Another technical design report has appeared on the arXiv, this time for the proposed Muon (g-2) experiment at Fermilab.

The name refers to the anomalous magnetic moment (g-2)/2 of the muon. A naive application of quantum electrodynamics says that the gyromagnetic ratio of elementary spin-1/2 particles should be g=2. When higher order corrections are considered, the result is slightly different from 2. It also turns out that these corrections can be calculated to high levels of accuracy and also measured with astounding precision. This makes measuring (g-2) for different particles (like electrons and muons) an excellent test of the Standard Model.

The muon anomalous magnetic moment is measured by creating a muon beam in a ring. As the muons decay, the resulting electrons are emitted in directions that are correlated with the direction of the muon’s spin. The muon’s spin direction precesses in an electromagnetic field at a frequency that’s different from the frequency at which muons travel around the ring. The precession frequency is related to the anomalous magnetic moment, so (g-2)/2 can be measured very accurately.