Measured Lepton Magnetic Moments

Abstract

The electron and muon magnetic moments have played, and continue to play, important roles in testing the fundamental mathematical description of physical reality called the Standard Model of particle physics (SM). The electron magnetic moment is the most precisely measured property of an elementary particle and the most precise SM prediction, setting up the most precise confrontation ever between experiment and theory. It enables the most precise test of quantum field theory, and of the fundamental CPT symmetry invariance of the SM with leptons. The stable electron is studied with quantum methods while the electron remains for months in its quantum ground and first excited states. The muon magnetic moment is one of the most precisely measured property of an unstable elementary particle. Although less precise measured than the electron, it provides greater sensitivity to physics beyond the Standard Model -- a powerful tool for testing the existence of new particles and forces. Because muons decay quickly, they must be studied as they orbit at nearly the speed of light in a large storage ring. The extremely high precision of the electron and muon magnetic moment measurements has driven major advances in theoretical physics, inspiring new techniques in quantum field theory, precision calculations, and lattice gauge theory. Only experimental limits currently exist on the size of the magnetic moments of the tau and neutrino leptons.

Figures (27)

• Figure 1: The electron and muon magnetic moments are measured within structures that differ in size by a factor of $10^3$, at energies and storage times that differ by a $10^{13}$ and $10^9$. Left: a roughly 14-mm cylindrical Penning trap cavity confines a single electron (or positron) for months, with excitations from its cyclotron ground state to its 0.6 meV excited state used to measure its magnetic moment. Right: a 14-m diameter storage ring within which the magnetic moments of approximately 5000 muons with a momentum of 3.1 GeV/c are stored for up to 700 $\mu s$ every 90 ms on average.
• Figure 2: Lowest cyclotron and spin energy levels of the quantum cyclotron.
• Figure 3: (a) Cryogenic system supports a 50 mK electron trap upon a 4.2 K solenoid to provide a very stable magnetic field. (b) Silver electrodes of a cylindrical Penning trap cavity. From NorthwesternMagneticMoment2023.
• Figure 4: The blackbody photons that stimulate cyclotron excitations for higher temperatures go away at lower trap cavity temperatures. From QuantumCyclotron.
• Figure 5: (a) Histogram of the measured time it takes for a blackbody photon to be absorbed by the quantum cyclotron in its ground state. (b) Histogram of the measured time it takes for a quantum cyclotron in its first excited state to decay to the ground state. From QuantumCyclotron.