Correlation Between Muon $g-2$ and $\mu\rightarrow{e}{\gamma}$

TL;DR

This paper examines the tension between explaining the muon g-2 anomaly and the constraints from the mu to e gamma decay, proposing mechanisms to reconcile these conflicting observations within new physics models.

Contribution

It summarizes one-loop contributions to muon g-2 and introduces two general methods, GIM mechanism and non-universal couplings, to resolve the conflict with mu to e gamma constraints.

Findings

GIM mechanism can suppress mu to e gamma decay

Non-universal couplings can reconcile g-2 with decay constraints

Scalar leptoquark model illustrates non-universal coupling approach

Abstract

While the muon $g-2$ anomaly can be successfully explained by some new physics models, most of them are severely constrained by the $\mu \to e \gamma$ bound. This tension is more transparent from the effective field theory perspective, in which the two phenomena are encoded in two very similar operators. However, with the ${\cal O}(1)$ Wilson coefficients, the current upper bound on $\mu \to e \gamma$ indicates a new-physics cutoff scale five orders smaller than that needed to eliminate the $(g-2)_\mu$ anomaly. By summarizing all the formulae from the one-loop contributions to the muon $g-2$ with the internal-particle spin not larger than 1, we point out two general methods to reconcile the conflict between the muon $g-2$ and $\mu \to e \gamma$: the GIM mechanism and the non-universal couplings. For the latter method, we use a simple scalar leptoquark model as an illustration.