Muon Anomaly: How A 'Last Hope' Effort Revolutionizes Physics

Earlier this month, physicists at the Fermi National Accelerator Laboratory (Fermilab) near Chicago reported the results of the Muon g-2 experiment where they observed the wobbling of elementary particles known as muons - and this wobbling has far-reaching implications for future studies.

The observation revealed that muons - elementary particles similar to electrons, only about 200 times more massive - wobbled more than originally predicted as they whizzed around a magnetized ring. However, a series of computations released on the same day casts a different light on the findings.

Confirming a Former Anomaly

Almost twenty years ago, physicists observed a supposed anomaly in the behavior of elementary particles, sparking hope of a groundbreaking discovery. An experimental measurement attempt was conducted at Brookhaven National Laboratory in 2004, providing a value for the muon magnetic moment that was considered anomalous in the context of the Standard Model of Particle Physics.

Fast forward sixteen years later, an international team of more than 170 physicists co-published what was tagged as "the most reliable prediction so far" for the theoretical value of the anomalous muon magnetic moment. However, the consensus value adherent to the Standard Model significantly varies with the 2004 Brookhaven observation.

Now, with the new Muon g-2 results at Fermilab, researchers now confirm that the wobbling of muons - their magnetic moment - aligns with the 2004 Brookhaven experiment and differs from the 2020 theoretical consensus.

Basically, the latest Muon g-2 findings suggest that there are still undiscovered particles that give muons this extra moment. Furthermore, additional confirmation of this muon behavior could eventually break down the Standard Model of Particle Physics, if not give birth to new physics. The Standard Model - which describes three of the known fundamental forces in the Universe, except for gravity - has been largely accepted as theoretically self-consistent for more than fifty years.

Slimming Chances of a New Physics

Adrian Cho, writing for the journal Science, also notes a concurrent report from the Budapest-Marseille-Wuppertal (BMW) collaboration contradicting the prevailing theoretical prediction with another set of calculations. Using a modern technique called "quantum chromodynamics," BMW physicists were able to calculate a value closer to the experimental results and effectively create a solid bridge between experimental and theoretical estimates on the muon magnetic moment. Researchers of the BMW collaboration published their findings in the journal Nature, in the article "Leading hadronic contribution to the muon magnetic moment from lattice QCD."

The perceived agreement between the latest Muon g-2 experimental results and the BMW theoretical values strongly suggests that the behavior of muon is exactly what is predicted by the Standard Model, lending credence to the prevailing particle physics model and dimming the chances of disproving or breaking it down.

In an article from Quanta Magazine, Aida El-Khadra, one of the co-organizers of the Theory Initiative behind the 2020 consensus and a particle theories with the University of Illinois, explains that the BMW computation should also be taken seriously. She notes that this approach was not integrated into the consensus because at the time, it still needed "vetting." Once the BMW calculation - originally posted online on the preprint repository arXiv last February 2020 - is independently verified by other research teams, it will be included by the Theory Initiative in the next assessment of the "anomalous" muon wobbling.

Dominik Stöckinger, who is a part of the Theory Initiative and the Fermilab Muon g-2 team, said that the BMW calculations create "an unclear status." He adds in the Quanta article that if the anomaly eventually disappears, it would be "the end of particle physics" as feared by some members of the particle physics community. Stöckinger called the muon g-2 experiment as their "last hope" of finding physics beyond the Standard Model, and that should it fail, many researchers might feel the need to "give up" and "do something else," himself included.

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