Hybrid Natural Inflation

TL;DR

This paper develops effective field theory models of Hybrid Natural Inflation, analyzing their potential, phenomenology, and compatibility with Planck data, highlighting the non-monotonic behavior of key inflationary parameters and the possibility of low-scale inflation.

Contribution

It introduces two simple EFT models of Hybrid Natural Inflation with detailed phenomenological analysis and fits to observational data, including the impact of non-monotonic slow-roll parameters.

Findings

Inflationary potential naturally arises with a pseudo-Nambu-Goldstone inflaton.

The tensor-to-scalar ratio r has a maximum due to non-monotonic epsilon.

Inflation can occur at scales as low as the electroweak scale.

Abstract

We construct two simple effective field theory versions of {\it Hybrid Natural Inflation (HNI)} that illustrate the range of its phenomenological implications. The resulting inflationary sector potential, $V=\Delta^4(1+a\cos(\phi/f))$, arises naturally, with the inflaton field a pseudo-Nambu-Goldstone boson. The end of inflation is triggered by a waterfall field and the conditions for this to happen are determined. Also of interest is the fact that the slow-roll parameter $\epsilon$ (and hence the tensor $r$) is a non-monotonic function of the field with a maximum where observables take universal values that determines the maximum possible tensor to scalar ratio $r$. In one of the models the inflationary scale can be as low as the electroweak scale. We explore in detail the associated HNI phenomenology, taking account of the constraints from Black Hole production, and perform a detailed fit to the Planck 2015 temperature and polarisation data.