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The large tensor spectrum recently observed by the BICEP2 Collaborationrequires a super-Planckian field variation of the inflaton in the single-fieldinflationary scenario. The required slow-roll parameter \epsilon \approx 0.01would restrict the e-folding number to around 7 in (sub-)Planckian inflationarymodels. To overcome such problems, we consider a two-field scenario based onthe natural assisted supersymmetric (SUSY) hybrid model ("natural SUSY hybridinflation" [1]), which combines the SUSY hybrid and the natural inflationmodels. The axionic inflaton field from the natural inflation sector can admitthe right values for the tensor spectrum as well as a spectral index of 0.96with a decay constant smaller than the Planck scale, f \lesssim M_P. On theother hand, the vacuum energy of 2 x 10^{16} GeV with 50 e-folds is provided bythe 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, andemploying the minimal Kahler potential.

Large tensor spectrum of BICEP2 in the natural SUSY hybrid model