The Fat, Wobbly, Nuisance of a Particle That Could Change How We Understand the Universe

What exactly happened in the physics world this week?

A water feature and sculpture outside the Fermilab.
Big news this week out of the Fermilab, pictured in May 2013. Raymond Boyd/Getty Images

This week, scientists at the Fermi National Accelerator Laboratory in Batavia, Illinois, announced a breakthrough result that could upend particle physics. It turns out that when you expose particles known as muons—aka “fat electrons”—to intense magnetic fields, they behave in ways that can’t be explained by physics as we know it, suggesting there’s a bunch of stuff happening in the quantum world for scientists to uncover.

The result has been highly anticipated among physicists for years, and one researcher at Fermilab described it to the New York Times as a “Mars rover landing moment,” but for most of us, a discovery that hinges on the difference between a measurement of 0.00116592040 and an expected one of 0.00116591810 is a little hard to parse.


To get a better sense of what muons are, why they do the things they do, and what muons doing the things they do means for the universe, I spoke with Emily Conover, a reporter for Science News who earned a Ph.D. in particle physics from the University of Chicago before turning to science writing. Our conversation has been edited and condensed for clarity.


Joshua Keating: First question: How do you say it, mew-on or moo-on?

Emily Conover: Mew-on, like the Greek letter μ.

OK, great. What is a muon?

Muons are basically like a heavy version of an electron. They are unstable particles so they have a lifetime of a fraction of a second after which they decay into other particles. They are actually always all around us, we just don’t realize it. They are produced commonly when a high energy particle like a proton hits the Earth’s atmosphere. There are these high energy particles that are blasting around space all the time and when they hit the atmosphere, they interact with the gas there and create other particles, including muons.


I have the picture in my head of the model of the atom. There’s the nucleus with the protons inside it, and then there are the little electrons whizzing around the outside. So, where are the muons?

One big concept that most people aren’t aware of is that there are lots of other particles that are out there that aren’t in atoms. Because they’re not stable, they don’t make up the matter that makes up everything we’re familiar with.


Got it. But they’re always around us?

Yeah, there’s lots of muons that are raining down on Earth’s surface all the time. Actually, it’s kind of a nuisance for most physics experiments, because most physics experiments don’t want to detect muons.


They’re weird. There’s a famous quote from a scientist when it was discovered: “Who ordered that?”* But in this experiment, they do want to study the muon, so they’re creating them artificially in order to study them.

So, what did we learn about them this week?

Muons have this property where they behave like little magnets. And basically, the strength of that magnet is something you can predict really well using our theories of particle physics—the “Standard Model” is what it’s called. So, you can predict how that magnet should respond within a magnetic field. People compare their behavior in the field to a top that’s spinning—the axis that the top rotates around, that’s that wobbling motion.


So the muons were exposed to a big magnet and then the scientists watched what they did?

Yes. So, they have this big doughnut-shaped magnet, and they send the muons into that magnet, and they circulate around inside of it, and as they’re circulating around, they’re also doing this whole wobbling, rotation thing.


How do the muons get into the magnet? 

So, this is done at Fermilab, and they use a particle accelerator to accelerate protons, and the protons slam into a target and interact with it, and then you have muons coming out the end of that. So, they accelerate the particles, and then they turn those into other particles, which are still moving in a fast beam, and then they’re able to corral those around this circular magnet.


They put the muons in the big magnet-tube, and then the muons did something weird. What did they do?

So, this magnet [muon] that wobbles about—it wobbled a little differently than they expected. This suggests that our theories of particle physics are missing something in the prediction of how much this muon should wobble [according to the Standard Model] vs. how much it did wobble.

Were they expecting to find this deviation, or did they honestly have no idea what would happen?

They were trying to see if they could confirm a previous deviation. There was an experiment at Brookhaven [National Laboratory] in the ’90s and early 2000s that saw a sign of this weird behavior of muons. But it wasn’t a strong enough signature of this to be really sure. So, the new experiment, the goal of it was to say, “Hey, was that a real thing that we saw earlier? Or was there some other explanation?” This was a souped-up version of the earlier experiment. They used the same magnet but improved all the other components of the experiment in various ways. And they’ll be able to measure it even better in coming years. They’ll be able to get a better handle on whether the muons really are behaving weird, or if there’s some other explanation.


The last time everyone got really excited about particle physics was a few years ago, when the Higgs boson was discovered. Does this have anything to do with that?

The Higgs boson is part of the Standard Model. In fact, people talked about it as the last missing piece of the Standard Model. The Standard Model is this theoretical mathematical model that explains how all the particles interact, but in order to make it work and explain how the particles in the standard model attain mass, you needed to have this other particle, the Higgs boson. So that’s why that was a big deal when it was discovered in 2012. It solidified the Standard Model picture that we had.


Just in time for it to get unsolidified!

It’s interesting. When they discovered the Higgs boson, of course it was very exciting and people were happy. But they also didn’t find anything that went beyond the Standard Model and a lot of people were hoping it would. I think some of the theoretical physicists felt a little let down that we didn’t find anything went beyond what we knew. So, if this new experiment is confirmed with more data, that could be the kind of thing that comes out of it.


OK, back to muons. What could be causing them to do the weird wobbly thing?

I’ll give you the general picture: Muons are constantly emitting and absorbing and interacting with other particles. There are these transient particles that exist in this weird quantum landscape that occurs. The strength of the muon’s magnet and the way it interacts with a magnetic field is not just a property of the muon itself, it has to do with how the muon interacts with all these other particles.


There could be particles out there that we don’t know about; they could be interacting with the muon and changing the strength of its magnet. So, this could be a sign that there’s a particle we haven’t identified out there that’s even weirder than the weirdest particles we know about it.

What does that have to do with upending the Standard Model?

The Standard Model is really useful for making really precise predictions. It explains all the crazy stuff we’ve seen at the Large Hadron Collider. It’s a really well-tested theory. But there are some things it doesn’t explain. It doesn’t explain dark matter—this matter that’s invisible but seems to be out there in the cosmos because you can see the gravitational effects of it on galaxies, but can’t be explained by any of the particles we know of in the standard model. And so, physicists are looking for something that’s a little bit beyond the Standard Model and they think there’s a deeper explanation that involves more particles or more forces.

The muon’s magnetism could provide a signature that points us to those. It could lead us to a theory that helps us better understand matter.

Correction, April 8: Due to a transcription error this article originally attributed a quotation to the discoverer of the muon that was said by another scientist.