The 2026 Breakthrough Prize in Fundamental Physics has been awarded to the “Muon g-2” collaborations, which over the years have worked on experiments at CERN, Brookhaven Laboratory and Fermilab, on an extremely high-precision measurement of a particular property of the muon: the so-called magnetic anomaly. In carrying out these sophisticated measurements, Italy, through INFN Italian National Institute for Nuclear Physics, has also played a significant role, in particular in the Muon g-2 experiment at Fermilab, which it has co-coordinated from 2020 to the present day.
“The Breakthrough Prize in Fundamental Physics this year has been awarded to three generations of the Muon g-2 experiment: the CERN experiment of the 1970s, the Brookhaven experiment in 2000, and finally the Fermilab experiment, which published its final results in June 2025, helping to establish what will remain for years the most precise measurement of the muon’s magnetic moment”, explains Marco Incagli, researcher at the INFN Pisa division who coordinates the Muon g-2 collaboration.
2025 video – “Final Muon g-2 Measurement at Fermilab”
Muons and the “g factor”
Muons, the protagonists of the Muon g-2 experiment, are fundamental particles similar to electrons, but about 200 times more massive. Like electrons, they possess the quantum property called “spin”, which makes them similar to tiny magnets: in the presence of an external magnetic field, they undergo a rotational motion known as precession, comparable to that of a spinning top tilted with respect to a vertical axis. The speed of precession in a magnetic field depends on the properties of the muon, described by a number called the “g factor”, to which nearly 100 years ago theoretical physicists assigned a value equal to 2 on the basis of what is described by the Standard Model of elementary particles. Very soon, however, experimental measurements showed that g deviates slightly from 2 (it is just a little larger), due to a quantity known as the muon magnetic anomaly (aμ), calculated as (g−2)/2. Measuring this anomaly with extremely high precision is the goal of the Muon g-2 experiment, which in fact takes its name from the formula (g−2)/2.
The effects of all the particles of the Standard Model depend on the muon magnetic anomaly, and a discrepancy between experiment and theory, such as that observed in the past, could indicate the presence of physical processes not foreseen, signalling the need to go beyond the Standard Model.
The Muon g-2 collaboration is made up of 176 scientists from 34 institutions in 7 countries. The Italian INFN group has taken an active part in the experiment since its very beginning, also holding leading roles, and has contributed on various fronts to its success, designing and building both a complex laser system for the absolute calibration of the calorimeters used in energy measurements, and a high-sensitivity optical magnetometer for measuring magnetic transients. The use of both these systems has made it possible to significantly reduce the overall uncertainty on the measurement of the muon magnetic anomaly. Italian researchers have also taken part extensively in the considerable data analysis effort that was necessary to obtain the final result.
The Italian institutions participating in the collaboration are the National Institute for Nuclear Physics, with its divisions of Pisa, Trieste, Rome Tor Vergata and Naples, and the National Laboratories of Frascati; the University, the Scuola Normale Superiore and INO-CNR in Pisa; the Universities of Trieste, Udine, Rome Tor Vergata, Naples and Molise.
