Challenges for an axion explanation of the muon $g-2$ measurement

Understand

The discrepancy between the muon $g-2$ measurement and the Standard Model prediction points to new physics around or below the weak scale.

• It is tantalizing to consider the loop effects of a heavy axion (in the general sense, also known as an axion-like particle) coupling to leptons and photons as an explanation for this discrepancy.
• We provide an updated analysis of the necessary couplings, including two-loop contributions, and find that the new physics operators point to an axion decay constant on the order of 10s of GeV.
• This poses major problems for such an explanation, as the axion couplings to leptons and photons must be generated at low scales.