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How a tiny, wobbling particle could unlock mysteries of the universe

The results of a new muon experiment are stirring up particle physics.

It’s an exciting time in particle physics. The results of a new experiment out of Fermilab in Illinois — involving a subatomic particle wobbling weirdly — could lead to new ways of understanding our universe.

To understand why physicists are so excited, consider the ambitious task they’ve set for themselves: decoding the fundamental building blocks of everything in the universe. For decades, they’ve been trying to do that by building a big, overarching theory known as the standard model.

The standard model is like a glossary, describing all the building blocks of the universe that we’ve found so far: subatomic particles like electrons, neutrinos, and quarks that make up everything around us, and three of the four fundamental forces (electromagnetic, weak, and strong) that hold things together.

But, as Jessica Esquivel, a particle physicist at Fermilab, tells Vox, scientists suspect this model is incomplete.

“One of the big reasons why we know it’s incomplete is because of gravity. We know it exists because apples fall from trees and I’m not floating off my seat,” Esquivel says. But they haven’t yet found a fundamental particle that conveys gravity’s force, so it’s not in the standard model.

Esquivel says the model also doesn’t explain two of the biggest mysteries in the universe: dark matter, an elusive substance that holds galaxies together, and dark energy, an even more poorly understood force that is accelerating the universe’s expansion. And since the overwhelming majority of the universe might be made up of dark matter and dark energy, that’s a pretty big oversight.

The problem is, the standard model works really well on its own. It describes the matter and energy we’re most familiar with, and how it all works together, superbly. Yet, as physicists have tried to expand the model to account for gravity, dark matter, and dark energy, they’ve always come up short.

That’s why Esquivel and the many other particle physicists we’ve spoken to are so excited about the results of a new experiment at Fermilab. It involves muons — subatomic particles that are like electrons’ heavier, less stable cousins. This experiment might, finally, have confirmed a crack in the standard model for particle physicists to explore. It’s possible that crack could lead them to find new, fundamental building blocks of nature.

Esquivel worked on the experiment, so we asked her to walk us through it for the Unexplainable podcast. What follows is a transcript of that conversation, edited for clarity and length.

Noam Hassenfeld

What was this muon experiment?

Jessica Esquivel

So at Fermilab, we can create particle beams of muons — a very, very intense beam. You can imagine it like a laser beam of particles. And we shoot them into detectors. And then by taking a super, super close measurement of those muons, we can use that as kind of a probe into physics beyond our standard model.

Noam Hassenfeld

So how, exactly, does this muon experiment point to a hole in the model, or to a new particle to fill that gap?

Jessica Esquivel

So the muon g-2 experiment is actually taking a very precise measurement of this thing that we call the precession frequency. And what that means is that we shoot a whole bunch of muons into a very, very precise magnetic field and we watch them dance.

Noam Hassenfeld

They dance?

Jessica Esquivel

Yeah! When muons go into a magnetic field, they precess, or they spin like a spinning top.

One of the really weird quantum-y, sci-fi things that happens is that when you are in a vacuum or an empty space, it actually isn’t empty. It’s filled with this roiling, bubbling sea of virtual particles that just pop in and out of existence whenever they want, spontaneously. So when we shoot muons into this vacuum, there are not just muons going around our magnet. These virtual particles are popping in and out and changing how the muon wobbles.

Noam Hassenfeld

Wait, sorry ... what exactly are these virtual particles popping in and out?

Jessica Esquivel

So, virtual particles, I ... see them as like ghosts of actual particles. We have photons that kind of pop in and out and they’re just kind of like there, but not really there. I think a really good depiction of this, the weirdness of quantum mechanics, is Ant-Man. There’s this scene where he shrinks down to the quantum realm, and he gets stuck and everything is kind of like wibbly-wobbling and something’s there, but it’s really not there.

That’s kind of like what virtual particles are. It’s just hints of particles that we’re used to seeing. But they’re not actually there. They just pop in and out and mess with things.

Noam Hassenfeld

So quantum mechanics says that there are virtual particles, sort of like ghosts of particles we already know about in our standard model, popping in and out of existence. And they’re bumping into muons and making them wobble?

Jessica Esquivel

Yes. But again, theoretical physicists know this, and they’ve come up with a really good theory of how the muon will change with regards to which particles are popping in and out. So we know specifically how every single one of these particles interacts with each other and within the magnetic field, and they build their theories based on what we already know — what is in the standard model.

Noam Hassenfeld

Got it. So even though there are these virtual ghost particles popping in and out, as long as they’re versions of particles we know, then physicists can predict exactly how the muons are going to wobble. So were the predictions off?

Jessica Esquivel

So what we just unveiled is that precise measurement doesn’t align with the theoretical predictions of how the muons are supposed to wobble in a magnetic field. It wobbled differently.

Noam Hassenfeld

And the idea is that you have no idea what’s making it do that extra wobble, so it might be something that hasn’t been discovered yet? Something outside the standard model?

Jessica Esquivel

Yeah, exactly. It’s not considered new physics yet because we as physicists give ourselves a very high bar to reach before we say something is potentially new physics. And that’s 5 sigma [a measure of the probability that this finding wasn’t a statistical error or a random accident.] And right now, we’re at 4.2 sigma. But it’s pretty exciting.

Noam Hassenfeld

So if it clears that bar, would this break the standard model? Because I’ve seen that framing in a bunch of headlines.

Jessica Esquivel

No, I don’t think I would say the standard model is broken. I mean, we’ve known for a long time that it’s missing stuff. So it’s not that what’s there doesn’t work as it’s supposed to work.

It’s just that we’re adding more stuff to the standard model, potentially. Just like back in the day when scientists were adding more elements to the periodic table ... even back then, they had spots where they knew an element should go, but they hadn’t been able to see it yet. That’s essentially where we’re at now. We know we have the standard model, but we’re missing things. So we have holes that we’re trying to fill.

Noam Hassenfeld

How exciting does all of this feel?

Jessica Esquivel

I think it’s like a career-defining moment. It’s a once-in-a-lifetime. We’re chasing new physics and we’re so close, we can taste it.

What I’m studying isn’t in any textbook that I’ve read or peeked through before, and the fact that the work that I’m doing could potentially be in textbooks in the future ... that people can be learning about the dark matter particle that g-2 had a role in finding ... it gives me chills just thinking about it!

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