Muon g−2 from Fermilab increases its (assumed) deviation in the anomalous magnetic moment of the muon to 5.1 sigmas

2023-08-10 21:26:16

The big news in particle physics in 2021 was the anomalous magnetic moment of the muon measured by Fermilab’s Muon g−2 experiment. He observed a discrepancy at 4.2 sigma with the consensus prediction of the standard model (LCMF, 07 Apr 2021). Today the new result of this experiment was announced at a press conference, a = (g−2)/g = 116 592 059(22) × 10−11, which with an error of 0.2 ppm raises the deviation to 5.1 sigmas. Can we talk about the discovery of new physics beyond the standard model? No, sorry, as the Standard Model prediction depends on the Hadronic Vacuum Polarization (HVP), which can be calculated by two methods that give two different results. The consensus forecast uses one of two methods. If the other is used, the deviation disappears (being less than two sigmas) and the experiment confirms the theory. The interpretation is opposite according to the preferred method. Therefore, from Fermilab, it is proposed to be cautious with the disclosure of this new result. The emphasis should be that the measurement made is very difficult, an experimental milestone, nothing more. No conclusions about the standard model should be drawn from the new measure.

As I already told you (LCMF, 10 May 2021), the estimation of the contribution of the hadronic vacuum has two components, the hadronic vacuum polarization (HVP) and the light-by-light hadronic scattering (HLbL). The calculation of the second (HLbL) with chromodynamics on the lattice (lattice QCD or LQCD) and by electron versus positron collision analysis (e⁺ e⁻ → π⁺ π⁻) leads to a similar result (a 0.15 ppm contribution to the value of g−2). However, for the calculation of the HVP there is a big difference between the LQCD prediction and the combination of the electron-positron collisions of BABAR, BESIII, CMD-3 and SND, the latter result being more accurate (which is the one used in the consensus prediction). However, an unexpected result was published in February 2023, CMD-3 published a value that disagrees with the others and is compatible with the LQCD estimate. In addition, new LQCD results were published earlier in the year that are compatible with each other. How do these results affect the consensus estimate? In September there is a scientific congress to evaluate what to do; to decide whether to incorporate the LQCD and CMD-3 measures on an equal footing (bringing the deviation from 5.1 sigmas down to about three sigmas), or whether such a decision should be delayed until after the summer of 2024 (By then there will be new LQCD results and new estimates from BABAR and BESIII which will make delaying it any further untenable.) You may be surprised that a Standard Model prediction is difficult to calculate, but the contribution of the hadronic vacuum to the g−2 of the muon requires accounting for non-perturbative effects (although perturbative effects suffice for the electron).

We are in a fascinating moment about the anomalous magnetic moment of the muon. All previous measurements (KLOE, SND, etc.) are being reanalyzed with modern techniques, new experimental results are expected, and new computational results are expected with LQCD. At this rate, in a couple of years we will have an estimate of the theoretical prediction that is much more reliable than the current one. Perhaps it coincides with Muon’s observation g−2, which will reinforce the standard model, that Achilles always defeats his enemies (until his heel arrives). The new paper is The Muon g − 2 Collaboration, “Measurement of the Positive Muon Anomalous Magnetic Moment to 0.20 ppm,” Fermilab, Aug 10, 2023 [PDF] (I guess it will appear on arXiv soon and be published on Physical Review Letters). The article on the surprising result of CMD-3 is CMD-3 Collaboration, «Measurement of the e+e→π+π⁻ cross section from threshold to 1.2 GeV with the CMD-3 detector,» arXiv:2302.08834 [hep-ex] (17 Feb 2023), doi: https://doi.org/10.48550/arXiv.2302.08834. . . . On the situation I recommend reading The Muon g−2 Theory Initiative, “The Status of Muon g−2 Theory in the Standard Model,” Aug 10, [web]. The third figure is taken from the presentation by Alexander Keshavarzi, “Muon g–2: Status of the Fermilab experiment and of the dispersive approach,” Lattice 2023, 04 Aug 2023 [indico].

If you want to know, feel free to delve into this blog: “What we expect from the Fermilab Muon g−2 experiment and other muon-related anomalies”, LCMF, Apr 03, 2021; “Fermilab g−2 muon increases deviation in muon anomalous magnetic moment to 4.2 sigmas”, LCMF, 07 Apr 2021; “G−2 Muon Anomaly Sparks Tsunami of Publications Among Theoretical Physicists”, LCMF, 10 Apr 2021; “The LQCD Estimation of the Chromodynamic Contribution to the Muon Anomalous Magnetic Moment”, LCMF, 10 May 2021; “The void is a substance… (my talk #Naukas21)”, LCMF, Sep 24, 2021; “New Hadronic Polarization (HVP) LQCD Calculations to Estimate Muon g−2 Value”, LCMF, 18 Feb 2023; among other.