Physicists fascinated by deepening the mystery of the magnetism of muonic particles

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Fermilab Muon g − 2 experiment uses this circular electromagnet to store muons, so that their magnetic moment can be measured with unprecedented precision.Credit: Brookhaven National Laboratory/SPL

The mysteries of the muon continue to leave physicists in suspense. Last year, an experiment suggested the elementary particle had inexplicably strong magnetism, possibly breaking a decades-long winning streak for the main theory in particle physics, known as the Standard Model. Now, calculations reviewed by several groups suggest that the theory’s prediction of muon magnetism may not be too far off from experimental measurements after all.

The new predictions are preliminary and do not fully justify the Standard Model. But by narrowing the gap between theory and experiment, they could make it easier to close the gap – while potentially creating another.

The muon is almost identical to the electron, except that it is 200 times heavier and short-lived, decaying millionths of a second after being created in particle collisions. Like the electron, the muon has a magnetic field that causes it to act like a tiny bar magnet. As muons travel, they generate various particles that briefly appear and disappear. These short-lived particles slightly increase the magnetism of the muon, called the magnetic moment. The big question is: by how much?

If the standard model already includes all the elementary particles of the Universe, it should be able to precisely quantify this additional magnetic contribution. But if the experiments prove that nature deviates from this prediction, they would point to the existence of hitherto unknown particles, whose fleeting appearances can distort the magnetic moment of the muon more than expected. Researchers have seen hints of such a discrepancy before and have spent decades trying to improve the accuracy of theory and experiments to confirm whether they give different results.

Conflicting results

In 2020, the theoretical physics community produced a consensus paper with the most accurate prediction yet for the magnetic moment of the muon1. This was largely based on calculations based on the fundamentals of the Standard Model, but researchers had to incorporate some experimental data to reflect the magnetic influence of particles such as quarks and gluons, which could not be adequately calculated. using only theory.

This calculation was soon joined by the most precise experimental measurement of the magnetic moment of the muon. In April 2021, the Muon g − 2 experiments at the Fermi National Accelerator Laboratory (Fermilab), outside of Chicago, Illinois, reported that the magnetic moment of the muon was significantly higher than the theoretical prediction2.

Yet on the same day, physicists from a collaboration called BMW unveiled separate calculations of the magnetic moment that did not require the assistance of experimental data. They used a technique called lattice quantum chromodynamics (lattice QCD) to simulate the behavior of quarks, gluons and other particles. This set the magnetic moment of the muon higher than the 2020 consensus document calculation, and closer to the muon g − 2 experimental value3.

Lattice QCD had not played a prominent role in the consensus document because at the time the technique’s predictions were not accurate enough. State-of-the-art mathematical techniques and supercomputing power then helped the BMW team give their networked QCD simulations enough momentum to kick into high gear. Since then, at least eight teams of physicists from around the world have raced to validate or improve BMW’s prediction. They started by focusing on a limited range of BMW-simulated particle energies.

Two preliminary results of this energy “window” were published on the arXiv preprint repository in April 2022: one by Christopher Aubin of Fordham University in New York and collaborators4and the other by Gen Wang at the University of Aix-Marseille in France5. Earlier this month, two other groups – one led by Hartmut Wittig of Johannes Gutenberg University in Mainz, Germany, the other by Silvano Simula of the National Institute of Nuclear Physics in Rome – announced their own window results at a muon conference in Los Angeles, CA. Simula’s group is writing a preprint and Wittig’s group submitted their preprint on June 146. All four calculations validated BMW’s own window results, even though their network techniques vary. “Very different ways of approaching the problem achieve a very similar result,” says Aubin.

New Consensus

“Over time, the different groups converge on a result that agrees with that of BMW, at least in the middle window,” says Davide Giusti, a physicist at the University of Regensburg, Germany, who is a former member of the collaboration of Simula, and who now works with another lattice-QCD group led by his colleague from Regensburg Christoph Lehner.

But the calculations are still preliminary, and could end up diverging once applied beyond the current window. “We don’t yet know if the lattice results from other collaborations agree with the BMW result for the other parts” of the calculation, says Aida El-Khadra, a theorist at the University of Illinois at Urbana-Champaign, who does part of another network-based QCD effort.

Moreover, the Muon g − 2 experimental result is always greater than the value calculated by lattice QCD, so it is too early to conclude that the standard model was correct all along. The Fermilab experiment plans to publish an updated value for the magnetic moment next year, but “even if the discrepancy between the theoretical prediction and the experiment turns out to be smaller – even if it doesn’t is only half – that would still be a big difference”. Witig said.

And if lattice QCD and experiments end up converging on the same value, physicists will still have to explain why the 2020 consensus document was so wrong, says Sven Heinemeyer, theoretical physicist at CERN, the European Laboratory for Particle Physics near from Geneva. , Swiss.

For now, physicists are scratching their heads. “It would be hard to believe that all of our network simulations were wrong,” says Aubin. But it’s also hard to imagine how calculations based on 2020 data could have gone wrong, he says.

Yet it is already clear that lattice QCD will have a significant impact on the question of muon magnetism, says Giusti. “This calculation is really exciting, and whatever the answer, it will be decisive.”

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