https://arxiv.org/abs/2105.07866
The model dependence of the magnetic anomaly makes it an incisive test of QED and a sensitive probe for new-physics contributions
[7] “Testing new physics with the electron g − 2,” JHEP 11, 113 (2012), arXiv:1208.6583 [hep-ph].
The interplay between theory and experiment played a decisive role in the development and validation of quantum electrodynamics [8].
[8] Silvan S. Schweber, QED and the Men Who Made It (Princeton University Press, Princeton, 1994). See in particular Chapter 5, “The Lamb Shift and the Magnetic Moment of the Electron.”
Beyond its role in challenging QED and probing for the virtual influence of undiscovered particles and forces, the anomalous magnetic moment provides stringent constraints on lepton substructure [9].
compositeness scale $M_l^*$ and the radius $R_l$ of the lepton to the magnetic anomaly defect
$$|\delta a_l|=\frac{m_l}{M_l^*}=m_lR_l$$ (no suppression) $$|\delta a_l|=(\frac{m_l}{M_l^*})^2=m_l^2R_l^2$$ (chiral model)
Electron
CPT invariance requires that the gyromagnetic ratios of electron and positron be identical: ge− = ge+ .
Circa: from Latin 'around, about, roughly, approximately'
one loop ($O(e^2)$)
$$a^{[2]}_e=\frac{\alpha}{2\pi}\approx 0.001162$$
it is of interest to ask whether a new light X boson, feebly coupled to the electron, might be responsible.
Muon
$$\langle a_\mu^{2021}\rangle -a_\mu^{[WP:2020]}=251(59)\times10^{-11}$$
$$a_\mu^{[EW]}=153.6(1.0)\times10^{-11}$$
At the Japan Proton Accelerator Research Complex in Tokai, J-PARC experiment E34 [53] will employ a very different technique, using a 300 MeV/c reaccelerated thermal muon beam with 50% polarization. The beam will be injected vertically into a solenoid storage ring with 1 ppm local magnetic field uniformity for the muon storage region with an orbit diameter of 66 cm. Compare the Brookhaven and Fermilab experiments, with muon momentum of 3.09 GeV/c and orbit diameter 14.224 m. The precision goal for aµ+ is a statistical uncertainty of 450 ppb, similar to the statistical weight of the BNL and FNAL-2021 samples, and a systematic uncertainty less than 70 ppb.
Tau
$$(\frac{m_\tau}{m_\mu})^2\approx 280$$