Harmonic Hybrid Inflation

Abstract

We present a mechanism for realizing hybrid inflation using two axion fields with a purely non-perturbatively generated scalar potential. The structure of scalar potential is highly constrained by the discrete shift symmetries of the axions. We show that harmonic hybrid inflation generates observationally viable slow-roll inflation for a wide range of initial conditions. This is possible while accommodating certain UV arguments favoring constraints $f\lesssim M_{\mathrm{P}}$ and $\Delta {\varphi }_{60}\lesssim M_{\mathrm{P}}$ on the axion periodicity and slow-roll field range, respectively. We discuss controlled ${\mathbb{Z}}_2$-symmetry breaking of the adjacent axion vacua as a means of avoiding cosmological domain wall problems. Including a minimal form of ${\mathbb{Z}}_2$-symmetry breaking into the minimally tuned setup leads to a prediction of primordial tensor modes with the tensor-to-scalar ratio in the range $10^{-4}\lesssim r\lesssim 0.01$, directly accessible to upcoming CMB observations. Finally, we outline several avenues towards realizing harmonic hybrid inflation in type IIB string theory.

16 pages, 7 figures, 1 appendix, LaTeX. V2 accepted in JHEP

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