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 $R^2$ 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.