Big Physics News: The Muon g-2 Experiment Explained

This is Scientific American’s 60-Second Science podcast. I’m Clara Moskowitz.

There are probably many more particles out there in the universe than the ones we know about. And today physicists got a hint about where they might be hiding. The finding comes from an experiment at Fermilab called Muon g-2, which looks at particles called muons that are heavier cousins of electrons.

It turns out their spins wobble more than the standard laws of physics say they should. Here to tell us all about it is David Hertzog of the University of Washington, one of the physicists on the experiment. By the way, this segment is on the longer side, dear listener. But hey, this is complicated physics.

David, thanks for being here.

“Thanks, Clara. This is a really exciting time for us.”

Okay. Let’s get grounded. Why are muons important? 

“Well, since the discovery of the muon, it’s played, actually, a rather unique and versatile role in subatomic physics. Topics that people use muons for range from fundamental constants of nature, basic symmetries, weak nucleon and nuclear interactions.

“And for us, what we care about the most is standard model tests and searches for new physics. That’s what we’re going to do with them. Now the muon is an unstable particle. It only lives for about two microseconds, but that’s sufficiently long to precisely study its properties. And yet it’s actually sufficiently short so that we have enough decays that we can also study a lot of tremendous information in the decay processes.

“Now, by a quirk of nature, which we call parody nonconservation or space nonconservation, muons are born what we call fully polarized, meaning they have spins in a direction that we go like tops do. And when they decay, we say they are a self-analyzing, which means we can figure out which way they were spinning when they decayed.

“And these two attributes are essential for the experiment that I’m going to talk to you about.”

So tell us why you ran this experiment in the first place. Why look at muon spin? 

“Well, when we measure the rate that the muon spin wobbles, or I use the word processes in a magnetic field, we learn directly about its own magnetism, which we call the magnetic moment.

“But you might ask, ‘What do we care about that for?’ Well, the laws of physics actually predict this magnetism very, very precisely. And the laws, if we think we know them completely, in turn, tell us the rate of that wobbling that we should expect in the magnet. So by measuring the rate, we can learn that the laws of physics are missing anything.”

Now tell us about the setup of this experiment in basic terms. How did it work? 

“It is a very, very complicated setup, Clara. But let me just try to break it down simply: We shoot big batches of muons into a large 14-meter-diameter superconducting magnet. All of them, we shoot in with their spins, kind of lined up in the direction they’re going like headlights on a car. When the muons begin to circulate and run around the magnetic ring, they sort of act like race cars going around a circular track. So as they go round and around and around. It turns out that the direction that their spins point no longer stays kind of lined up with the way they were when they were injected.

“And every 29 times around the track, the spin direction actually makes an extra full turn. So this difference is what we measure. We measure the difference between the spin direction and the direction the muons were going. That signal, then, is all tied up in the comment I made about parity violation earlier—in self-analyzing spin, I said. We record the products of the muon decays when they spin around like that. And we collect them into a spectrum that ends up with a kind of a modulation exactly at that lapping frequency. So the lapping frequency is the ticket. How fast the spin goes around faster than the muon runs around.”

And what did you find?

“Well, we found that that wobble frequency was faster than the prediction—and we found, also interestingly enough, that the same kind of level faster than what had been measured 20 years ago at Brookhaven National Lab. So we confirmed this, this value that was out there for about 20 years, that people were kind of like ‘Is that right?’”

So what does this mean, and why is it so exciting? 

“Oh, it’s truly exciting because the significance of the difference now, between the prediction and the experiments, is so high that it looks like it might be revealing something. Twenty years ago, it was just a soft difference between the prediction, but we have a higher-precision experiment now.

“And when we can bind that with the measurement from 20 years ago, the precision is pretty high. And we’re really at the level where people begin to think this starts to look a bit like a discovery. So at the very beginning, Clara, what you said was perhaps there are more particles in the universe than the ones we know about, and it’s those extra particles which would cause this spin to go faster than the prediction. 

“It is such a complicated experiment. We’re publishing four papers at once—probably 100 pages in the journals—to explain the whole thing. I’ve been doing this for about 30 years. So you also realize we do this blinded, right?”

Right. So when did you find out?

“Just about a month ago. So, you know, just enough to put in the numbers into the final plots and to write the beginning and ends of the paper—90 percent of the papers written before we know their result. And basically, you have 170 people sitting on a zoom meeting that all have to be satisfied and vote. And then we reveal these secret envelopes, and then we decode the clock frequency, and suddenly we see the results. It’s really unnerving, but it absolutely means we are not biased as to what we’re going to get, because we have no way to change the number after we type in the secret code.”

And what did you feel when you saw that number? 

“To be honest, we all screamed in excitement, but maybe for different reasons. For me having been involved in this so long ago and also being involved in the previous experiment, I was extremely happy that we were verifying that the previous experiment was correct.

“But then the second emotion comes along that the two of them together now push the difference to what’s actually called 4.2 standard deviations, which means it’s about a one in 40,000 chance or so of being a fluke. And that really is exciting because we’re all looking for new physics.”

That’s amazing. So we might actually be seeing the work of particles that we never knew about before.

“It sure does, but we do have more work to go. We’re just sitting on the smallest pile of the data so far, in terms of the results. We have a lot more data that we’re taking as we speak. And only then, when we analyze all of it, might we actually know, you know, the final truth to this. But the other thing that makes it kind of interesting is, from all of the students and postdocs and young people still working on this with so much more data to go, basically, we’re not quite over the line of what they call discovery at five standard deviations.

“So this is very motivating for us to finish the job since we have so much more data that we can look at, all of it sort of fell in the right place to keep it—keep it kind of cool.”

Well then I’m going to stay tuned. Thank you so much, David.

“You bet. It was enjoyable.”

For Scientific American’s 60-Second Science podcast, I’m Clara Moskowitz.

—Clara Moskowitz

[The above text is a transcript of this podcast.]
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