A new measurement of the magnetic anomaly 𝑎𝜇≡(𝑔𝜇−2)/2 of the positive muon has been obtained by the Muon 𝑔 −2 Experiment at Fermi National Accelerator Laboratory (FNAL). This result is based on the third and final dataset (years 2020 to 2023) of the experiment, which contains over 2.5 times the total statistics of the previous results.
The precise determination of the magnetic anomaly 𝑎𝜇 has long been awaited as it’s a powerful indicator for possible cracks in the Standard Model, a discrepancy between theory and experiment giving access to fundamental “new physics”.
From the ratio of the precession frequencies for muons and protons in a storage ring magnetic field, together with precisely known ratios of fundamental constants, the value 𝑎𝜇=1 165 920 710⁢(162)×10−12 (139 ppb) was determined for the new dataset, and 𝑎𝜇=1 165 920 705⁢(148)×10−12 (127 ppb) when combined with the previous results of the 2021 and 2023 publications. Altogether, these measurements at FNAL have improved the precision on the world average by over a factor of 4 and will remain the most precise determination of 𝑎𝜇 for many years to come.
This precise determination of 𝑎𝜇 has boosted also the efforts on the extremely complex calculations of the theoretical value of the muon anomaly.
The Muon 𝑔 −2 collaboration counts about 176 scientists from 34 institutions in 7 countries. The Italian group, with scientists from INFN, universities, Scuola Normale and CNR, has contributed on both the experiment and the data analysis. On the experimental side, the Italian group has conceived, built and maintained a laser-based gain monitoring system that continuously calibrated the nearly 1300 silicon photomultipliers (SIPM) of the 24 positron detectors for almost 10 years of data taking. This system significantly contributed reducing the systematic error budget. Ino-Pisa researchers C. Ferrari, A. Fioretti and C. Gabbanini were central to the development of this very peculiar laser system.
These latest results were recently published in Phys. Rev. Lett. 135, 101802, as Editor’s suggestion, featured in Physics, and discussed in Physics Research news of the same day. In the picture, PRL’s issue cover showing Muon 𝑔 −2 storage ring at FNAL.

https://journals.aps.org/prl/abstract/10.1103/7clf-sm2v
https://muon-g-2.fnal.gov/
https://muon-gm2-theory.illinois.edu/