What's this about a muon experiment potentially upending Standard Model of physics? We speak to one of the scientists involved

'It’s an exciting prospect, but too early to say so definitively' professor tells us


Physicists are this week giddy with excitement after a decade-long experiment looking at the inner-workings of a muon, a type of particle similar to the electron, hints that there may be another fundamental particle or force waiting to be discovered.

The Muon g-2 experiment, spun up at the US Department of Energy’s Fermi National Accelerator Laboratory in 2011, appears to support what researchers have suspected for a long time: the Standard Model of particle physics may be incomplete.

The Standard Model describes how the four fundamental forces in the universe – the electromagnetic, strong, weak, and gravitational* forces – function in terms of particle interactions. Physicists should be able to predict how particles behave and what properties they should have accurately with the model if it’s, indeed, correct.

But initial results from the experiment have found that the muon, a particle typically found in cosmic rays, is a tiny bit more magnetic than the model predicts.

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Like their electron cousins, muons have a spin and a magnetic moment. When the muons are placed in a strong magnetic field, they thus wobble around an axis and behave like a spinning top. The rate at which one wobbles, or precesses, is related to its magnetic moment.

Physicists can use observations of the wobbles to determine the muons' g-factor, which is the ratio between their magnetic moment and spin. The results from the Muon g-2 experiment show the g-factor is bigger than the Standard Model says it should be.

Some variations in the g-factor are expected, yet the deviation is more than the Standard Model predicts, which suggests something else unknown and possibly fundamental is in play.

Though the difference is tiny, it has significant consequences, David Kawall, a physics professor at the University of Massachusetts Amherst, who was involved in the experiment, explained to The Register.

“The precession rate can be predicted very accurately, knowing the electric charge and mass of the muon, and the strength of the storage magnetic field,” he said. “The interesting part is that the precession rate also depends on the interactions of the muon with the other particles of the Standard Model.

“The muon is never really alone in the vacuum, it interacts with a sea of virtual particles that appear and disappear out of existence. These interactions effectively change the strength of the muon’s internal magnet. Theorists can predict the influence of the known particles on muon’s magnetic properties. By carefully comparing the observed magnetic properties of the muon with the predicted properties, we can test whether the theory is complete - or whether something has been left out.”

So what's the big deal?

Since the strength of the muon’s magnetic field is larger than predicted, it suggests that it could be interacting with particles or a type of force physicists don’t yet know about.

At the moment, however, they’re reluctant to confirm this. “It’s an exciting prospect, but too early to say so definitively,” Prof Kawall told us.

It should be said that we know the Standard Model is incomplete. For instance we’re convinced of the existence of dark matter, but it doesn’t fit in the Standard Model. So we need a more complete theory

“It should be said that we know the Standard Model is incomplete. For instance we’re convinced of the existence of dark matter, but it doesn’t fit in the Standard Model. So we need a more complete theory, and such theories naturally tend to include new particles that could be causing the difference observed in the Fermilab result between prediction and experiment.”

For what it's worth, the experiment's team of 200 physicists at 35 academic institutions across seven countries has only analyzed six per cent of the results from the experiment. Specifically, they've analyzed the motion of about eight billion muons.

And the test is still running. It involves forming an energetic beam of muons by smashing protons together in an accelerator.

The beam of muons are then funneled into a storage ring made of a superconducting magnet measuring 50 feet across. The particles circulate and decay into combinations of particles, including positrons.

These positrons carry information about the muons, and by measuring the rate these positrons are produced as well as their energies, the researchers can measure how much the muons were wobbling. That value gives the team a vital piece of the puzzle to calculate the muons' g-factor.

An eight-minute video by the national lab describing the work is below, or you can catch an 75-minute scientific seminar on the findings here.

Youtube Video

What exactly is making a muon more magnetic than predicted is still a mystery the boffins are still trying to figure out.

“Ideally we’ll have a second result out within a year of so that should have uncertainties reduced, perhaps by a factor of two or so,” Prof Kawall said. "We’re still taking data, and reaching the target of a factor of four improvement over the Brookhaven result is still at least three years away. The good news is that the experiment is performing very well, we just need more data."

As for how useful this information is right now, the professor told us:

I think in practical terms it means very little in the short term. In the long term, it will become part of the march of science towards a better understanding of the universe. New theories of physics will have to be consistent with the measurement results. Some theories won’t survive, others will, and this winnowing process leads to better, more accurate theories of nature.

In the long term, these more accurate and complete theories of nature might have practical implications, though it’s hard to see that now.

"I think also there is something thrilling about the whole endeavor," he concluded, "that we can make predictions about the physical world with staggering accuracy, but a problem in the eighth decimal place could actually be an earthquake that requires a new paradigm." ®

* "Although not yet found, the 'graviton' should be the ... force-carrying particle of gravity", CERN says of the standard model


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