Some particle decay measurements show tiny deviations from Standard Model predictions. Could be new physics or systematic errors, experiments ongoing.
Reidar Hahn, CC BY-SA 4.0, via Wikimedia Commons
The muon's magnetic puzzle
Deep within the suburban area of Batavia, Illinois, an experiment is measuring a fundamental property of a particle called the muon. Muons are similar to electrons but about 200 times more massive. They have a quantum property called spin, which makes them behave like tiny internal magnets. When placed in a magnetic field, this internal magnet "wobbles," or precesses, like a spinning top. The speed of this wobble is determined by a number called the g-factor.
According to P.A.M. Dirac's theory of relativistic quantum mechanics from the 1920s, the g-factor for a fundamental particle like the muon should be exactly 2. However, the vacuum of space is not empty; it fizzes with "virtual" particles that pop in and out of existence. These fleeting particles interact with the muon and alter its magnetic moment, causing the g-factor to deviate slightly from 2. The size of this deviation, called the anomalous magnetic moment, can be predicted with extreme precision by the Standard Model of particle physics.
The Muon g-2 experiment at Fermilab measures this value by sending a beam of muons into a 15-meter (50-foot) diameter superconducting storage ring. As the muons race around the ring at nearly the speed of light, their wobble is meticulously recorded by detectors. An earlier version of this experiment at Brookhaven National Laboratory (BNL) in New York, which finished in 2001, found a slight disagreement with the theoretical prediction. To improve the precision, the huge electromagnet was transported 3,200 miles from BNL to Fermilab. After years of upgrades, the new experiment began taking data in 2017. On August 10, 2023, the collaboration released its analysis of the first three years of data, confirming the earlier discrepancy. The combined experimental result now differs from the primary theoretical prediction by a statistical significance of 5.1 sigma. In particle physics, a 5-sigma result is the conventional threshold to claim a discovery.
Other cracks in the model
The muon's wobble is not the only hint of physics beyond the Standard Model. For years, another set of anomalies appeared in the decays of particles called B-mesons at the Large Hadron Collider beauty (LHCb) experiment at CERN. The Standard Model predicts that certain B-mesons should decay into muons and electrons at virtually identical rates. However, early data suggested these decays happened at different rates, a potential violation of a principle called lepton universality.
One particular decay, B⁰→K*μ⁺μ⁻, showed a persistent tension with predictions. By 2022, some of these B-meson measurements showed deviations from the Standard Model at a significance of around 4 sigma. This suggested a 1-in-16,000 chance that the result was simply a statistical fluctuation.
The scientific process is rigorous. As the LHCb collaboration collected and analyzed more data, the B-meson anomaly began to shrink. The latest analyses, incorporating much larger datasets from LHC Run 2 (2016-2018), now show results consistent with the Standard Model's predictions. While this specific anomaly has faded, the persistent and growing significance of the Muon g-2 result at Fermilab keeps physicists searching for an explanation. The discrepancy could be evidence of undiscovered particles or forces, or it could point to subtle, unaccounted-for issues in the incredibly complex theoretical calculations.
💡Fun Facts
To move the 15-meter-wide electromagnet from Brookhaven, NY to Illinois, it was primarily transported on a barge, traveling down the East Coast, around Florida, and up the Mississippi and Illinois rivers.
The magnetic field inside the Muon g-2 storage ring is about 1.45 Tesla, which is approximately 30,000 times stronger than the Earth's magnetic field.
The term "g-factor" was introduced in 1921 by German physicist Alfred Landé to describe unexpected splitting patterns in the spectra of atoms in a magnetic field, years before the concept of electron spin was fully developed.
The Standard Model prediction for the muon's anomalous magnetic moment is one of the most precisely calculated quantities in all of science, involving complex quantum loop calculations.
The outdoor areas of the Fermilab site are open daily from dawn to dusk. The Lederman Science Center and parts of Wilson Hall have more restricted public hours, typically Monday-Friday during business hours.
Admission
Free for self-guided tours of public areas.
Accessibility
The Fermilab site is a large, 6,800-acre campus. Main buildings like the Lederman Science Center and Wilson Hall atrium are accessible. Outdoor areas include paved paths and natural trails.
