ACT-Era Constraints on Single-Field Inflation in $f(T)$ Teleparallel Gravity

We reassess single-field slow-roll inflation in teleparallel gravity with $f(T)=C\,T^{2δ+1}$, motivated by recent measurements from the Atacama Cosmology Telescope (ACT) that indicate a modest upward shift in the scalar spectral index $n_s$. Using analytic approximations together with high-precision numerical calculations, we compute primordial predictions for representative potentials: power-law monomials, hilltop models, and $E$-type plateaus. We find that torsional corrections controlled by $δ$ generically suppress $r$ while keeping $n_s$ near its slow-roll value. As a result, modest positive $δ$ can restore viability to sub-quadratic monomials and hilltop models that are disfavored in general relativity (GR), whereas $E$-type plateaus remain compatible only in a limited range of $δ$: small $δ$ may improve the fit but moderate $δ$ drives the dynamics toward quadratic-like behaviour and increases $r$. These signatures are observationally testable: improved cosmic microwave background (CMB) B-mode measurements will further discriminate among potential classes and place quantitative bounds on torsional deviations from GR.