Scientists from the Muon g-2 experiment, hosted at the U.S. Department of Energy’s Fermi National Accelerator Laboratory and involving a broad international collaboration, including numerous researchers from Italy’s National Institute for Nuclear Physics (INFN), have announced their third and final measurement of the muon’s anomalous magnetic moment. This result is consistent with those published in 2021 and 2023 but achieves significantly greater precision: 127 parts per billion, surpassing the original experimental design goal of 140 parts per billion.

Muons, the focus of the Muon g-2 experiment, are fundamental particles similar to electrons but approximately 200 times more massive. Like electrons, they possess a quantum property called ‘spin,’ which endows them with a magnetic moment, making them behave like tiny magnets. When exposed to an external magnetic field, muons undergo a rotational motion known as precession, akin to the wobble of a spinning top tilted relative to a vertical axis. The precession frequency in a magnetic field depends on the muon’s properties, described by a number called the ‘g-factor.’ Nearly a century ago, theoretical physicists predicted a g-factor value of 2 based on the Standard Model of particle physics. However, experimental measurements soon revealed that g slightly exceeds 2 due to a quantity known as the muon’s anomalous magnetic moment (a_μ), calculated as (g−2)/2.

Measuring this anomaly with the highest possible precision is the objective of the Muon g-2 experiment, which derives its name from the formula (g−2)/2.

The Muon g-2 collaboration comprises 179 scientists from 37 institutions across seven countries. The Italian INFN group has been actively involved since the experiment’s inception, playing leading roles and contributing significantly to its success. They designed and implemented two systems that substantially reduced the overall uncertainty in measuring the muon’s anomalous magnetic moment: an absolute laser calibration system for the calorimeters used in energy measurements, and a high-sensitivity optical magnetometer for detecting magnetic transients. Additionally, they played a crucial role in the extensive data analysis efforts leading to the final result.

‘Thanks to the deployment and synergy of diverse expertise, from optics and laser specialists to computing and data analysis experts, the Italian INFN group was critically important to the success of the measurement,’ concludes Giovanni Cantatore, physicist at UniTS and the INFN Trieste Section, and leader of the Italian Muon g-2 team.

The muon’s anomalous magnetic moment is influenced by all particles within the Standard Model. A discrepancy between experimental results and theoretical predictions, as observed in the past, could indicate the presence of physical processes not accounted for in the current theoretical framework, suggesting the need to revise or even extend the Standard Model. Accordingly, the international Muon g-2 Theory Initiative has worked in parallel with the experimental group to refine theoretical calculations. In addition to techniques based on data from various experiments, which previously yielded values in tension with those from Fermilab, a computational approach leveraging high-performance computing has recently been adopted. This method has produced a theoretical prediction closer to the experimental measurement, though not yet fully aligned with it.

While the primary analysis of the Muon g-2 experiment has concluded, the extensive dataset collected over the past six years offers opportunities for further exploration. In the future, the collaboration plans to measure another muon property known as the electric dipole moment and to test a fundamental symmetry in physical laws known as CPT symmetry (charge, parity, and time reversal).