Flavour alignment of New Physics in light of the $(g-2)_\mu$ anomaly

TL;DR

This paper explores the conditions under which new physics models can explain the muon g-2 anomaly while remaining consistent with flavor violation constraints, highlighting the need for precise flavor alignment in theoretical models.

Contribution

It analyzes flavor alignment requirements in SMEFT for explaining muon g-2 and discusses the challenges in models with broken flavor symmetries.

Findings

Semileptonic four-fermion operators must be tightly aligned with lepton Yukawas.

Achieving this alignment is feasible with specific flavor symmetries.

Many models addressing B anomalies face tension with these flavor alignment conditions.

Abstract

We investigate the flavour alignment conditions that New Physics (NP) models need to satisfy in order to address the $(g-2)_\mu$ anomaly and, at the same time, be consistent with the tight bounds from $\mu \to e \gamma$ and $\tau \to \mu \gamma$. We analyse the problem in general terms within the SMEFT, considering the renormalisation group evolution of all the operators involved. We show that semileptonic four-fermion operators, which are likely to generate a sizeable contribution to the $(g-2)_\mu$ anomaly, need to be tightly aligned to the lepton Yukawa couplings and the dipole operators in flavour space. While this tuning can be achieved in specific NP constructions, employing particular dynamical assumptions and/or flavour symmetry hypotheses, it is problematic in a wide class of models with broken flavour symmetries, such as those proposed to address both charged- and neutral-current $B$ anomalies. We quantify this tension both in general terms, and in the context of explicit NP constructions.