On 10 August 2023 the Muon g-2 Collaboration based at the US Department of Energy’s Fermi National Accelerator Laboratory (Fermilab) outside Chicago announced the discovery of a discrepancy in the Standard Model of particle physics. The discrepancy involves the sub-atomic particles known as muons that are the most stable form of strange matter in the Standard Model. The main particles found in the cosmic rays that bathe the surface of the Earth, muons can be seen with the naked eye using a century-old device known as a cloud chamber.
At the science seminar announcing the results, James Mott from the Muon g-2 collaboration revealed a five standard deviation discrepancy between theory and experiment that surpasses the conventional threshold in particle physics for the discovery of a new phenomenon. However, the muon tension with the Standard Model goes back more than two decades to experiments done from 1997 to 2001 at the US Department of Energy’s Brookhaven National Laboratory outside New York City. As the longest-standing tension in the Standard Model of particle physics, the newly revealed discrepancy strongly suggests there is a slight shift at the part per million level of precision in the strength of the electromagnetic force between muons and the particles of light energy known as photons.
Within the Standard Model of particle physics, the electromagnetic force between muons and photons must be the same as the electromagnetic force between photons and the sub-atomic particles known as electrons whose motion is the basis of modern electronic technology. Yet the measurements at Brookhaven and now at Fermilab, using a beam with thousands of muons filling a volume the size of a large dog, seem to show that the strength of the coupling between muons and photons within the beam grows with the volume that the beam fills and with the number of muons within the beam (See Dark Matter Makes Light Bend in the Lab). By contrast, the measurements of the electron-photon coupling with the best precision typically probe a single electron trapped in a microscopic volume.