Constraining Hybrid Natural Inflation with recent CMB data

TL;DR

This paper evaluates the Hybrid Natural Inflation model against recent CMB data, finding it requires a large positive spectral running to fit BICEP2 results, which may lead to primordial black hole overproduction.

Contribution

It demonstrates how the HNI model can be constrained using current CMB observations and highlights the implications of BICEP2 data on its viability.

Findings

HNI model compatible with CMB data under certain conditions

Large positive spectral running needed for BICEP2 compatibility

Potential overproduction of primordial black holes in the model

Abstract

We study the Hybrid Natural Inflation (HNI) model and some of its realisations in the light of recent CMB observations, mainly Planck temperature and WMAP-9 polarization, and compare with the recent release of BICEP2 dataset. The inflationary sector of HNI is essentially given by the potential $V(\phi) = V_0(1+a\cos (\frac{\phi}{f} ) )$, where $a$ is a positive constant smaller or equal to one and $f$ is the scale of (pseudo Nambu-Goldstone) symmetry breaking. We show that to describe the HNI model realisations we only need two observables; the spectral index $n_s$, the tensor-to-scalar ratio, and a free parameter in the amplitude of the cosine function $a$. We find that in order to make the HNI model compatible with the BICEP2 observations, we require a large positive running of the spectra. We find that this could over-produce primordial black holes in the most consistent case of the model. This situation could be aleviated if, as recently argued, the BICEP2 data do not correspond to primordial gravitational waves.