Speaker: Mario Fernandez Navarro (University of Zurich)

The QCD axion provides a compelling solution to the strong CP problem. In this talk I will discuss a minimal origin for the otherwise unspecified Peccei-Quinn symmetry of the QCD axion based on an extra-dimensional gauge theory, which nevertheless retains much of the simplicity of traditional KSVZ/DFSZ anomaly sectors. I will then consider more general scenarios involving additional PQ-breaking sources compatible with the strong CP solution, and/or multiple axion states originating from extra PQ symmetries. I will show that the photon-coupling phenomenology is qualitatively controlled by two structural properties: whether a linear combination of PQ symmetries broken only by QCD exists, and whether the anomaly coefficients are universal. This leads to a general sum rule governing the (multi-)axion system, helping to connect a potential experimental signature with the underlying axion potential and its UV completion.