“The particle you thought was a bare muon is actually a muon plus a cloud of other things that appear spontaneously,” said Chris Polly, another leader of the Fermilab Muon g-2 experiment. “They change the magnetic moment.”….
“A muon constantly emits and reabsorbs temporary “virtual: particles. This entourage affects the muon’s magnetic moment, or how much torque it feels while moving through a magnetic field.”
One-Photon correction: “The simplest scenario occurs when a muon emits and reabsorbs a virtual photon. This is the most common possibility, so it has the largest effect on the muon’s magnetic moment.”