The anomalous magnetic moment of the muon was last measured by the E821 Muon g-2 experiment at Brookhaven National Laboratory. Data was taken between 1997 and 2001. An uncertainty of 540 parts per billion (ppb) was achieved. But the result left the scientific community with a long-standing, larger-than-3σ deviation from the prediction obtained within the Standard Model of Particle Physics (SM).
The comparison of the experimental result with the high precision SM prediction provides as stringent test of the fundamental assumptions of the model. This discrepancy is taken as a strong hint of new physics.
Over the years the SM prediction was continously refined and verified. In 2017 the Muon g-2 Theory Initiative was founded to provide a comprehensive SM prediction in light of the upcoming new Muon g-2 experiment at Fermi National Accelerator Laboratory (FNAL), Batavia, IL, USA. Their most recent result is summarized in the White Paper (https://muon-gm2-theory.illinois.edu/) and has an uncertainty of 350 ppb.
The Muon g-2 experiment E989 at FNAL aims to improve the experimental uncertainity by a factor of four to less than 140 ppb.
As a postdoctoral researcher at the Univeristy of Washington's Center for Experimental Nuclear Physics and Astrophysics Martin Fertl was heavily involved in the construction of the precision magnetic field measurement instrumentation that is required to ultimately measure the magnetic field in the muon storage volume. To achieve the ultimate sensitivity goal an uncertainty of 70 ppb has to be reached.
After relocating to JGU Mainz and the PRISMA+ Cluster of Excellence in 2019, Martin Fertl's group is now providing leading contibutions to the continued improvement of the magnetic field and the analysis of the acquired data.
NEWS (April 7th, 2021): The newest measurement of the anomlaous magnetic moment form the FNAL E989 experiment can be found here (after the embargo has dropped).
A poster summarizing the new result is linked here.