Studying G-axion Inflation model in light of PLANCK

TL;DR

This paper demonstrates that a simple Galileon modification to axion inflation models can make them compatible with Planck data, with predictions independent of the axion decay constant, and explores post-inflation dynamics and reheating mechanisms.

Contribution

Introducing a Galileon-type modification to axion inflation models to align their predictions with Planck data and analyzing the post-inflation dynamics and reheating processes.

Findings

Modified G-axion models fit Planck data within 1σ range.

Post-inflation dynamics depend on the axion decay constant.

Instant preheating is inefficient for sub-Planckian models.

Abstract

With the Planck 2015 result, most of the well known canonical large field inflation models turned out to be strongly disfavored. Axion inflation is one of such models which is becoming marginalized with the increasing precession of CMB data. In this paper, we have shown that with a simple Galileon type modification to the marginally favored axion model calling G-axion, we can turn them into one of the most favored models with its detectable prediction of $r$ and $n_s$ within its PLANCK $1\sigma $ range for a wide range of parameters. Interestingly it is this modification which plays the important role in turning the inflationary predictions to be independent of the explicit value of axion decay constant $f$. However, dynamics after the inflation turned out to have a non-trivial dependence on $f$. For each G-axion model there exists a critical value of $f_c$ such that for $f>f_c$ we have the oscillating phase after inflation and for $f<f_c$ we have non-oscillatory phase. Therefore, we obtained a range of sub-Planckian value of model parameters which give rise to consistent inflation. However for sub-Planckian axion decay constant the inflaton field configuration appeared to be singular after the end of inflation. To reheat the universe we, therefore, employ the instant preheating mechanism at the instant of first zero crossing of the inflaton. To our surprise, the instant preheating mechanism turned out to be inefficient as opposed to usual non-oscillatory quintessence model. For another class of G-axion model with super-Planckian axion decay constant, we performed in detail the reheating constraints analysis considering the latest PLANCK result.