Higgs-like inflation under ACTivated mass

Abstract: Recent analyses that combine the latest data from the Atacama Cosmology Telescope (ACT) with cosmic microwave background observations by BICEP/Keck and Planck, together with the DESI baryon acoustic-baryonic-oscillation (BAO) measurements, have tightened the limits on inflationary scenarios. The joint data set yields a spectral index of primordial scalar perturbations $n_s = 0.9743 \pm 0.0034$ and an upper bound on the tensor-to-scalar ratio of $r < 0.038$. This slight upward shift in $n_s$ puts the previously favored Starobinsky model, and the conventional metric Higgs(-like) inflation--based on a quartic potential with a large non-minimal coupling in the Jordan frame--under tension with observations. In metric Higgs-like inflation the attractor behavior makes the predictions remarkably stable against higher-order operators, so modifying $n_s$ through such terms is difficult. In this paper, I show that adding a quadratic mass term to the Jordan-frame potential can raise $n_s$ and restore compatibility with the new data. I also discuss how this mass term can naturally arise from threshold effects in Higgs inflation.