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Special Feature

The tussle over the muon g-2

A short-lived particle continues to throw the world of physics in a tizzy. Physicists, experimental and theoretical, are equally invested in studying the particle.

Experimental physicists often work long and hard to verify the claims of theoretical physicists. Gravitational waves were predicted by Einstein's general theory of relativity in 1916, but physicists took a century to build the equipment to detect them. The Higgs boson was predicted in 1964, but discovered only in 2012. Sometimes, however, experimental physicists turn the table on their theorist friends — as in the case of the muon, a heavy, electron-like particle that lives for a fraction of a second before disintegrating.

For about 50 years, particle physics laboratories have been measuring the magnetic moment of the muon with increasing accuracy. Magnetic moment is a measure of the influence of a magnetic field on a particle. Since, in practice, a muon is influenced by particles around it, precise measurements of its magnetic moment can be used to verify theories of particles and forces in physics. The reigning champion of the field is the Standard Model (, and recent muon experiments have been testing the resilience of this theory.

The latest measurements of the muon's anomalous magnetic moment were announced by Fermilab, the U.S. particle physics and accelerator laboratory, on August 10, 2023. These showed a discrepancy with the Standard Model calculations, possibly hinting at physics yet to be understood.

"The result was shocking," says Timothy Chupp, Professor of Physics at the University of Michigan and the principal investigator of the magnetic field analysis. "It was just amazingly consistent. It agreed with everything that had been measured before, but it was much more precise."


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