Heavy Field Effects on Inflationary Models in Light of ACT Data

Abstract: Recent results from the Atacama Cosmology Telescope (ACT), when combined with Planck and DESI datasets, indicate a scalar spectral index $n_s$ larger than that reported in the Planck 2018 baseline, thereby challenging conventional Starobinsky-type ($\alpha$-attractor) inflationary scenarios at the $2\sigma$ level. In addition, the positive running of the spectral index $\alpha_s$ implied by the data provides strong constraints on these models. In this paper, we explore the possibility that the presence of an additional heavy field during inflation, with a mass of order the Hubble scale and a sizable mixing coupling to the inflaton, can reconcile such inflationary models with the ACT results by increasing both $n_s$ and $\alpha_s$, particularly in the strong-mixing regime. Furthermore, we extend this framework to traditional inflation models such as chaotic inflation and natural inflation, which have already been excluded by Planck alone, and show that they can be revived in certain regions of parameter space. Inflationary observables, including the spectral index $n_s$, the tensor-to-scalar ratio $r$, and the running $\alpha_s$, are computed within the single-field EFT approach, which is applicable even in the presence of a heavy field with large mixing. We also discuss the non-Gaussianity signatures arising from the heavy field, noting that parts of the parameter space are already excluded or can be tested in future observations. Finally, we present concrete model realizations that allow for such a large mixing.