we arrive at the run 1 to six. So the bottom red number which is our kind of definitive measurement and if you combine it with all of the previous experiments as well you get the purple point at the bottom but you can see it's totally dominated by our single most precise point. It's hard to kind of get it from this plot but this is really an amazing achievement. It's a measurement that's accurate to 127 parts per billion. Like how do you quantify that number? Uh, one way to think about that is if you tried to measure how many people are in London right now and you got it correct to within one person, that's the same level of accuracy. Or if you measured like the length of a football field and you got it correct to within the width of a human hair, that's that's the same level of accuracy. Um, it's the most precise measurement that's ever been made a particle accelerator. And it was really like decades of progress from three generations of this like iterate iterative improvements on this experiment and the method. Um the the final result um there combined with all the previous efforts that went into it was awarded the breakthrough prize. So that's the 3 million prize that's in the title of my talk. Um they call themselves the Oscars of science. I don't know. I'm not sure anyone else calls them that. Um, but it was it was a really nice recognition of, you know, what is really an amazing scientific achievement. The reason it's hard to interpret on this plot is because I'm not showing you what the prediction is yet. So, here's just like we've just gone and measured a number. Is that interesting? Nobody knows. What number did we expect to measure? That's where it gets interesting. So, now let's compare to the theory. So, this is a slightly different graph. So this um now shows um how far the prediction is away from the experiment and the experiment measure is shown as the red band. So the band is the uncertainty on our measurement and then the green lines are some theory predictions and you can see that they don't agree and they don't agree if you're into stats at all. They don't agree by more than five standard deviations which is normally the point in my field where you would say you have discovered something and what could it have been? So it comes back to this fact that you know G is just this number but it has many many things influencing it. And if you do a calculation taking into account all the things that we know you will get one number but nature will include everything that is possible. And so there may be some new physics effects in our measurement which are not in our calculation. And so the fact that they don't agree suggests that that's that we have discovered something. But [sighs] then there was a different way to do this calculation. Um so the the top ones, the green points, um the calculation is actually very hard and and it's to do with this thing in particular. Um you can ask me more in the Q&A afterwards if you want. Um so there's a different way to calculate this number. Um the top ones the green ones use data inputs to calculate it and the bottom one is what's called latis QCD. So this is almost like a first principle calculation. It's done on supercomputers. And the latis predictions are shown in blue. And now you can see that they agree actually rather well with our experimental measurement. So you know science um it's the scientific method in action I suppose we um have learned something about how to calculate this property we haven't discovered anything fundamentally new it seems unfortunately uh but no there is still a bit of a question as to like why these two theory calculations disagree so I think when we put our first result out there was a lot of excitement because we didn't have the latis points so the latis point had only just come out. So we at first we thought we'd really discovered something and now it seems like um there's a lot of work to be done on the theory side in understanding why these two methods disagree. So there might still be something unexpected hiding in the data which is behind the green points but we'll have to see how am I doing for time. Oh I've got I've gone quite fast then right um because so that that brings me to the end. So this is really the status um of this experiment. Um I tried to think of like what is kind of a tagline for what we do and it's kind of like um zooming in to see the big picture in a way. I mean by measuring something so very very precisely you can learn about effects that are that kind of influence the whole universe in a way. We're we're trying to see if there are some there's some influence on this number to like the nth decimal place which is caused by some extremely heavy particles which existed right at the start of the universe and might explain why we have the standard model that we do why the universe looks the way it does. Um obligatory nod to my to the funders. Um if you're interested and want to learn more I wrote a book a few years ago. You can find out more about it. I feel like I would be remiss if I didn't um stop talking about science for a minute and started to talking about politics. Um last year um we were all very excited about our scientific results. This year we're all spending all of our time talking about funding. Um there is a funding crisis in our field. Um there's been lots of headlines about it. Um, I'm obviously have a very strong opinion on [laughter] the value of fundamental science. I think fundamental science is incredibly important. I think it inspires people to do scientific subjects. I think it leads to um kind of transfer of technology and knowledge and so on techn because you know in when you do fundamental science you end up developing technologies which didn't exist before because you're trying to do something completely new and those spill out those spill out into industry and so on eventually. Um, and what's happening right now in the UK is that the the funding for fundamental science has been cut very dramatically over the last uh 3 or 4 years. Um, to the point where it's becoming a bit of an existential crisis in the UK and there's a real danger that many of the kind of exciting things that we do are just not going to be possible here anymore and they will be done in other countries. Um, I'll try and end on a slightly happier note. >> [laughter] >> That's all I had. Um I hope you that gave you some sense of um what this this experiment does and maybe why it was exciting. Um we have to leave here so they can set up for the next talk. But I'm happy I'll go to the Q&A tent. I'm very happy if anyone wants to ask me anything about any of this or just have a chat about particle physics or anything in general. I'll be over there. So thank you. [applause]