Electroweak constraints on flavorful effective theories

TL;DR

The paper develops a model-independent EFT framework for dimension-6 operators with arbitrary flavor structure and derives electroweak precision constraints from on-shell Z and W observables, focusing on the mass shift $\delta m$ and vertex corrections $\delta g$. By working in the mass-eigenstate basis and using a linearized, tree-level treatment, the authors map the pole observables onto 20 diagonal and a few off-diagonal vertex corrections, plus $\delta m$, and examine several well-motivated flavor scenarios (Flavor Universality, Alignment, MFV, and Anarchic vector-like fermions). They provide quantitative bounds showing leptonic Z couplings are tightly constrained at $\mathcal{O}(10^{-3})$, while many quark couplings are less constrained, with some directions remaining relatively weakly constrained in a completely generic setting; specific flavor assumptions impose characteristic patterns and correlations among the vertex corrections. The results are translated to Warsaw and SILH' operator bases, highlighting flat directions and the necessity of combining pole data with off-pole observables to fully constrain the operator space. Overall, the work informs model building and guides experimental searches by quantifying robust, flavor-dependent bounds on new physics that couples to electroweak gauge bosons.

Abstract

We derive model-independent constraints arising from the Z and W boson observables on dimension six operators in the effective theory beyond the Standard Model. In particular, we discuss the generic flavor structure for these operators as well as several flavor patterns motivated by simple new physics scenarios.