Some background information on the "g-2" value of the muon: The muon is an elementary particle, ~206 times heavier than the electron. Like the electron, the muon has an internal magnet, a magnetic moment, related to its angular momentum called spin 1/n

The proportionality constant between magnetic moment and spin is called the gyromagnetic ratio g. The simple expectation (in Dirac theory) for the value of g is 2, however we know that its value is slightly larger. This is called the anomalous magnetic moment of the muon 2/n

The difference (hence the name g-2) comes from the fact that the vacuum is not truly empty space, but filled with virtual particles (photons, electrons, heavier particles) which can briefly emerge from the vacuum, interact with the muon and contribute to its value of g 3/n

In the Standard Model (SM) of particle physics, the value of g-2 can be calculated very precisely. The relative deviation from 2, a= (g-2)/2, is a_theo =116591810(43)x10^-11, see this recent impressive work by the Muon g-2 Theory Initiative https://arxiv.org/abs/2006.04822  (165 pages) 4/n

A past experiment at the Brookhaven National Laboratory ( @BrookhavenLab) however reported in 2006 a smaller value, a discrepancy compared to the SM prediction, namely a_exp=116592089(63)x10^-11. The difference is 279(76)x10^-11 (corresponding to a 3.7-sigma effect) 5/n

This is exciting, and could be a sign of new, yet unobserved elementary particles. However the measurement needs confirmation with increased precisions. A new experiment, the Fermilab Muon g-2 experiment, will have 4 times the precision of the past Brookhaven experiment 6/n

The Muon g-2 experiment at Fermilab ( @Fermilab) will thus be able to measure the relative deviation from 2, a = (g-2)/2, with a precision of 140 parts per billion. It injects a polarised muon beam in a storage ring with a uniform magnetic field B and an electric field E 7/n

Inside the ring the muons will be subject to both spin precession and cyclotron frequency: the difference between the two is called the anomalous precession frequency. The latter depends on the mass of the muon, on the magnetic field B and on the relative deviation a 8/n

Thus, by measuring the anomalous precession frequency and the B-field very precisely, one can extract the value of a, and hence the value of g. The anomalous precession frequency is extracted from the decay products of the (positively charged) muons 9/n

The observed decay products are positrons (positively charged electrons), which are preferentially emitted in the muon-spin direction (due to parity violation in weak interactions). The magnetic field B is mapped precisely by using proton nuclear magnetic resonance probes 10/n

The new experiment uses a 14-m diameter storage ring from Brookhaven, which was moved to Fermilab ( https://www.bnl.gov/newsroom/news.php?a=24180), together with the Fermilab accelerator complex, providing about 10^5 polarised positive muons per second 11/n

The magnetic field inside the storage ring has a magnitude of 1.5 T and is kept as uniform as possible. Twenty four electromagnetic calorimeters (each made of 54 lead fluoride crystals read out by silicon photomultipliers) detect the positrons form the decay of stored muons 12/n

The Fermilab experiment started taking data in the spring of 2018. If the discrepancy with the Standard Model is confirmed with higher statistics, this could be an exciting hint of "new physics", of particles beyond those known in the Standard Model! 13/n

New results (from the first experimental run) are expected to be announced this spring, possibly as early as March. Please see the official website of the experiment for some nice pictures and much more detailed information: https://muon-g-2.fnal.gov/index.html  14/14

Finally some advertisement for the Zurich Physics Colloquium ;-) Here is the recording of David Hertzog's talk from November 11, 2020 on "Nearly First Results and Up-to-Date Status of the Fermilab Muon g-2 Experiment": https://video.ethz.ch/speakers/zurich_physics_colloquium/40ab90f2-b9a9-4928-8486-38f28f4829c6.html