Muon g-2 Experiment Challenges Standard Model with Landmark Discovery

On June 3, 2025, the Muon g-2 collaboration at Fermilab announced the final results of their extensive six-year experiment, confirming that muons exhibit behavior deviating from the Standard Model of particle physics. This landmark finding suggests the potential existence of new physics beyond our current understanding.

The experiment involved measuring the precession, or "wobble," of muons—subatomic particles similar to electrons but approximately 200 times more massive—as they traveled through a magnetic field. The observed precession rate differed from theoretical predictions, indicating that the Standard Model may be incomplete and pointing toward undiscovered particles or forces.

The Muon g-2 experiment aimed to measure the anomalous magnetic dipole moment of the muon with high precision. Muons, like electrons, act as tiny magnets. The "g-factor" indicates the strength of this magnet and its rate of gyration in an external magnetic field. The experiment sought to determine if the observed g-factor aligns with Standard Model predictions.

Previous experiments at CERN (1959-1979) and Brookhaven National Laboratory (1997-2001) provided measurements that hinted at discrepancies between experimental values and Standard Model predictions. Fermilab's Muon g-2 experiment, initiated in 2013, aimed to achieve higher precision to investigate these anomalies.

The experiment utilized a 50-foot-diameter superconducting magnetic storage ring, originally from Brookhaven, transported to Fermilab in 2013. Muons were injected into this ring, and their precession in the magnetic field was measured using electromagnetic calorimetric detectors. The goal was to achieve a precision of 0.14 parts per million (ppm). The final results surpassed this benchmark, reporting a precision of 0.127 ppm.

The experiment observed a precession rate differing from theoretical predictions, suggesting potential new physics beyond the Standard Model. The results suggest the existence of undiscovered particles or forces. Theoretical explanations include supersymmetry, which posits partner particles for each known particle, and the existence of new heavy or light particles interacting with muons.

Graziano Venanzoni, co-spokesperson of the Muon g-2 experiment and physicist at the Italian National Institute for Nuclear Physics, remarked:

"Today is an extraordinary day, long awaited not only by us but by the whole international physics community."

Marco Incagli from the National Institute for Nuclear Physics in Italy stated:

"This measurement will remain a benchmark ... for many years to come."

The confirmed deviation of the muon's behavior from Standard Model predictions has profound implications, suggesting the existence of undiscovered particles or forces. New particles could influence early universe dynamics, potentially affecting the rate of cosmic expansion and offering insights into dark matter.

The final results from Fermilab's Muon g-2 experiment represent a significant milestone in particle physics, challenging the completeness of the Standard Model and opening avenues for new physics. As theoretical and experimental efforts continue, the scientific community stands on the brink of potentially groundbreaking discoveries that could reshape our understanding of the universe.