Will Fermilab muon experiment usher in new physics?

A new measurement of the muon’s anomalous magnetic moment at the US Department of Energy’s Fermi National Acceleratory Laboratory (Fermilab) challenges our current theoretical understanding of matter and forces in the universe known as the Standard Model of Particle Physics.

Known as the Muon g-2 (pronounced gee minus two) experiment, the Fermilab investigation has been looking for indications of new particles and new forces by examining how muons interact in a magnetic field.

Iterations of the Muon g-2 experiment have been running at labs around the world since they began at CERN in 1959.

The Fermilab Muon g-2 experiment began to create in 2018.

What are muons?

Subatomic particles can be either made up other particles (for example protons and neutrons made up of quarks), or are elementary, like electrons.Muons are subatomic particles sitting in the same group of particles as electrons known as leptons. Muons, however, are much heavier than the familiar electron – more than 200 times heavier, in fact.

There are two types of lepton: those with -1 charge (electrons, muons and tauons), and those with 0 charge (electron neutrino, muon neutrino and tau neutrino). All subatomic particles possess a quantum mechanical intrinsic spin.

In the charged leptons, this spin interacts with the particle’s charge to produce an inherent magnetism, like a bar magnet, in the particles.

In the presence of a magnetic field, this internal magnet’s direction precesses, or wobbles like a spinning top.

How fast the muon precesses in a magnetic field depends is the muon anomalous magnetic moment, represented by the letter g. Physics theory predicts g should equal 2 for a muon.

Deviations from this value can be attributed to “virtual” particles around the muon that pop in and out of existence as the muon interacts with the fundamental forces of nature.

As the muon intermingles with its virtual particle dance partners, the value of g changes for the muon. In theory, the Standard Model accounts for all these changes. Any anomalous measurements not explained by the Standard Model may suggest new particles or even a fifth unknown fundamental force of nature, perhaps related to enigmatic phenomena such as dark matter or dark energy.

What did Fermilab discover?

The power of the Fermilab results lies in its precision.

In 2021, Fermilab released its first results of the Muon g-2 experiment. Now, it has released an updated value of g which includes an additional two years of data and is more than twice as accurate,  with a precision of 0.20 parts per million.

That’s like measuring the straight-line distance between Adelaide and Melbourne and being only 13 centimetres off.

The result was announced in a press release and submitted to the Physical Review Letters.

“This measurement is an incredible experimental achievement,” says Peter Winter, co-spokesperson for the Muon g-2 collaboration. “Getting the systematic uncertainty down to this level is a big deal and is something we didn’t expect to achieve so soon.”

The previous most sensitive result for the muon anomalous magnetic moment was recorded in 2001 at the Brookhaven National Laboratory, New York. That experiment had a total error of 0.7 parts per million.

What does it mean?

The Brookhaven result deviated from theoretical predictions, suggesting new physics, more than 20 years ago.

The experiment was moved to Fermilab 10 years after those results were published in a bid to produce an even more accurate value.

Fermilab’s result did just that. What is more, it too deviates from theory by about 2 parts in a million.

Recently, however, physicists have suggested using computer simulations to predict the value of g – comparing these results with the latest experimental value sees the discrepancies all but vanish.

Whether or not the Fermilab results indicate new, unknown physics remains to be seen. All that may come out in the wash as the Muon g-2 experiment continues to run. The collaboration expects its uncertainties to reduce further to just 0.14 parts per million by the time the final results are published in 2025.