The standard model of particle physics is our best explanation for how the building blocks of the universe interact, but it doesn’t explain everything. That’s where Fermilab’s Brendan Kiburg and Brendan Casey come in. They’re part of a team of 200 scientists from 33 institutions around the world who have been observing the magnetic wobble of a subatomic particle known as a muon, using a powerful magnet at Fermilab. This ongoing experiment, known as Muon g−2, involves the acceleration of muons around a superconducting “ring” a thousand times at nearly the speed of light. In August, Fermilab announced that the team had measured the muon’s wobble with higher precision than ever before. The results confirm with even greater confidence that this wobble is faster than the standard model says is possible.

What are the implications of that exactly? Does this upend our understanding of the universe? Are physicists on the verge of discovering a new particle, or perhaps a fifth force of nature, or even a new dimension? The tantalizing answer: Scientists aren’t sure, at least not yet. But Kiburg and Casey are among those trying to find out.

Some have speculated that Fermilab’s muon results could indicate a fifth force of nature. What are you finding?

CASEY: When you talk to high-energy physicists and say “fifth force,” it means something very specific. In the ’80s, people discovered something they thought meant gravity was breaking down, and they called that the “fifth force.” This could really be anything. It could be a new particle, it could be a new interaction, it could be a new force, it could be an extra dimension. It could be modifications to space-time itself.

KIBURG: The cool thing about the muon is that it’s sensitive to all these particles popping in and out of existence. So as you do this more and more precisely, you can see the effects of particles we know about. And what we’re hunting for is an effect where some particle that we don’t know about is slightly tickling the muon and making it precess, or spin, a little faster in that magnetic field. So as experimentalists, our hypothesis is that there might be some new particles out there, and if we do this measurement really precisely, we might be able to see them.

“This could be a new particle, a new interaction, a new force, an extra dimension. It could be modifications to space-time itself.”

Brendan Casey, senior scientist at Fermilab

You are using a storage-ring magnet originally built for similar experiments at Brookhaven National Laboratory in New York — the results of which were released in 2001. What’s different about Fermilab’s experiment?

KIBURG: The collaboration recognized that we had infrastructure at Fermilab to put more muons in and increase the number of times we perform this measurement by a large factor. So we brought the magnet here in 2013. Brookhaven’s result motivated taking a closer look because it was discrepant with the theoretical prediction by about 3.5 standard deviations. Our threshold for stating that we’ve discovered something in science is to look for five standard deviations, which means this is very unlikely to happen just by chance. So either it’s evidence of something new or there’s some sort of hidden mistake in your system that nobody could figure out was there. Our experimental result was consistent with Brookhaven’s experimental result, and that gave us confidence that the experimental value was indeed larger than what the theory was predicting. And as we take more and more data, analyze more and more data, the error bars continue to shrink. We have about 70 percent of our data, and the error bars are gonna go down again by roughly a factor of two the rest of the way.

If the muons are indeed doing what you think they are, that would be a big deal, wouldn’t it?

CASEY: I would say so. We’re at a very big branch point in the field right now because the LHC [Large Hadron Collider] has not made any discoveries besides the Higgs [boson, an elemental particle identified a decade ago]. And so we have to decide what we’re doing next [in high-energy physics]. And what we learned from this experiment will go a very long way in informing that. If we could say something very conclusively, that would be a huge thing. Unfortunately, right now we can’t.

What are the challenges of communicating the implications to the public?

CASEY: You need a very good prediction, and you need a very good measurement. And if the prediction didn’t have problems right now, we would be making a much bigger deal out of this. We’d be making an enormous deal out of this. Now what we know is that if we get the prediction in a little bit better shape, then there’s potential — there’s a huge amount of potential here. We are trying not to oversell that at this moment. G−2 [the standard model’s theoretical value of the wobble] is something we used as a tool to understand physics for over a hundred years now. So we’re at this moment where things haven’t exactly lined up, but nobody thinks it’s gonna stay that way. You’d have to be extremely pessimistic based on the history to think, “OK, we’re now stuck. We’re not gonna be able to move forward from where we are now.” And none of us are pessimists.

KIBURG: The other really cool aspect about the g−2 experiment is that it marries a bunch of different niches within the physics community. Typically the people that build the detectors are looking at flashes of light and crystals or the tickling of gases and straws — that’s one sort of skill set. Then there are people who really focus on electronics. And there are people who focus more on computing. We also have a large community that usually focuses on tabletop experiments that you might do with three or four people that are ultraprecise. But this experiment marries them all, and they’re all needed to get this one number that tells us secrets about the universe.