The muon, an elementary particle similar to the electron but significantly heavier, has been at the center of a physics puzzle for two decades. Discrepancies between experimental measurements of its anomalous magnetic moment (g-2) and theoretical predictions based on the Standard Model of particle physics hinted at the possibility of undiscovered particles or forces. However, new research from the BMW Collaboration suggests the gap between theory and experiment might be smaller than previously thought, potentially diminishing the prospect of new physics lurking in the muon’s behavior.
The BMW Collaboration’s latest findings, currently available on the arXiv preprint server, build upon their previous work published in Nature in 2021. Using advanced lattice quantum chromodynamics (QCD) simulations on finer lattices, the team achieved a more precise calculation of the muon’s g-2. Their results show an anomalous magnetic moment that deviates from the experimental average by only 0.9 standard deviations, significantly narrowing the previously observed discrepancy.
The g-2 storage-ring magnet at Fermilab.The g-2 storage-ring magnet at Fermilab. Photo: Reidar Hahn / Wikimedia Commons
Understanding the Muon’s Magnetic Anomaly
The muon’s g-2 describes the quantum mechanical contribution to its wobble in a magnetic field. For years, experimental measurements of this property have differed from predictions made by the Standard Model, leading physicists to speculate about the existence of new particles or forces influencing the muon’s behavior. This discrepancy spurred significant research efforts, including both theoretical calculations and experimental measurements.
The BMW Collaboration’s approach differs from traditional experimental methods that rely on particle collisions. Instead, they utilize large-scale lattice QCD simulations, essentially creating a gridded spacetime and simulating QCD interactions. According to study co-author Zoltan Fodor, a theoretical particle physicist at the University of California, San Diego, this method requires no experimental input, relying solely on the principles of QCD. The team’s findings suggest that the apparent mismatch between the predicted and measured anomalous magnetic moment of the muon might not be as significant as previously believed.
From CERN to Fermilab: A History of Muon g-2 Measurements
Early measurements of the muon’s anomalous magnetic moment at CERN in the 1960s lacked precision. In 2006, the E821 experiment at Brookhaven National Laboratory provided more refined measurements, revealing a discrepancy with the Standard Model exceeding two standard deviations. Subsequent calculations further widened this gap to more than three standard deviations.
This discrepancy spurred further investigations. The 2021 results from the Muon g-2 Collaboration at Fermilab showed a 4.2 standard deviation difference, seemingly strengthening the case for new physics. However, subsequent experimental results from the CMD-3 accelerator in Russia suggested a smaller discrepancy, creating a more complex picture.
Muon g-2 experiment ringMuon g-2 Experiment Ring. Photo: Reidar Hahn / Wikimedia Commons
The Quest for New Physics and the Future of Muon Research
Andreas Crivellin, a theoretical physicist at the University of Zurich and the Paul Scherrer Institute, notes that explaining the muon’s g-2 with new physics isn’t straightforward. He emphasizes the difficulty in constructing theoretical models that generate a sufficiently large effect to account for the observed discrepancies. The five sigma threshold, often considered the gold standard for claiming a discovery in particle physics, remains elusive in the case of the muon’s g-2.
While the BMW Collaboration’s findings and the CMD-3 results appear to lessen the likelihood of new physics explaining the muon’s g-2, the story isn’t over. Muon colliders, proposed as a powerful tool for probing the muon’s properties, are gaining momentum. Recent research on muon beams has advanced the feasibility of these future colliders. Additionally, the Fermilab Muon g-2 experiment is expected to release its final results next year, potentially adding another crucial piece to this ongoing puzzle.
Conclusion: The Muon’s Mystery Continues
The muon’s anomalous magnetic moment continues to be a subject of intense scrutiny. While the latest theoretical calculations from the BMW Collaboration suggest a closer alignment with the Standard Model, the quest for a definitive answer persists. Future experimental results, including the final data from Fermilab, and the development of muon colliders promise further insights into this enigmatic particle and its potential to reveal new frontiers in physics.
