Natural-Scalaron Inflation
Abstract
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 R2 term in the action, which leads to another inflaton candidate because of the equivalence between the R2 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.