Harmonic Hybrid Inflation
  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 $f\lesssim M_{\rm
P}$ and $\Delta\phi_{60}\lesssim M_{\rm P}$ 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.
