Physicists Mesmerized by Deepening the Mystery of Muon Particle Magnetism

Theoretical predictions move closer to experimental outcomes, but questions stay concerning possible gaps in the standard model of particle physics.

The muon’s mysteries still leave physicists mesmerized. Last year, an experiment suggested that the fundamental particle had inexplicably strong magnetism, possibly breaking a decades-long streak of triumphs for the leading theory of particle physics, known as the standard model. Currently, modified calculations by several groups suggest that the theory’s prediction of muon magnetism might not be very far from the experimental dimensions after all.

The new predictions are initial and do not entirely vindicate the standard version. However, by narrowing the space between theory and experiment, they might make it simpler to resolve the discrepancy- while potentially creating another one.

The muon is approximately identical to the electron, except that it is 200 times heavier and short-term, decaying millions of a second after being developed in particle collisions. Like the electron, the muon has a magnetic field making it act like a tiny bar magnet. As muons travel, they produce numerous particles that briefly pop in and out of presence. These ephemeral particles slightly increase the muon magnetism, referred to as its magnetic moment. The big question is: by how much?

If the standard model already consists of all of the universe’s fundamental particles, it should have the ability to precisely measure this additional magnetic contribution. However, if experiments confirm that nature deviates from that prediction, it would point to the existence of hitherto unknown particles, whose fleeting looks can skew the muon magnetic moment more than expected. Researchers have already seen tips of such a disparity and have spent decades attempting to boost the precision of both theory and experiments to validate whether they do provide distinct results.

Conflicting outcomes

In 2020, the theoretical-physics community created a consensus paper with the most accurate prediction yet for the muon’s magnetic moment. This mainly relied on calculations based on the basic principles of the standard model. However, the researchers were required to plug in some experimental data to show the magnetic impact of particles such as gluons and quarks, which could not be calculated adequately utilizing theory alone.

This calculation was soon joined by the most accurate experimental measurement of the muon’s magnetic moment. In April 2021, the Muon g– 2 experiment at the Fermi National Accelerator Laboratory (Fermilab), outside Chicago, Illinois, reported that the muon magnetic moment was considerably higher than the theoretical prediction.

Yet, on the same day, physicists in a collaboration called BMW unveiled separate calculations of the magnetic moment that did not call for the support of experimental data. They used a technique known as lattice quantum chromodynamics (lattice QCD) to simulate the behavior of gluons, quarks, and other particles. This pegged the muon magnetic moment higher than the calculation in the 2020 agreement paper and closer to the Muon g- 2 experimental value.

Lattice QCD had not played an essential part in the agreement paper due to the fact that, at that time, the method’s predictions were not precise enough. State-of-the-art mathematical methods and sheer supercomputing power subsequently assisted the BMW team in providing their lattice-QCD simulations sufficient of a boost to make the grade. Since then, a minimum of 8 teams of physicists worldwide have been racing to validate or improve on the BMW prediction. They have started by concentrating on a restricted range of the particle energies that BMW simulated.

Two initial results from this energy ‘window’ were published in April 2022 on the arXiv preprint repository: one by Gen Wang at the University of Aix-Marseille in France and the other at Fordham University in New York City by Christopher Aubin and his collaborators.

Previously this month, two more groups– one led at Johannes Gutenberg University in Mainz, Germany by Hartmut Wittig, and the other by Silvano Simula at the National Institute for Nuclear Physics in Rome– announced their own window results at a muon conference in Los Angeles, The Golden State. A Preprint is being written by Simula’s group, and Wittig’s group submitted its preprint on 14 June. All four calculations validated BMW’s own window outcomes, even though their lattice techniques differ. “Very distinct means of approaching the issue are obtaining a very similar outcome,” states Aubin.

New agreement

“As time passes, the distinct groups are converging on a result that agrees with BMW’s, a minimum of in the intermediate window,” states Physicist Davide Giusti in Germany at the University of Regensburg, who is a former member of Simula’s collaboration, and who currently works with another lattice-QCD group led by his Regensburg colleague Christoph Lehner.

However, the calculations are still preliminary and could end up diverging once they are used beyond the present window. “We do not yet know if lattice results from other collaborations agree with the BMW result for the other pieces” of the calculation, states Aida El-Khadra, a theorist at the University of Illinois at Urbana-Champaign, who is part of another lattice-QCD effort.

Additionally, the Muon g– 2 experimental outcome is still higher than the value determined by lattice QCD. So, it is early to conclude that the standard version was correct all along. The Fermilab experiment expects to post an updated value for the magnetic moment the following year; however, “even if the gap between theoretical prediction and experiment turns out to be smaller– even if it is only fifty percent as a lot– it would still be a large disparity,” Wittig states.

Moreover, if lattice QCD and experiments do end up converging on the same value, physicists would still need to describe why the 2020 consensus paper was so off the mark, says Sven Heinemeyer, a theoretical physicist at CERN, the European particle physics laboratory outside Geneva in Switzerland.

In the meantime, physicists are left scratching their heads. “It would be difficult to believe that all of our lattice simulations were wrong,” states Aubin. However, it is also difficult to imagine how the data-driven calculations from 2020 could have gone awry, he says.

Still, it is already clear that lattice QCD will have a significant influence on the muon magnetism question, states Giusti. “This calculation is truly exciting, and whatever the response is, it will be decisive.”

Read the original article on Nature.