Bounds for the scale of inflation and the tensor-to-scalar ratio in Hybrid Natural Inflation

Abstract: Recently we have studied in great detail a model of Hybrid Natural Inflation (HNI) by constructing two simple effective field theories. These two versions of the model allow inflationary energy scales as small as the electroweak scale in one of them or as large as the Grand Unification scale in the other therefore covering the whole range of possible energy scales. The inflationary sector of the model is of the form $V(\phi)=V_0 \left(1+a \cos(\phi/f)\right)$ where $0\leq a<1$ and the end of inflation is triggered by an independent waterfall field. One interesting characteristic of this type of models is that the tensor-to-scalar ratio $r$ is a non-monotonic function of $\phi$ presenting a {\it maximum} close to the inflection point $\phi_I=\pi/2$ of the potential. Because the scalar spectrum $\mathcal{P}s(k)$ of density fluctuations when written in terms of the potential is inversely proportional to $r$ we find that $\mathcal{P}_s(k)$ presents a {\it minimum} at $\phi{min}$. We use this property of HNI together with the observation that the spectrum is decreasing during the first 8 e-folds of observable inflation to determine bounds for the inflationary energy scale $\Delta$ and for the tensor-to-scalar ratio $r$.