James D. Wells

I believe the measurement of the electron's g-factor was one of the early triumphs of QED. You may wondering whether there is some reason why new, unknown particles would interact with muons but not electrons. Or whether the electron's g-factor also affected, but at too low a level to show up in those early experiments. All valid questions.

You might also think you need a theory that violates lepton universality (not accommodated in any way by the Standard Model) in order to explain an effect that influences the muon g-2 and not the electron g-2. Or you might think it's more the case that the electrons g-factor is unaffected, rather than it being affected at too low a level to detect. Also valid questions.

Of course, as almost always usual in physics, reality is stranger than theory. There are candidate particles such as leptoquarks which can do this sort of job for you, where electrons and muons are affected differently. Because the muon is around 200 times heavier than the electron, heavy new particles sitting in the loop could influence the muons g-2 value and not the electrons. So you don't explicitly need lepton universality violation as I theorised above to get an electron vs. muon difference. LHCb has looked at bottom quarks decaying to a strange quark and two charged leptons, like b -> d µ+ µ- and b -> s µ+ µ-. The muon and electron versions of this process should happen at exactly the same rate, but the muon / electron ratio has been measured to be below 1. In 2017 this hinted at the same sort of new physics as the muon g-2 run at Fermilab in 2021 confirmed at 4.2-sigma accuracy.

The interest in 2021 comes from the fact that theory and experiment are in tension for the muon g-2, because the theory calculation already includes the extra Standard Model stuff I alluded above to (electroweak and hadronic). I've always wondered how sure we are that the extra stuff contributing to the muon g-2 is well controlled in the theory calculation. Unfortunately, the hadronic bit is a bugger, as you might expect. It can't be calculated from first principles so they take it from measurements from other experiments. There are 2 different, independent, ways of doing it and they agree very nicely so people are pretty confident it’s good enough to rely on.

This is an excellent book if you want to delve into the details of the Standard Model. Maybe we’ll get more juice as soon as a second edition comes out with the Fermilab results…… (meer)