ACT DR6 Insights on the Inflationary Attractor models and Reheating

Abstract

We investigate the observational constraints on $\alpha$-attractor inflationary models and their post-inflationary reheating dynamics in light of the latest CMB data from ACT DR6 combined with Planck18, BICEP/$Keck$ 2018, and DESI (P-ACT-LB-BK18). Focusing on both E- and T-type attractor potentials, we analyze how inflationary observables, namely the scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$, are indirectly influenced by reheating parameters such as the reheating temperature $T_{\text{RH}}$, the inflaton EoS $w_\phi$, and consequently the inflaton's couplings to SM particles. We incorporate constraints from PGW overproduction via $\Delta N_{\rm eff}$ bounds. Additionally, we include theoretical constraints from CW one-loop radiative corrections, as well as inflaton self-resonance, a non-perturbative effect that can occur even in the absence of inflaton-SM interactions and imposes a strong lower limit, particularly significant for $1/3\lesssim w_\phi\lesssim 0.7$. Our analysis shows that E-models allow a broad range of reheating scenarios, including matter-like reheating ($w_\phi=0$), whereas the T-model yields more restrictive results and remains viable only for $w_\phi>1/2$ for all three non-gravitational interaction channels, $\phi\to\bar{f}f$, $\phi\to bb$, and $\phi\phi \to bb$ are considered. For stiffer EoS, an intermediate coupling window survives, bounded from above by CW corrections or $2\sigma$ observational limits, and from below by the combined PGW and self-resonance constraints. We derive updated bounds on inflaton couplings for both decay and scattering channels and identify parameter regions consistent with successful perturbative reheating. These results establish a robust connection between inflationary dynamics, reheating physics, and particle interactions, and provide concrete targets for upcoming precision CMB observations.