Archive for muon magnetic moment

The magnetic moment of the muon revisited

Posted in The Universe and Stuff with tags , , , , on June 10, 2025 by telescoper

A couple of years ago I posted an item about a measurement of the anomalous magnetic moment of the muon, a quantity usually known as (g-2). At the time this appeared to be in conflict with the predictions of the standard model (SM)of particle physics but the general opinion among experts was that the problem was likely to lie with the calculations rather than the experiment.

Well, catching up on things I missed during examination marking season, I refer you to a substantial paper (188 pages long) that appeared on arXiv on 27th May 2025, with the abstract:

If this is too small to read I direct yourself to the salient point, namely that “there is no tension between the SM and experiment at the current level of precision”.

It seems the Standard Model survives to fight another day…

Leave a comment »

Anomalous Media Coverage

Posted in The Universe and Stuff with tags , , , on August 14, 2023 by telescoper

Via a characteristically garbled piece in the Grauniad, bizarrely entitled Scientists may be on brink of discovering fifth force of nature, I found out that there has been an announcement of a new measurement of the anomalous magnetic dipole moment – known to its friends as (g-2) – of the muon. This new measurement is consistent with one made a couple of years ago, but with a smaller range of uncertainty. It’s an impressive result from an experimental point of view.

There is a news item explaining the new result here, which says

Precision test of particle’s magnetism confirms earlier shocking findings — but theory might not need a rethink after all.

That’s pretty much the opposite of what the Guardian piece says.

There’s also a video produced by the Fermilab Muon g-2 team that explains what has changed from 2021 to now:

My own view is that these recent experimental measurements of g-2, which seem to be a bit higher than theorists expected, can be straightforwardly reconciled with the predictions of the standard model of particle physics by simply adopting a slightly lower value of 2 in the theoretical calculations.

P.S. The classical value is of course  g-2 ≈ 7.81 m s-2.

3 Comments »