Nonminimal coupling in light of ACT data

The latest ACT data release disfavors the attractor $n_s=1-2/N$. In inflationary models with nonminimal coupling, such attractors typically arise in the strong coupling limit. To align with observational constraints, we focus on nonminimal coupling models with small coupling constants. For the model with the coupling function $\Omega \left(\varphi \right)=1+\xi f\left(\varphi \right)$ and the potential $V\left(\varphi \right)={\lambda }^2{f}^2\left(\varphi \right)$, we find that observational data constrain the parameters as $0.1\lesssim \xi \lesssim 35$ and $0\lesssim k\lesssim 1.5$ for $f\left(\varphi \right)={\varphi }^k$ at the $1\sigma$ confidence level. With the help of the nonminimal coupling $\Omega \left(\varphi \right)=1+\xi {\varphi }^2$, the hilltop inflation and power-law inflation models with power indices $2/3$ and $1/3$ can be consistent with observational data within the $1\sigma$ range. We also give the viable parameter regions of $\xi$ for these three models.