G-factor

But over the next two decades, experiments found that the electron’s so-called g-factor was not quite 2—it was off by about a tenth of a percent.

If they don’t, then they violate lepton flavor universality—and the unexpected g-factor measurement suggested that’s just what was happening.

Things started to change in 2004, when the E821 experiment at Brookhaven National Laboratory on Long Island announced its measurement of a property of the muon—a heavy version of the electron—known as its g-factor.

Physicists quantify it with a g-factor, which we can predict very precisely with the Standard Model.

In such a magnetic field, the muon wobbles in a manner determined by an intrinsic number known as the g-factor.