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Muon Magnetism

Updated: Jan 28, 2022

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According to our current understanding of the universe, everything in it is made up of two types of elementary particles: quarks, which combine to form particles such as protons and neutrons that are known as hadrons, and leptons, which do not respond to the “strong force” which bonds quarks together and therefore do not form larger particles. There are six varieties, or ‘flavours’, of leptons: electrons, taus, and muons, which have electric charge, and electron-neutrinos, tau-neutrinos, and muon-neutrinos, which do not have charge.

Muons are a type of elementary particle that possess an electric charge. Muons are produced from the decay of particles called pions, which are produced when cosmic rays interact with atomic nuclei in the upper atmosphere. They have a lifespan of 2.20 millionths of a second before they decay into one electron, one electron-neutrino, and one muon-neutrino.

In April 2021, the Muon G-2 experiments conducted at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, concluded that muons are slightly more magnetic than the Standard Model of Particle Physics predicted they would be.

The Muon G-2 experiments were conducted to test the muon’s ‘magnetic moment’, or the muon’s magnetic strength. Muons were placed in a ring fifteen metres in diameter fitted with magnets made of a metal core surrounded by superconducting wire. The electric current that the wires conducted created the necessary magnetic field. The muons’ magnetic field interacted with the magnetic field in the ring, and the interaction caused the muons to rotate. The kinetic energy and direction of rotation of the muons were measured by detectors positioned around this ring, and the results inform us of the muon’s ‘g-factor’, a number linking the spin, or inherent rotation, and magnetic moment of a particle that describes the strength of the muon’s magnetic field.

The strength of the muon’s magnetic field affects how quickly the muon rotates when interacting with another magnetic field, which is why an artificial magnetic field was used during the Muon G-2 experiments. However, this strength, or g-factor, was found to be higher than quantum physics and the Standard Model of Particle Physics - the theory describing weak (breaking apart), strong (coming together) and electromagnetic interactions - predicted they would be. Quantum physics predicts that muons should have a g-factor equal to 2, or precisely 2.00233183620, and a magnetic moment of 0.00116591810. However, muons’ proven g-factor is 2.00233184122, and their magnetic moment is 0.00116592061. The disparity between the two values is 4.2 standard deviations.

This changes what the Standard Model tells us about electromagnetic interaction, and as space is never truly empty, but rather always filled with particles whose existence cannot be truly proven, it is speculated that muons are interacting with these virtual - unprovable - particles that could be affecting muons’ magnetic moment and g-factor.

Could these measurements prove the existence of virtual particles? What kinds of new discoveries could they lead to? And will the Muon G-2 experiments change the world of physics as we know it?

Work Cited:

[1] Maura. “First Results from Fermilab’s Muon g-2 Experiment Strengthen Evidence of New Physics.” News, 29 June 2021,

[2] Nature Editorial. “Long-Awaited Muon Physics Experiment Nears Moment of Truth.” Nature, Accessed 8 Oct. 2021.

[3] “First Results from Fermilab’s Muon g-2 Experiment Strengthen Evidence of New Physics.” NSF - National Science Foundation, Accessed 8 Oct. 2021.

[4] Author, No. “The Muon’s Theory-Defying Magnetism Is Confirmed by New Experiment –.” Physics World, 8 Apr. 2021,

[5] “Simple Science: Particles.” Imperial College London, Accessed 8 Oct. 2021.

[6] “Leptons.” HyperPhysics, Accessed 8 Oct. 2021.

[7] Cho, Adrian. “Particle Mystery Deepens, as Physicists Confirm That the Muon Is More Magnetic than Predicted.” Science.Org, 7 Apr. 2021,


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