Single-Field Inflation in Light of ACT DR6: A New Potential and the Challenge of Positive Running

The ACT DR6 combination considered in this work yields a higher scalar spectral index than the Planck 2018 baseline, motivating us to examine whether canonical single-field inflation can accommodate the ACT-preferred region. We study the rational plateau potential [Formula: see text] which interpolates between power-law and plateau regimes. We compute the scalar spectral index [Formula: see text] and tensor-to-scalar ratio [Formula: see text] at leading potential slow-roll order and the running [Formula: see text] at its leading nonzero order. For [Formula: see text], the model gives [Formula: see text] placing the prediction within the [Formula: see text] region of the ACT constraint on [Formula: see text]. The running is analytically negative for all [Formula: see text], [Formula: see text], and [Formula: see text] within the adopted slow-roll approximation. The ACT central value [Formula: see text] represents only a [Formula: see text] preference for positive running and is therefore consistent with zero. These results show that the rational plateau potential can approach the ACT-preferred scalar tilt while maintaining a small tensor amplitude, whereas a statistically significant future detection of positive running would disfavour the model.