A novel assessment of muon magnetism has emerged, yet its implications remain uncertain.

 

Muons appear to exhibit unexpected behaviors, but the scientific community remains divided on what these behaviors signify.

Through meticulous analysis of how these subatomic particles deviate within a magnetic field, physicists have refined their understanding of muon's intrinsic magnetism to an unprecedented degree. The findings were shared on August 10 during a seminar hosted by Fermilab in Batavia, Illinois, as part of the Muon g−2 experiment.

Prior investigations into muons' magnetic properties have not aligned with the predictions of well-established scientific theories, particularly the standard model of particle physics. This model is revered for its accuracy and comprehensive testing in explaining subatomic particles and the fundamental forces governing them.

Numerous physicists have held onto the anticipation that the discrepancy observed in muon behavior might indicate a potential flaw within the enduring theory, opening avenues for a more profound comprehension of the cosmos. However, a series of recent unforeseen scientific findings have added complexity to the theoretical projection of the muon's minuscule magnetic strength. This complexity has made it increasingly challenging to determine whether the measurement signifies groundbreaking new physics or an existing prediction-related enigma that remains unresolved.

For a long time, measurements of muon magnetism have subtly indicated the potential existence of unidentified particles. Muons, akin to electrons within the particle family, possess roughly 200 times the mass. These transient particles behave as miniature magnets, each carrying its individual magnetic field. The intensity of this magnet is modulated by an intriguing quantum physics phenomenon. Empty space is perpetually bustling with particles that momentarily emerge before vanishing. These evanescent entities, termed "virtual" particles, wield tangible effects. They temporarily alter the strength of the muon's magnet, a modification that can be calculated within the bounds of the standard model.

The precise quantification of this alteration, denoted as the anomalous magnetic moment or "g−2" in the realm of physics equations, has perplexed scientists.

Intriguingly, particles that remain undiscovered by scientific exploration could potentially influence the measured value of g−2. Thus, the earlier indications of discord between predictions of the standard model and the observed outcomes have sparked significant interest among physicists.

Brynn MacCoy, a researcher on the Muon g−2 project and a physicist at the University of Washington in Seattle, elucidates, "The behavior of muons that we are measuring is influenced by all the forces and particles in the cosmos. It's essentially granting us a direct insight into the functioning of the universe."

The initial hint of a disparity between the forecasted and observed g−2 values emerged from an experiment at Brookhaven National Laboratory in Upton, N.Y., over twenty years ago. Subsequently, in 2021, the Muon g−2 experiment, situated at Fermilab, unveiled its inaugural outcomes, corroborating the incongruity.

At present, the Muon g−2 team has achieved twice the precision in an updated magnetism assessment, as disclosed in the Fermilab seminar and a paper released on August 10 via the Muon g−2 collaboration's website.

Physicist Carlos Wagner from the University of Chicago, who was not part of the experiment, commends, "To attain such a degree of precision is genuinely unprecedented and immensely impressive. I am utterly amazed." This latest measurement incorporates fourfold data compared to the previous one, alongside various enhancements that have bolstered accuracy.

Researchers aspire to compare this ascertained value with the standard model's projection. Yet, defining precisely what the standard model forecasts entails intricacies.

There exists a complex phase in the calculation of the g−2 value.

In 2020, after meticulous deliberation, a collective of theoretical physicists known as the Muon g−2 Theory Initiative reached a consensus prediction for comparison with measurements. However, subsequent to that, conflicting data from other experiments and theoretical computations have emerged, as elaborated in an announcement released on August 9 on the Muon g−2 Theory Initiative's website. This influx of information has introduced uncertainty into the prediction.

Tom Blum, a theoretical physicist from the University of Connecticut in Storrs, highlights the current challenge: "At this juncture, making a definitive comparison to ascertain whether the standard model concurs or diverges from the experimental data is unfeasible."

This uncertainty hinges on a particularly intricate facet of the g−2 calculation, referred to as the hadronic vacuum polarization. This term pertains to the adjustment brought about by a virtual photon emitted by the muon, which subsequently splits into a quark and its antimatter counterpart, an antiquark. Quarks are a fundamental particle category that constitute larger particles, including protons and neutrons. The quark and antiquark interact before mutually annihilating and transforming back into a virtual photon.

Two primary approaches have been devised for calculating this hadronic vacuum polarization factor. The traditional method entails incorporating specific experimental data into the calculation. These data stem from experiments that scrutinize the collisions between electrons and their antimatter counterparts, positrons, leading to the production of hadrons. The outcomes of such experiments are generally deemed to be well comprehended.

However, a recent experiment conducted at the VEPP-2000 particle collider in Novosibirsk, Russia, called CMD-3, has diverged from the outcomes of these other experiments. As reported in February on arXiv.org, researchers from CMD-3 presented findings that disagree with the results of those preceding experiments. If this particular outlier, CMD-3, is accurate, it could imply that the indications of disparity between muon measurements and the prediction might be less pronounced than initially perceived.

An alternative approach to estimating the intricate hadronic vacuum polarization term involves a technique known as lattice quantum chromodynamics. This methodology entails dividing spacetime into a mathematical grid to facilitate more manageable calculations. It has only recently become possible to achieve calculations with sufficient precision for meaningful comparisons.

In 2021, a group informally referred to as "BMW" published their computation of the contribution of the hadronic vacuum polarization in the journal Nature. This estimation indicated a closer alignment between the prediction and measurement of g−2 and conflicted with the data-driven method. However, this technique required verification. Subsequently, other scientists conducted their own lattice calculations to validate a portion of the BMW result. These independent teams arrived at similar outcomes to BMW, thus bolstering confidence in the lattice approach.

The focus has now shifted from scrutinizing the experimental measurement to investigating the discord among various theoretical methods.

"The experiment has provided its results," remarks theoretical physicist Thomas Teubner from the University of Liverpool in England, who is part of the Muon g−2 collaboration. He states that now, in order to determine whether muons conform to the standard model or challenge it, the onus lies on the theoretical physicists. "We need to reconcile our understanding," he asserts.