Radiatively Corrected Starobinsky Inflation and Primordial Gravitational Waves in Light of ACT Observations

Abstract: Nonminimal coupling between the inflaton and the Ricci scalar plays a crucial role in shaping the predictions of single-field inflationary models. While a quartic potential with such coupling represents one of the simplest realizations compatible with cosmological observations, it generically receives important radiative corrections when the inflaton interacts with other fields, particularly those involved in the reheating process. In this work, we focus on radiative corrections arising from bosonic scalar couplings and study their impact on inflationary dynamics within the nonminimally coupled quartic potential framework. We demonstrate that bosonic corrections, unlike fermionic ones, yield predictions more compatible with the latest constraints from the Atacama Cosmology Telescope (ACT) Data Release 6, especially when combined with Planck and BICEP/Keck data. We begin with a general model involving a gauge-singlet real scalar inflaton coupled to a complex scalar field, which could be the Standard Model Higgs or a GUT Higgs. As a concrete realization, we also investigate the case where the inflaton serves as a dark matter candidate through a Higgs portal interaction while highlighting its potential to generate observable levels of primordial gravitational waves. Notably, the nonminimally coupled inflation model studied here is field-theoretically equivalent to the Starobinsky model, and the inclusion of quantum corrections from scalar fields leads to characteristic imprints in the $(n_s, r)$ plane, allowing for refined constraints on scalar couplings from current and future observations.