Evading the Lyth Bound in Hybrid Natural Inflation
  Generically, the gravitational-wave or tensor-mode contribution to the
primordial curvature spectrum of inflation is tiny if the field-range of the
inflaton is much smaller than the Planck scale. We show that this pessimistic
conclusion is naturally avoided in a rather broad class of small-field models.
More specifically, we consider models where an axion-like shift symmetry keeps
the inflaton potential flat (up to non-perturbative cosine-shaped modulations),
but inflation nevertheless ends in a waterfall-regime, as is typical for hybrid
inflation. In such hybrid natural inflation scenarios (examples are provided by
Wilson line inflation and fluxbrane inflation), the slow-roll parameter
$\epsilon$ can be sizable during an early period (relevant for the CMB
spectrum). Subsequently, $\epsilon$ quickly becomes very small before the
tachyonic instability eventually terminates the slow roll regime. In this
scenario, one naturally generates a considerable tensor-mode contribution in
the curvature spectrum, collecting nevertheless the required amount of
e-foldings during the final period of inflation. While non-observation of
tensors by Planck is certainly not a problem, a discovery in the medium to long
term future is realistic.
