Past Event: Vernon W. Hughes Lecture - David Hertzog, University of Washington

Muons: Particles of the Moment

In April 2021, a measurement of the muon’s magnetism shook the particle physics world, making front-page news worldwide. How could one of nature’s tiniest magnets cause such a stir—and what’s happened since? The story begins long ago with early explorations of magnetism and Ampère’s molecular currents, experiments that even drew in Einstein. Before quantum mechanics revealed its secrets, the ratio of a system’s magnetic moment to its angular momentum—the “g-factor”—was predicted to be 1. Early measurements seemed to support this, until refined experiments showed a value of 2, beautifully explained by Dirac’s relativistic quantum theory.

Precision studies of the electron’s g-factor later revealed it to be slightly larger than 2, a discovery that helped forge quantum electrodynamics—the most precise theory in science. When the muon was discovered as a heavier cousin of the electron, physicists realized it offered another exacting test of the Standard Model. For decades, experiment and theory advanced in step, until our 2004 muon measurement came in over 3σ above prediction—hinting at possible new physics. To probe further, we built a more sensitive experiment at Fermilab, recently completing data collection and analysis with a final precision of 127 ppb. Meanwhile, theoretical predictions have evolved dramatically, especially with advances in lattice QCD. I will trace this remarkable story—from its historical roots to the latest results announced just last month—at a level accessible to all.

Host: Karsten Heeger