1 00:00:00,000 --> 00:00:06,000 Nobody wants to do this. Come on, somebody be brave. Tell me. I've got two theorists here and an experimentalist. 2 00:00:06,000 --> 00:00:15,000 No one knows why. I mean, we know 3 00:00:15,000 --> 00:00:20,000 Hello everybody and welcome to Early Morning Coffee at CERN. My name is Steven Goldfarb. 4 00:00:20,000 --> 00:00:26,000 I'm Claudia Cornella. My name is Simon Kuberski. And I'm Fred Gray. And we have an amazing show for you 5 00:00:26,000 --> 00:00:32,000 today. It's something that is a result that really impressed me. We just had a presentation made last week I think it 6 00:00:32,000 --> 00:00:38,000 was on the these results and it might not sound familiar to you as a physics 7 00:00:38,000 --> 00:00:44,000 topic. It's from the muon g-2 experiment at Fermilab. But the topic 8 00:00:44,000 --> 00:00:49,000 is really really nice. And I think the thing that caught my eye was the degree 9 00:00:49,000 --> 00:00:56,000 of precision, the amount of work that went into getting this measurement, a measurement of the magnetic moment of 10 00:00:56,000 --> 00:01:02,000 the muon, which is weird. So I want to start out with that. I want to try to understand 11 00:01:02,000 --> 00:01:07,000 first of all to explain what is a magnetic moment. 12 00:01:07,000 --> 00:01:14,000 Well, when I think of magnetic moments, I actually go back and think about bar magnets, the kind that everybody played 13 00:01:14,000 --> 00:01:21,000 with as a kid. And you know that some of them are stronger than others. And so the strength of a bar magnet reflects 14 00:01:21,000 --> 00:01:27,000 its magnetic moment. And you know, why is that magnet magnetized? It's 15 00:01:27,000 --> 00:01:32,000 magnetized because it's built up of a lot of subatomic particles. and the 16 00:01:32,000 --> 00:01:39,000 spins of the electrons inside all add up to the magnetic moment of the whole 17 00:01:39,000 --> 00:01:46,000 bar magnet. Okay, there there's a lot in that. Okay, first of all, a muon is 18 00:01:46,000 --> 00:01:54,000 just a simple particle, right? It's an elementary particle. It's like an electron but more massive, heavier, 19 00:01:54,000 --> 00:02:00,000 right? There's electrons, muons, taus. Those are three leptons. I don't know Claudia maybe you can explain to us 20 00:02:00,000 --> 00:02:06,000 what why would a muon have a magnetic moment? Because muons have spin 21 00:02:06,000 --> 00:02:12,000 and if you want to some extent to spin is like a rotation but it's not a 22 00:02:12,000 --> 00:02:17,000 rotation in a physical space it's really a quantum property of the particle 23 00:02:17,000 --> 00:02:23,000 I don't know if that helps well I think it does. But let me 24 00:02:23,000 --> 00:02:30,000 first of all we should acknowledge that this is the international year of quantum science science and technology which is in the 25 00:02:30,000 --> 00:02:35,000 spirit of of our previous podcast. Of course everything we do here at CERN 26 00:02:35,000 --> 00:02:42,000 basically or at Fermilab has to do with quantum mechanics, quantum field theory. But that's a strange thing, though. 27 00:02:42,000 --> 00:02:50,000 You have to admit that's a strange thing. So an elementary particle is something which has no structure to it. You you can't really think of it as 28 00:02:50,000 --> 00:02:58,000 spinning in in space, but it has some property that that's spin. I mean, why? 29 00:02:58,000 --> 00:03:04,000 Why? And how do we figure that out? 30 00:03:04,000 --> 00:03:10,000 Nobody wants to do this. Come on, somebody be brave. Tell me. I got two theorists here and an experimentalist. 31 00:03:10,000 --> 00:03:17,000 No one knows why. I mean, we know that it's the case, but no one knows why they have spin. I mean we just 32 00:03:17,000 --> 00:03:24,000 found out by the way they interact that there has to be this property that is like a tiny magnetic I mean like 33 00:03:24,000 --> 00:03:29,000 a rotation but there can't be right and this is what we have to live with I mean sometimes it's really just 34 00:03:29,000 --> 00:03:35,000 difficult and hard to understand but it is what the theory tells us but it it is a really important property 35 00:03:35,000 --> 00:03:41,000 in the end right because this is what defines the properties of 36 00:03:41,000 --> 00:03:48,000 material versus say the force carriers right force carriers have integral spin or in the case of the Higgs 37 00:03:48,000 --> 00:03:56,000 Boson, zero spin which is still integral that's carrying the force and then electrons and muons and taus and their 38 00:03:56,000 --> 00:04:03,000 neutrinos have spin 1/2 so there's a certain that's the amount of spin they have but I guess we learned 39 00:04:03,000 --> 00:04:08,000 about this because you can add that spin onto angular momentum somehow right you 40 00:04:08,000 --> 00:04:13,000 can add that on to to that value but okay so this is somehow 41 00:04:13,000 --> 00:04:19,000 magically this elementary particle has this property and that gives it a 42 00:04:19,000 --> 00:04:26,000 magnetic moment. And how would you with theory tell me what that magnetic 43 00:04:26,000 --> 00:04:32,000 moment should be? What would you guess it would be? Well, one thing to say is that spin is 44 00:04:32,000 --> 00:04:39,000 not enough in general to have a magnetic moment in the case of an elementary particle. You also need a particle which is 45 00:04:39,000 --> 00:04:45,000 charged. Okay. Instead if you have a composite particle so a particle which 46 00:04:45,000 --> 00:04:51,000 is made out of smaller building blocks that can have a magnetic moment 47 00:04:51,000 --> 00:04:57,000 also if it's electrically neutral for example the neutron is made of up and down quarks which are 48 00:04:57,000 --> 00:05:02,000 individually charged. Okay. And you can imagine them to move a bit in the in the nucleus. 49 00:05:02,000 --> 00:05:08,000 Okay. So the charge distribution of the neutron even though the neutron is overall neutral 50 00:05:08,000 --> 00:05:13,000 has a non-trivial shape and this in combination with the spin is 51 00:05:13,000 --> 00:05:20,000 what allows you to have a magnetic moment. So not every particle has a magnetic moment. Not every particles but the leptons 52 00:05:20,000 --> 00:05:29,000 apparently do because they do. Yeah. They all do. So we have electrons and muons and taus and why maybe I can 53 00:05:29,000 --> 00:05:35,000 ask Fred why did you guys at Fermilab and first at Brookhaven decide you 54 00:05:35,000 --> 00:05:41,000 wanted to measure the magnetic moment of the muon. The muon is really kind of ideal because 55 00:05:41,000 --> 00:05:49,000 it has the the perfect combination of mass and lifetime. So it turns 56 00:05:49,000 --> 00:05:55,000 out that if you're looking for the contributions of new physics to the 57 00:05:55,000 --> 00:06:03,000 magnetic moment of a particle it helps if that particle is more massive and the muon is about 200 times more 58 00:06:03,000 --> 00:06:09,000 massive than the electron and that amplifies its sensitivity to new physics 59 00:06:09,000 --> 00:06:17,000 by a factor of about 200 times squared. And so that means that it's a 60 00:06:17,000 --> 00:06:23,000 great laboratory for searching for new physics but also we have to have 61 00:06:23,000 --> 00:06:28,000 them around long enough to be able to study them. So the lifetime has to be long 62 00:06:28,000 --> 00:06:35,000 enough. Of course electrons live forever, right? They're stable particles and the magnetic moment of the electron 63 00:06:35,000 --> 00:06:42,000 has been measured in the lab incredibly precisely. But it's not really so sensitive to new physics. Meanwhile, the 64 00:06:42,000 --> 00:06:47,000 lifetime of the tau is incredibly short. It's not long enough to be able 65 00:06:47,000 --> 00:06:53,000 to study it effectively. And so, the lifetime of the muon is long 66 00:06:53,000 --> 00:06:58,000 enough that we can circulate them around an accelerator, at rest a muon lives on 67 00:06:58,000 --> 00:07:04,000 average a little more than two microseconds. In our experiment about 64 microseconds 68 00:07:04,000 --> 00:07:10,000 because they're moving at close to the speed of light which actually increases their lifetime from our point 69 00:07:10,000 --> 00:07:16,000 of view because their clocks are running slowly. This is a really interesting thing from special 70 00:07:16,000 --> 00:07:23,000 relativity. So there's a secret to longevity if everybody if you really want to live long move really super fast. And I 71 00:07:23,000 --> 00:07:30,000 mean this is why we know about muons, right? Muons hit us over the head all the time. Not to scare you but while 72 00:07:30,000 --> 00:07:36,000 we've been talking hundreds of muons have gone through you through your head from interactions in our in our upper 73 00:07:36,000 --> 00:07:42,000 atmosphere and that was one of the things that got us started out on the whole path of doing particle physics right we saw 74 00:07:42,000 --> 00:07:50,000 mother nature's doing this all the time I saw a calculation once that said that mother nature had done the whole LHC 75 00:07:50,000 --> 00:07:57,000 program about 10,000 times already in our upper atmosphere so another reason to see that it's a safe thing this 76 00:07:57,000 --> 00:08:05,000 has happened we've done this, but we just didn't have our detectors around to measure everything that was happening at the time. Okay, so let's get 77 00:08:05,000 --> 00:08:12,000 back to to the muon. Well, actually, we should mention this that there's that lifetime, but the tau is much more 78 00:08:12,000 --> 00:08:20,000 massive. So, that's the difference between these guys, electrons, muons, and taus. And it turns out that when you're more massive, as my wife 79 00:08:20,000 --> 00:08:26,000 tells me you're going to decay more quickly. But it's true for elementary particles. The more the 80 00:08:26,000 --> 00:08:33,000 heavier they are, they have more ways they can decay in. Right. Decay is a terrible word. I hate to use decay 81 00:08:33,000 --> 00:08:38,000 because they don't really decay. They change. Transform. They transform from one type to another. 82 00:08:38,000 --> 00:08:45,000 They don't actually It's not like there's a muon. There's no electrons inside a muon, right? It's still an elementary particle. Okay. So, you've 83 00:08:45,000 --> 00:08:51,000 made this really nice measurement. Not just you. your whole collaboration. 176 of us. 84 00:08:51,000 --> 00:08:57,000 176 of you working on this from all from the US because it's in Fermilab. No, it's very much an international 85 00:08:57,000 --> 00:09:04,000 collaboration with a lot of major contributions from places including 86 00:09:04,000 --> 00:09:10,000 Italy, Germany, the United Kingdom, South Korea and China. Wow okay so it's it's as international 87 00:09:10,000 --> 00:09:16,000 as many of our even larger collaborations that are here at CERN and it didn't even start at Fermilab 88 00:09:16,000 --> 00:09:22,000 did it. I mentioned Brookhaven And it started there, right? I would even say before that it 89 00:09:22,000 --> 00:09:28,000 started with a series of three experiments here at CERN. And then after that came an experiment at Brookhaven 90 00:09:28,000 --> 00:09:34,000 that I worked on for my PhD. So I've been doing this for quite a long time now. We were taking that data back in 91 00:09:34,000 --> 00:09:40,000 the late 1990s, early 2000s. And we had an opportunity to relocate the 92 00:09:40,000 --> 00:09:45,000 experiment to Fermilab long after we were done at Brookhaven. Because at 93 00:09:45,000 --> 00:09:52,000 Fermilab they really had some great ideas about how to with their accelerator complex serve up an enormous 94 00:09:52,000 --> 00:09:57,000 number of muons to the experiment in small little batches for us which turns 95 00:09:57,000 --> 00:10:03,000 out to matter and so with many many muons because 96 00:10:03,000 --> 00:10:09,000 really our experiment is still limited by the number of muons that we've counted by the statistics 97 00:10:09,000 --> 00:10:15,000 and with them divided up so that they don't pile up with each other as they 98 00:10:15,000 --> 00:10:20,000 decay so that we can count each muon decay separately. Wow. 99 00:10:20,000 --> 00:10:27,000 So that's the motivation for moving it there to Fermilab. So and that was easy right? You just put 100 00:10:27,000 --> 00:10:33,000 it in a box and ship it you know UPS orÉ Well, when we first got started 101 00:10:33,000 --> 00:10:40,000 talking about the logistics we were actually talking about picking it up with a helicopter and moving it with a 102 00:10:40,000 --> 00:10:45,000 helicopter to a barge in the port. It turns out the helicopter idea was not so 103 00:10:45,000 --> 00:10:51,000 practical. So it went onto a truck to a barge around the tip of Florida into 104 00:10:51,000 --> 00:10:58,000 the Gulf and then up through the rivers and up to the port of Lemont near 105 00:10:58,000 --> 00:11:05,000 Chicago and then back onto a truck on the tollways in the Chicago 106 00:11:05,000 --> 00:11:10,000 area out to Fermilab. So I bet quite a production. I bet everybody loved the traffic jam. 107 00:11:10,000 --> 00:11:16,000 I got to ride along with it one night and yeah, you could certainly sense that 108 00:11:16,000 --> 00:11:21,000 at the closed on ramps to the tollway, people were a little bit annoyed, but at 109 00:11:21,000 --> 00:11:27,000 the same time, I think it was worth it for the science that we got. Sure. Well, I mean, we we've been 110 00:11:27,000 --> 00:11:33,000 through that here. I remember taking my son to school actually at CERN has a little school for when 111 00:11:33,000 --> 00:11:38,000 they're very young and and us being slowed down because there was a blue dipole. My son at three was already 112 00:11:38,000 --> 00:11:45,000 saying, "Look, dad, a blue dipole." So, we we've had that. So, when you when you do these major things and when we build 113 00:11:45,000 --> 00:11:51,000 our future accelerators in the local areas, we'll have to make sure we all understand it's a slight sacrifice, but 114 00:11:51,000 --> 00:11:59,000 it's worth it for the for the science that's being done. So, I mentioned the amazing precision and I think 115 00:11:59,000 --> 00:12:06,000 that's important to note here because I love to wax poetic about the experiment I work on. I work on the Atlas experiment and it's an enormous thing. 116 00:12:06,000 --> 00:12:12,000 It's some half of a football field in length. And so over 117 00:12:12,000 --> 00:12:18,000 these 46 meters we also line up our detector at the 118 00:12:18,000 --> 00:12:24,000 far ends to tens of microns. Right. So that means we have 119 00:12:24,000 --> 00:12:29,000 one in a million precision. Now your measurement gives us a precision of what amount? 120 00:12:29,000 --> 00:12:36,000 A little better than 140 parts in a billion. And so that is the 121 00:12:36,000 --> 00:12:42,000 analogy that I've heard recently from one of my colleagues is that they have these big animals a little bit bigger 122 00:12:42,000 --> 00:12:48,000 than cows at Fermilab, Bison. They're famous and so the precision is about 123 00:12:48,000 --> 00:12:54,000 one sunflower seed out of a whole bison. Okay. So if a bison were to eat one more 124 00:12:54,000 --> 00:12:59,000 sunflower seed, you could tell the difference within that level of precision. That's right. That that's a good definition of 125 00:12:59,000 --> 00:13:06,000 precision. I think the only the only experiment that beats us I can tell you there is one that beats us. That would be LIGO, right? Their measurements of 126 00:13:06,000 --> 00:13:12,000 gravitational waves. Impossible. I don't know how they do it. That's impossible. But you did a lot of work to get that 127 00:13:12,000 --> 00:13:18,000 far. But what I'd like to know actually I turn to theory now. Okay. Because Simon's also working on trying to 128 00:13:18,000 --> 00:13:23,000 improve theory. This is a very interesting thing because you guys have them beat a little bit here. The 129 00:13:23,000 --> 00:13:30,000 precision of the measurement. Tell us about that Simon. What what's going on? Why is theory not able to to have a 130 00:13:30,000 --> 00:13:35,000 smaller error bar than the experiment? I mean the reason is not that we were not trying. So actually I mean before 131 00:13:35,000 --> 00:13:40,000 these guys built their experiments there was this g-2 theory initiative. So 132 00:13:40,000 --> 00:13:48,000 really hundreds of people from all over the world coming together with the idea that we should give our best to have the 133 00:13:48,000 --> 00:13:55,000 most precise prediction of the theory. to really do the best doing calculations 134 00:13:55,000 --> 00:14:01,000 to be able to compare to experiment and it was clear that we have to improve and that the experiment would be 135 00:14:01,000 --> 00:14:07,000 very precise, but it's an incredibly difficult calculation to do 136 00:14:07,000 --> 00:14:13,000 so I mean the question is what is happening there and the point is that at 137 00:14:13,000 --> 00:14:20,000 this precision we're talking about everything matters really everything in the sense that this muon and magnetic 138 00:14:20,000 --> 00:14:26,000 field doesn't only talk to the magnetic fields which is composed out of photons 139 00:14:26,000 --> 00:14:32,000 but they're all these quantum fluctuations and it's really talking to all the forces in the standard model which are the weak interaction the 140 00:14:32,000 --> 00:14:38,000 strong interaction and the electromagnetic interaction. Okay, that's a strange thing. Let's just 141 00:14:38,000 --> 00:14:43,000 try to wrap our heads around this for a second here. So everything's going on, right? You have a certain amount of 142 00:14:43,000 --> 00:14:49,000 energy. I know that if we take our protons and accelerate them, it turns out there's things popping in and out of 143 00:14:49,000 --> 00:14:55,000 the vacuum out of nowhere. And perhaps the big bang was us just popping out of nowhere. I don't know. 144 00:14:55,000 --> 00:15:01,000 But that's happening right at high energies. So, maybe Claudia, can you explain to me a little. 145 00:15:01,000 --> 00:15:07,000 But how does this work? How how is it that we don't get an answer that's 146 00:15:07,000 --> 00:15:17,000 precise that we have to sort of expand, I guess. How does that work? 147 00:15:17,000 --> 00:15:25,000 Okay. It's complicated to explain. It's complicated. I'm gonna I'm gonna try. Okay. Well, in general, when we try 148 00:15:25,000 --> 00:15:30,000 to make a prediction for an observable to make a a calculation that then can be 149 00:15:30,000 --> 00:15:35,000 compared to the experiment, ideally we want to try to do it in the 150 00:15:35,000 --> 00:15:40,000 simplest way possible. And 151 00:15:40,000 --> 00:15:46,000 depending on the precision of the experiment, we need to get a certain precision on our theory prediction too. 152 00:15:46,000 --> 00:15:51,000 We kind of want to match that precision. If the experiment is not very precise, 153 00:15:51,000 --> 00:15:58,000 it might be enough to make some we can call it some sort of approximation which is good enough to get a prediction of 154 00:15:58,000 --> 00:16:05,000 the same precision as the experiment. But as experiments get more and more precise, 155 00:16:05,000 --> 00:16:11,000 we cannot do these approximations anymore. And then things become very easily much 156 00:16:11,000 --> 00:16:16,000 more complicated. Now in general, depending on the type of interactions 157 00:16:16,000 --> 00:16:22,000 you're looking at it might be possible that at every 158 00:16:22,000 --> 00:16:28,000 step the effect gets smaller and smaller. So by doing the 159 00:16:28,000 --> 00:16:35,000 first-order computation you already get the bulk of the result. But what these guys in practice deal 160 00:16:35,000 --> 00:16:40,000 with is a situation where everything matters at the same time. There is no 161 00:16:40,000 --> 00:16:45,000 approximation. Everything everywhere all at once. I guess what's going on here? 162 00:16:45,000 --> 00:16:52,000 So I mean it is a very interesting because most people would think all right and and it's very normal to think 163 00:16:52,000 --> 00:16:58,000 that you have a theory and the theory will tell you well it should be. 164 00:16:58,000 --> 00:17:05,000 You know you have a simple thing. You have a muon here it is it's got a charge and a spin and you do your calculation but it 165 00:17:05,000 --> 00:17:11,000 turns out because of the energy things can pop in and out of a 166 00:17:11,000 --> 00:17:17,000 vacuum, you end up having to do and this is this This is what actually drew 167 00:17:17,000 --> 00:17:24,000 me into the field I have to admit in quantum field theory was that when you have an interaction there are a lot of 168 00:17:24,000 --> 00:17:30,000 different possible things going on right so I always mention this when I talk to people about what goes on when we 169 00:17:30,000 --> 00:17:36,000 collide protons and and these two quark let's say or two gluons interact 170 00:17:36,000 --> 00:17:42,000 there's many different ways that that can happen and we're not even allowed to see that right we never know so you guys 171 00:17:42,000 --> 00:17:48,000 never really get to get inside there and see what's what exactly happened with this muon becoming an electron. And it 172 00:17:48,000 --> 00:17:55,000 could have gone through many different paths. Some of those very complex, but the more complex they are, the smaller the probability. And some of 173 00:17:55,000 --> 00:18:02,000 them straightforward. And you end up with this sort of infinite sums, right? You get something that we learn if 174 00:18:02,000 --> 00:18:08,000 you go far enough in math at school, you'll find these sort of expansions. 175 00:18:08,000 --> 00:18:14,000 and there you see that the bigger the further you go 176 00:18:14,000 --> 00:18:20,000 the smaller the numbers are but as you're saying, in this world in the 177 00:18:20,000 --> 00:18:27,000 world of the the muon here you have to go out pretty far to get the precision and Simon's just not getting it done! 178 00:18:27,000 --> 00:18:32,000 Well no, it's really complicated I mean so there's the frustrating thing maybe is 179 00:18:32,000 --> 00:18:37,000 that 99.99% of this theory prediction is is based on 180 00:18:37,000 --> 00:18:44,000 this electromagnetic interaction and people can compute it very nicely and it's super precise and then there's this 181 00:18:44,000 --> 00:18:50,000 small piece it's called the hadronic vacuum polarization contribution and that's basically I mean there's the muon 182 00:18:50,000 --> 00:18:57,000 and then there's a photon that tells the muon how strong the magnetic field is and then this photon 183 00:18:57,000 --> 00:19:03,000 makes a quark anti-quark pair and as soon as we have this everything happens it's there are tons of particles and 184 00:19:03,000 --> 00:19:08,000 everything happens at the same time and we cannot just sit down and write a formula and get the result and we 185 00:19:08,000 --> 00:19:14,000 cannot even go to a computer and press a button and get the result. 186 00:19:14,000 --> 00:19:21,000 So we have to go to a computer but it takes years and years and the biggest computers in the world to get to results 187 00:19:21,000 --> 00:19:27,000 that we have to understand this. There's a name for this right? Lattice QCD. It's called Lattice QCD and it's really 188 00:19:27,000 --> 00:19:33,000 the only method that we know that reliably can tackle these this strong interactions, these hadronic properties of 189 00:19:33,000 --> 00:19:39,000 a theory. Yeah. So just to recap I mean the strong interaction but there's for some reason there are people out there who don't 190 00:19:39,000 --> 00:19:45,000 know about QCD and the strong interaction they ought to. That's you know the interaction between quarks 191 00:19:45,000 --> 00:19:52,000 Quarks are what put we put together to make protons and we make neutrons out of them and there's a whole lot of other 192 00:19:52,000 --> 00:19:57,000 hadrons our friends at LHCb are finding new combinations all the time, including pentaquarks 193 00:19:57,000 --> 00:20:03,000 and all these different excited states of these but in this case you can produce these these pairs and 194 00:20:03,000 --> 00:20:10,000 when you do that lots of different things can can happen. So, so I guess I mean I should note that that we had to 195 00:20:10,000 --> 00:20:16,000 bring on two theorists because the experimentalist has done his job over there. But it seems 196 00:20:16,000 --> 00:20:21,000 the theory is really more than twice as hard, I have to say. Yeah. Okay. It's a very 197 00:20:21,000 --> 00:20:28,000 hard job to try to get this down. And one thing that's important to note is at no point are either of you guys trying 198 00:20:28,000 --> 00:20:34,000 to be like the other, right? So you're not saying, "Oh, my result is over here." And for a while it's been like 199 00:20:34,000 --> 00:20:39,000 that, right? The result of the measurement has been a little ways away 200 00:20:39,000 --> 00:20:47,000 from theory which makes it intriguing and interesting. But all you're trying to match up is the precision. Try to get 201 00:20:47,000 --> 00:20:53,000 as good a precision or better than the other and then you'll see what nature tells you if they agree. So maybe 202 00:20:53,000 --> 00:20:58,000 Fred you can expand a little bit more on how because you guys did an amazing job to get the precision that you have. What 203 00:20:58,000 --> 00:21:05,000 did you have to do? Right. Well, the first key was simply measuring an awful lot of muons. So, 204 00:21:05,000 --> 00:21:12,000 we recorded a little more than 300 billion final events that made it 205 00:21:12,000 --> 00:21:20,000 into our final data set. And to do that, we actually had to store more than a trillion muons in the storage ring. So, 206 00:21:20,000 --> 00:21:26,000 that was really the first key was simply having this incredibly high 207 00:21:26,000 --> 00:21:31,000 statistical power. And then we really had to pay attention to like you said 208 00:21:31,000 --> 00:21:36,000 before everything everywhere all the time. So we had to focus on really 209 00:21:36,000 --> 00:21:42,000 improving every possible systematic error. And you can see going from one 210 00:21:42,000 --> 00:21:49,000 year to another the kinds of things that we improved. So for example in our 211 00:21:49,000 --> 00:21:56,000 experiment the technique really relies on what's called the magic momentum which is to say that the muons 212 00:21:56,000 --> 00:22:03,000 have exactly the right momentum that the electric fields don't rotate their spin 213 00:22:03,000 --> 00:22:09,000 so that we can use large electric fields to confine and focus them inside the storage ring. 214 00:22:09,000 --> 00:22:15,000 And yeah on the other hand not every muon is going to have exactly the same 215 00:22:15,000 --> 00:22:22,000 momentum. Some will be going a little fast and some will be going a little bit slow. So for this most recent result, we 216 00:22:22,000 --> 00:22:29,000 just had to work much harder on measuring the distribution of momentum including building a whole new detector 217 00:22:29,000 --> 00:22:34,000 like some of my colleagues at the University of Washington did. They call it the mini sci-fi. And so we introduced 218 00:22:34,000 --> 00:22:40,000 this in order to get just that much better precision on what we knew about 219 00:22:40,000 --> 00:22:47,000 the momentum. And you know for example, we need to know the magnetic field incredibly precisely that 220 00:22:47,000 --> 00:22:53,000 the muons experience and so we've been 221 00:22:53,000 --> 00:22:59,000 measuring the magnetic field in a sort of static way. So the part of the 222 00:22:59,000 --> 00:23:06,000 magnetic field that doesn't vary with time by pulling a trolley full of magnetic field probes all around the 223 00:23:06,000 --> 00:23:14,000 inside of the vacuum chambers at a time when the muons aren't there. But for this most recent measurement, we also 224 00:23:14,000 --> 00:23:21,000 were able to measure the time dependent magnetic fields from when we fire the kickers that kick the muons onto the 225 00:23:21,000 --> 00:23:27,000 correct orbit and from when we energize the quadrupole plates that make the big 226 00:23:27,000 --> 00:23:33,000 electric fields that confine the particles. So, you know, very 227 00:23:33,000 --> 00:23:39,000 precise measurements of those things were included progressively as we moved from the analysis of one run into 228 00:23:39,000 --> 00:23:44,000 another to get to better and better precision. And we're at the point now where really 229 00:23:44,000 --> 00:23:51,000 we would again have to improve everything if we wanted to continue to make progress here. We've really gotten 230 00:23:51,000 --> 00:23:58,000 those systematic errors down as small as we practically can. I think there's a 231 00:23:58,000 --> 00:24:03,000 few technical terms I should mention here you kickers for example or quadruples but these are all sort of 232 00:24:03,000 --> 00:24:10,000 means of using electromagnetic fields. You have an electric field to push things along make them go higher 233 00:24:10,000 --> 00:24:15,000 energies and then and then your quadrupoles also will do they focus or 234 00:24:15,000 --> 00:24:20,000 they are for focusing that's right and actually we don't have anything that 235 00:24:20,000 --> 00:24:27,000 that changes the energy of the particle once it gets into the storage ring, the kicker just redirects the particles 236 00:24:27,000 --> 00:24:32,000 From an orbit that would come around and hit the injection point again onto 237 00:24:32,000 --> 00:24:39,000 an orbit that would be stored in the vacuum chambers that sort of centers it right in the middle of the 238 00:24:39,000 --> 00:24:44,000 the quadrupoles. Okay. For those watching actually on YouTube, you have a picture of it. 239 00:24:44,000 --> 00:24:49,000 It's a relatively small accelerator compared to what you see here at CERN. Usually 240 00:24:49,000 --> 00:24:55,000 14 meters in diameter. But it's an amazing thing and actually it has an 241 00:24:55,000 --> 00:25:00,000 interesting part to it. I noticed that when the muons come in there's like a gap, right? There's like a space between the 242 00:25:00,000 --> 00:25:06,000 the magnets. And that it's fine. Muons just keep going. Right. 243 00:25:06,000 --> 00:25:12,000 Right. Over a short distance that's certainly true. Although over a long enough distance you certainly want to 244 00:25:12,000 --> 00:25:17,000 have them in a vacuum so that they don't scatter off of the air. 245 00:25:17,000 --> 00:25:23,000 Beautiful. It was a beautiful, beautiful experiment and congratulations on getting such amazing results. I do 246 00:25:23,000 --> 00:25:29,000 want to ask go back to the slight discrepancy that there was and maybe it's disappearing. We don't know. 247 00:25:29,000 --> 00:25:35,000 What can we learn when we do a precision measurement like this? Maybe 248 00:25:35,000 --> 00:25:42,000 I'll ask Claudia if you do a precision measurement, how does it help us when we compare 249 00:25:42,000 --> 00:25:48,000 it to theory? What can we gain from that? Well, whenever there is a discrepancy in 250 00:25:48,000 --> 00:25:53,000 a precision measurement, especially if this discrepancy is 251 00:25:53,000 --> 00:25:58,000 statistically significant like it was in the case of the g-2, we as 252 00:25:58,000 --> 00:26:04,000 theorists, all we want to do is to try to find out which type of new particles 253 00:26:04,000 --> 00:26:09,000 and forces could explain that discrepancy. So, precision measurements are the 254 00:26:09,000 --> 00:26:15,000 perfect venue to do this. In some sense 255 00:26:15,000 --> 00:26:23,000 by looking at a discrepancy, we can learn several things. We can learn something about possibly the scale, the 256 00:26:23,000 --> 00:26:30,000 mass of the particle that explains the discrepancy. And also something about its coupling. 257 00:26:30,000 --> 00:26:35,000 Okay. Coupling being how they interact. How they interact. Yes. So for the 258 00:26:35,000 --> 00:26:42,000 case of the g-2, two of the most common explanations were one in terms of 259 00:26:42,000 --> 00:26:49,000 very light particles called axion-like particles. These are particles that arise for 260 00:26:49,000 --> 00:26:55,000 example in models trying to explain dark matter or to solve another big problem of the standard model which is called 261 00:26:55,000 --> 00:27:01,000 the strong CP problem. Or also other types of particles like 262 00:27:01,000 --> 00:27:07,000 for example heavy bosons like heavy Z bosons that couple differently maybe 263 00:27:07,000 --> 00:27:14,000 between electrons and muons and taus, etc. Then in practice when when we build 264 00:27:14,000 --> 00:27:19,000 one of those models we also have to pay attention because in general the 265 00:27:19,000 --> 00:27:25,000 moment you have a new particle with new interactions it's not just going to pop up in one measurement but it's also 266 00:27:25,000 --> 00:27:32,000 going to arise somewhere else and then you have to make sure that your model doesn't contradict other data. So 267 00:27:32,000 --> 00:27:39,000 somehow from these small discrepancies, if they are confirmed if they are 268 00:27:39,000 --> 00:27:46,000 statistically relevant we can definitely learn something about what is beyond the theory that we 269 00:27:46,000 --> 00:27:51,000 know right now. So and we know that there is something beyond it because there are several problems it doesn't address. 270 00:27:51,000 --> 00:27:58,000 Exactly. And that it's important to note that we have this amazing theory and we keep testing it 271 00:27:58,000 --> 00:28:04,000 and we'll be doing many more precision tests here. For example, at CERN, we're always doing precision tests 272 00:28:04,000 --> 00:28:09,000 as well. We're not just looking out there. Is there a new particle out there? We do that and we love that. But 273 00:28:09,000 --> 00:28:14,000 we will be in fact in a year tearing apart the LHC and our 274 00:28:14,000 --> 00:28:21,000 experiments. At least Atlas and CMS are going to be refurbished to go to high luminosity and in which case when we get 275 00:28:21,000 --> 00:28:27,000 many more collisions like you need lots of muons, we need lots of proton collisions and with that we get 276 00:28:27,000 --> 00:28:35,000 the higher statistics, we get higher precision and we can look for very rare things but also look for these discrepancies because we're missing what 277 00:28:35,000 --> 00:28:41,000 95% of the universe. the standard model's great butÉ 278 00:28:41,000 --> 00:28:48,000 Then in the future there are many different ideas out there what we want to do in the future beyond the LHC after I've retired 279 00:28:48,000 --> 00:28:55,000 for sure and that you know includes electron-positron colliders 280 00:28:55,000 --> 00:29:00,000 maybe a huge FCC maybe linear colliders also possible muon colliders for those 281 00:29:00,000 --> 00:29:08,000 of you in the muon business a lot of different ideas out there but precision is one of the things we really want to to look for so before we 282 00:29:08,000 --> 00:29:14,000 finish up actually I would like to to hear a little bit more from Simon about you know what are the next steps. You 283 00:29:14,000 --> 00:29:19,000 mentioned getting more computing that's something everybody wants that you know chatgpt 284 00:29:19,000 --> 00:29:25,000 will work better too with more computing and we can use the same computers as the ones training chatgbt so it would be 285 00:29:25,000 --> 00:29:31,000 great if they just hand it over to us. Let's just ask chatgbt could you 286 00:29:31,000 --> 00:29:37,000 hand it over? But beyond that what else what other work can you do to improve move these models? 287 00:29:37,000 --> 00:29:43,000 What work is going on so really I mean there are two ways to compute this HVP contribution that is 288 00:29:43,000 --> 00:29:50,000 really carrying all the uncertainty and lattice QCD has only recently I mean last five years we were able all I mean to 289 00:29:50,000 --> 00:29:57,000 reduce our uncertainties by a factor of five or so and now we're really able to make this prediction 290 00:29:57,000 --> 00:30:03,000 and we're very certain that we can improve but as Claudia said as as soon as you want to get to better 291 00:30:03,000 --> 00:30:09,000 precision you always have to be more careful about tiny effects that you have to control. So it's certainly getting bigger 292 00:30:09,000 --> 00:30:14,000 computers, getting better algorithms. I mean be smart about how you want to 293 00:30:14,000 --> 00:30:21,000 compute something. And also have new ideas. And then there's a second way one can 294 00:30:21,000 --> 00:30:27,000 compute it has been done historically. It's called the Ratio method. And if they I mean they can also improve with 295 00:30:27,000 --> 00:30:34,000 new experimental input actually. Okay. because some sometimes a theory has to have for various values that you 296 00:30:34,000 --> 00:30:39,000 use for the input. Before we go, one last thing I'm gonna turn over to Fred here. 297 00:30:39,000 --> 00:30:46,000 What's next? You said you're not going to do it yourself, right? Are you handing the baton over to 298 00:30:46,000 --> 00:30:51,000 anybody else? So, at Fermilab, we still do have a few more papers to come out of our 299 00:30:51,000 --> 00:30:57,000 experiment. So, we're going to look at the potential for new physics to cause 300 00:30:57,000 --> 00:31:04,000 our result to change as a function of time. Maybe oscillating, getting a little bigger and smaller over time as a 301 00:31:04,000 --> 00:31:09,000 result of coupling to things like dark matter fields or violations of the 302 00:31:09,000 --> 00:31:17,000 postulates of special relativity. So, we'll be looking for that. We'll be looking for the electric dipole moment 303 00:31:17,000 --> 00:31:24,000 of the muon. we should be able to set a limit that's stronger than the current limit on how big that could be should be 304 00:31:24,000 --> 00:31:30,000 zero in the standard model. So if we were to see something that's not zero then that would be exciting. But 305 00:31:30,000 --> 00:31:36,000 then the next muon g-2 experiment is going to be at J-Park in Japan and they 306 00:31:36,000 --> 00:31:42,000 have a technique there that is like ours but with muons that are not at the 307 00:31:42,000 --> 00:31:49,000 magic momentum. So they can't use electric fields to confine them. Instead, they cool their muon beam to 308 00:31:49,000 --> 00:31:56,000 the point where they don't have to use electric fields and they can use a much smaller magnet. So, they will be able to 309 00:31:56,000 --> 00:32:01,000 do a complimentary experiment with smaller uncertainties. I should also point out that the reason 310 00:32:01,000 --> 00:32:08,000 that I'm here at CERN is to work on an experiment called Muon that we're setting up at the CERN SPS. And so, what 311 00:32:08,000 --> 00:32:14,000 we are doing there is getting experimental input for a third way of 312 00:32:14,000 --> 00:32:19,000 doing the theory. So you've heard from Simon about two ways right from 313 00:32:19,000 --> 00:32:26,000 electron positron collision data the R ratio method and lattice gauge theory. 314 00:32:26,000 --> 00:32:32,000 Well we're observing the hadronic vacuum polarization effects in the interaction 315 00:32:32,000 --> 00:32:38,000 between a muon and an electron when a high energy muon beam hits the electrons in a target. 316 00:32:38,000 --> 00:32:45,000 Okay great. So I should say SPS by the way many people might know that super proton synretron which brought us the 317 00:32:45,000 --> 00:32:51,000 W and the Z beson many years back about 1983 I think it was way back and we 318 00:32:51,000 --> 00:32:57,000 still use these and we have some older accelerators than that even that we still use as well. Well thank you very 319 00:32:57,000 --> 00:33:06,000 much. Fred Gray is professor of physics at Regis University in beautiful Colorado. Simon 320 00:33:06,000 --> 00:33:12,000 Kuberski and Claudia Cornella are both fellows here at CERN working in 321 00:33:12,000 --> 00:33:18,000 the theory division. So thank you for joining us. This has been early morning 322 00:33:18,000 --> 00:33:23,000 coffee at CERN, a podcast by the scientists of CERN (or in fact not just CERN) 323 00:33:23,000 --> 00:33:29,000 about the science of CERN. You can find all of our episodes on the 324 00:33:29,000 --> 00:33:34,000 CERN YouTube channel and anywhere you get your podcast. Guaranteed. Go ahead. 325 00:33:34,000 --> 00:33:40,000 Find the place you get your podcast. You can find us. Be sure to do whatever you're supposed to do. Like and share 326 00:33:40,000 --> 00:33:45,000 and I don't know, send it to your friends. There's a lot of things you're supposed to do. So, do those things and 327 00:33:45,000 --> 00:33:51,000 subscribe. Our editor and producer today is Melanie Arnold. Hi, Melanie. 328 00:33:51,000 --> 00:33:59,000 Good job. Thanks. Our executive producer is Jacques Fichet. Jacques Fichet and Piotr Traczyk 329 00:33:59,000 --> 00:34:07,000 are our technical leads, while Ron is away getting fixed up. Our studio manager is Max Price. Sound 330 00:34:07,000 --> 00:34:12,000 engineering is by Piotr. Our original theme comes from the Canettes Blues Band with 331 00:34:12,000 --> 00:34:19,000 piano by Wojt play-it-in-any-key Krajewski. Many thanks to Paula Catapano, Matthew 332 00:34:19,000 --> 00:34:26,000 Chalmers, and Arnaud Marsollier for all of their advice and strategic planning. Big thanks to the entire Education 333 00:34:26,000 --> 00:34:32,000 Communication and Outreach team here at CERN for providing us with access to beautiful Wire Chamber Studio and all 334 00:34:32,000 --> 00:34:39,000 the help that comes with it. I also want to take a moment to thank Cetna Krishna who's no longer with us. (That sounds 335 00:34:39,000 --> 00:34:46,000 Terrible!) It's because she finished up her fellowship here at CERN. She 336 00:34:46,000 --> 00:34:52,000 along with Joni Pham were really two of the people who came up with the whole idea of doing this, and I get to have a lot of fun and I hope that they're doing 337 00:34:52,000 --> 00:34:58,000 well as well. Opinions expressed here are our own and do not necessarily 338 00:34:58,000 --> 00:35:04,000 reflect those of our colleagues or of CERN although we think they ought to. My name is Steven Goldfarb. This has 339 00:35:04,000 --> 00:35:05,000 been Early Morning Coffee at CERN. We'll see you next month.