We consider a scenario in which the inflaton $\varphi$ is a pseudoscalar field nonminimally coupled to gravity through a term of the form $\mathcal{X}R{\varphi }^2$. The pseudoscalar is also coupled to a $U(1)$ gauge field (or an ensemble of $\mathcal{N}$ gauge fields) through an axial coupling of the form $\varphi F\tilde{F}$. After [M. M. Anber and L. Sorbo, Phys. Rev. D 81, 043534 (2010)], it is well known that this axial coupling leads to a production of gauge particles which acts as a friction term in the dynamics of the inflaton, producing a slow-roll regime even in presence of a steep potential. A remarkable result in this scenario is that the spectrum of the chiral gravitational waves sourced by the scalar-gauge field interplay can be enhanced due to the nonminimal coupling with gravity, leading to measurable signatures, while maintaining agreement with current observational constraints on $n_s$ and $r$. The inclusion of nonminimal coupling could be helpful to alleviate tensions with non-Gaussianity bounds in models including axial couplings.

• Received 2 July 2018

DOI:https://doi.org/10.1103/PhysRevD.98.083519