We study the effects of dimension-four and dimension-six nonminimal Higgs couplings to the Ricci scalar $R$ in the $R^2$-Higgs inflation model in light of the recent ACT and SPT observations. We show that the dimension-six operators $|\mathrm{\Phi }{|}^2{R}^2$ and $|\mathrm{\Phi }{|}^{4}R$ can accommodate the enhanced scalar spectral index $n_s$ preferred by the combined $\mathrm{CMB}+\mathrm{BAO}$ analyses. Using a doubly covariant formalism, we find that the same region of parameter space that explains the observed value of $n_s$ can also induce rapid preheating through the production of the Goldstone modes. If thermalization proceeds efficiently through this preheating mechanism, it may help match the inflationary scale with the CMB reference scale.

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• Evolution of the Universe
• Inflation
• Quantum cosmology