Muons
Muons are fundamental particles belonging to the lepton family, similar to electrons but approximately 207 times more massive. They are produced when cosmic rays collide with Earth’s atmosphere and decay into electrons and neutrinos with a mean lifetime of about 2.2 microseconds. Despite their brief existence, muons penetrate deep into Earth’s atmosphere and reach the surface, making them detectable in particle physics experiments.
Magnetic Properties and the g-2 Measurement
A key property of muons is their anomalous magnetic moment, often expressed as the g-factor or more commonly as the quantity g-2, which measures the difference between the muon’s observed magnetic moment and its theoretical prediction. This measurement is highly sensitive to unknown particles and interactions that could affect the muon’s behavior. Precisely determining g-2 therefore serves as a test of the Standard Model of particle physics and can reveal discrepancies that might point to new physics.
The Fermilab Experiment
The Muon g-2 experiment at Fermilab measures the muon’s anomalous magnetic moment using a technique in which muons circulate in a strong magnetic field. The experiment builds on decades of earlier measurements and aims to achieve unprecedented precision. Results from data collected in 2021 and subsequent runs are expected in 2025, which may either confirm existing measurements or reveal tensions with theoretical predictions that could indicate physics beyond the Standard Model.