Studying G-axion Inflation model in light of PLANCK
  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.
