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Register a new userNonminimal Couplings and the Forgotten Field of Axion Inflation
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We study the multifield dynamics of axion models nonminimally coupled to gravity. As usual, we consider a canonical $U(1)$ symmetry-breaking model in which the axion is the phase of a complex scalar field. If the complex scalar field has a nonminimal coupling to gravity, then the (oft-forgotten) radial component can drive a phase of inflation prior to an inflationary phase driven by the axion field. In this setup, the mass of the axion field is dependent on the radial field because of the nonminimal coupling, and the axion remains extremely light during the phase of radial inflation. As the radial field approaches the minimum of its potential, there is a transition to natural inflation in the angular direction. In the language of multifield inflation, this system exhibits ultra-light isocurvature perturbations, which are converted to adiabatic perturbations at a fast turn, namely the onset of axion inflation. For models wherein the CMB pivot scale exited the horizon during radial inflation, this acts to suppresses the tensor-to-scalar ratio $r$, without generating CMB non-Gaussianity or observable isocurvature perturbations. Finally, we note that the interaction strength between axion and gauge fields is suppressed during the radial phase relative to its value during the axion inflation phase by several orders of magnitude. This decouples the constraints on the inflationary production of gauge fields (e.g., from primordial black holes) from the constraints on their production during (p)reheating.
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