Speaker
Description
The asymptotically flat plateau of tree-level Higgs inflation can sustain a transient ultra-slow-roll (USR) phase for specific pre-inflationary initial conditions. We systematically scan the phase space of initial field kinematics and identify trajectories where this non-attractor phase occurs near the horizon crossing of the largest observable CMB modes. Because USR violates the slow-roll attractor, we compute the primordial power spectrum by exact numerical integration of the Mukhanov–Sasaki equations, going beyond the slow-roll approximation. We find a localized suppression of the scalar power spectrum at $k \sim 5\times10^{-4}\ \mathrm{Mpc}^{-1}$, which reduces the CMB quadrupole by up to 34% relative to ΛCDM prediction while preserving the acoustic peak structure. The resulting temperature spectrum improves the low-$\ell$ Planck fit while remaining statistically indistinguishable from ΛCDM across the full spectrum ($\Delta\chi^2 = +40.7$ over 2507 degrees of freedom). We also show that standard slow-roll estimates of the spectral index become unreliable near the onset of the USR transient, with deviations up to $\Delta n_s \sim 0.03$. These results reveal that even the minimal Higgs inflation setup can depart from attractor evolution and produce observable imprints at the largest cosmological scales.