The KSVZ Atlas: A Unified SMEFT-ALP Framework

Abstract

We develop a general framework for matching KSVZ-like ultraviolet completions featuring vector-like fermions and a spontaneously broken $\mathrm{U}(1)_{\mathrm{PQ}}$ symmetry onto the Standard Model Effective Field Theory and the low-energy axion-like particle effective theory. The framework applies to arbitrary vector-like fermion representations and PQ-charge assignments, and systematically captures the effective interactions generated in both sectors. We then perform a comprehensive phenomenological analysis of the resulting SMEFT operators, based on global fits to electroweak precision, Higgs, and flavor observables, obtaining robust bounds on the corresponding Wilson coefficients that are largely independent of the details of the ultraviolet realization. These constraints can subsequently be translated into the QCD axion and ALP parameter space, providing indirect probes of ALP couplings. We further investigate several representative examples of the interplay between the SMEFT and ALP sectors, illustrating how direct ALP searches and indirect precision and flavor observables provide complementary information on the same underlying dynamics. We find that, over large regions of parameter space, indirect constraints derived from the SMEFT analysis dominate over direct ALP probes, except in scenarios where the PQ-charge assignment permits mass mixing with Standard Model fermions. Overall, our results establish a unified framework for connecting ultraviolet completions, SMEFT analyses, and ALP searches, enabling both the interpretation of existing constraints and the exploration of future signals within a common theoretical setting.