Large tensor spectrum of BICEP2 in the natural SUSY hybrid model
  The large tensor spectrum recently observed by the BICEP2 Collaboration
requires a super-Planckian field variation of the inflaton in the single-field
inflationary scenario. The required slow-roll parameter \epsilon \approx 0.01
would restrict the e-folding number to around 7 in (sub-)Planckian inflationary
models. To overcome such problems, we consider a two-field scenario based on
the natural assisted supersymmetric (SUSY) hybrid model ("natural SUSY hybrid
inflation" [1]), which combines the SUSY hybrid and the natural inflation
models. The axionic inflaton field from the natural inflation sector can admit
the right values for the tensor spectrum as well as a spectral index of 0.96
with a decay constant smaller than the Planck scale, f \lesssim M_P. On the
other hand, the vacuum energy of 2 x 10^{16} GeV with 50 e-folds is provided by
the inflaton coming from the SUSY hybrid sector, avoiding the eta problem.
These are achieved by introducing both the U(1)_R and a shift symmetry, and
employing the minimal Kahler potential.
