Natural-Scalaron Inflation
  A pseudo Nambu-Goldstone boson (such as an axion-like particle) is a
theoretically well-motivated inflaton as it features a naturally flat potential
(natural inflation). This is because Goldstone's theorem protects its potential
from sizable quantum corrections. Such corrections, however, generically
generates an $R^2$ term in the action, which leads to another inflaton
candidate because of the equivalence between the $R^2$ term and a scalar field,
the scalaron, with a quasi flat potential (Starobinsky inflation). Here it is
investigated a new multifield scenario in which both the scalaron and a pseudo
Nambu-Goldstone boson are active (natural-scalaron inflation). For generality,
also a non-minimal coupling is included, which is shown to emerge from
microscopic theories. It is demonstrated that a robust inflationary attractor
is present even when the masses of the two inflatons are comparable. Moreover,
the presence of the scalaron allows to satisfy all observational bounds in a
large region of the parameter space, unlike what happens in pure-natural
inflation.
