Reaction to the announcement of a new measurement of (g-2)

Today’s announcement of a new measurement of the anomalous magnetic dipole moment – known to its friends as (g-2) of the muon – has been greeted with excitement by the scientific community, as it seems to provide evidence of a departure from the standard model of particle physics (by 4.2σ in frequentist parlance).

My own view is that the measurement of g-2, which seems 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 for 2 in the theoretical calculations.

P.S. According to my own (unpublished) calculations, the value of g-2 ≈ 7.81 m s-2.


11 Responses to “Reaction to the announcement of a new measurement of (g-2)”

  1. Anton Garrett Says:

    Spoken like a true cosmologist.

    Are you allowed to differentiate with respect to 2?

  2. nannacecilie Says:

    More seriously, it does not change the status – the main issue remains the theoretical uncertainties in the hadronic contributions.

    • Anton Garrett Says:

      Where can I find a discussion of this for physicists but not for specialists in field theory, please – something at the Physics World/Physics Today level?

    • telescoper Says:

      But perhaps it’s easier to make up some “new physics” than it is to do the old physics properly?

      • Anton Garrett Says:

        The Fermilab announcement puts error bars on the theoretical calculation; are you (Peter, cecilie) questioning those?

        I’d welcome a reference for the error bars.

      • Jonivar Skullerud Says:

        That is actually true to a large extent. Doing the old physics properly means doing a lattice QCD+QED calculation of the hadronic vacuum polarisation (and of the subleading hadronic light-by-light scattering contribution), which is indeed hard and requires a lot of computational resources. It is only in recent years that we have become able to do those calculations with sufficient precision. There was a paper out yesterday [arXiv:2104.02632] of the light-by-light scattering confirming previous results, but the hadronic vacuum polarisation requires more work. Intriguingly, arXiv:2002.12347 suggests no new physics is needed (and that the old theory results are wrong), but that would need confirmation by other groups.

        Anton: I do not know of a good discussion of the theory uncertainty (or theory calculation) but if you allow the use of Feynman diagrams as crutches the principles and where the uncertainties come from could be outlined fairly easily.

      • Jonivar Skullerud Says:

        In direct reply to the last question from Anton: the “error bars” on the standard model prediction are wrong. Those are uncertainties evaluated within one particular method, and cannot catch the systematic uncertainties associated with using that method.

      • Anton Garrett Says:

        Thank you, Jonivar. I was presuming that lattice QCD had to be involved and it isn’t easy.

        Nothing wrong with Feynman diagrams, it’s just that we now know they are not the whole story even when the perturbation expansion converges.

  3. I remember when my electrodynamics professor was discussing g—2 for the electron, and noted the good agreement between experiment and theory. There was disagreement in the last digits. I asked if that is perhaps real disagreement or whether the measurement just weren’t good enough and thus the agreement could potentially be perfect. His reply was neither; it is too difficult to do more-precise calculations.

  4. […] In cosmology we have only a handful of free parameters and this process has worked pretty well providing a model in which these parameters are tightly constrained by a host of observational results. This is the standard model and although there a “tensions”, most prominently concerning the Hubble Constant, the model has survived. The same situation holds for the standard model of particle physics, though there is tension concerning the muon magnetic moment that  I blogged about here. […]

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