Constraining Reheating Temperature, Inflaton-SM Coupling and Dark Matter Mass in Light of ACT DR6 Observations

Abstract: We explore the phenomenological implications of the latest Atacama Cosmology Telescope (ACT) DR6 observations, in combination with Planck 2018, BICEP/Keck 2018, and DESI, on the physics of inflation and post-inflationary reheating. We focus on the $\alpha$-attractor class of inflationary models (both E- and T-models) and consider two reheating scenarios: perturbative inflaton ($\phi$) decay ($\phi \rightarrow bb$) and inflaton annihilation ($\phi \phi \rightarrow bb$) into Standard Model (SM) bosonic particles ($b$). By solving the Boltzmann equations, we derive bounds on key reheating parameters, including the reheating temperature, the inflaton equation of state (EoS), and the inflaton-SM coupling, in light of ACT data. To accurately constrain the coupling, we incorporate the Bose enhancement effect in the decay width. To ensure the validity of our perturbative approach, we also identify the regime where nonperturbative effects, such as parametric resonance, become significant. Additionally, we include indirect constraints from primordial gravitational waves (PGWs), which can impact the effective number of relativistic species, $\Delta N_{\rm eff}$. These constraints further bound the reheating temperature, particularly in scenarios with a stiff EoS. Finally, we analyze dark matter (DM) production through purely gravitational interactions during reheating and determine the allowed mass ranges consistent with the constrained reheating parameter space and recent ACT data. Our results highlight the constraining power of next-generation CMB datasets on inflationary dynamics, reheating, and the DM parameter space.