Phenomenology of Axion Models with a Light Radial Mode

The QCD axion is one of the most well-motivated extensions of the standard model. It promises an elegant solution to the strong CP problem while also providing a non-thermal mechanism for the production of dark matter. Conventionally, the axion is envisioned as the light, angular mode of a complex scalar field where the heavy, radial mode is integrated out. In this thesis, we investigate how a sufficiently light radial mode can impact the dynamics of a QCD axion, supported by lattice simulations for non-linear evolution. We then prove that the radial mode is itself a viable dark matter candidate and investigate its observational signatures when it couples to the Standard Model via the Higgs portal. Finally, we critically assess the conditions required to realize such a light scalar and discuss possible explanations from the perspective of naturalness.