Isocurvature-induced features in multi-field Higgs-$R^2$ inflation

TL;DR

This paper investigates how multifield dynamics in Higgs-$R^2$ inflation with kinetic mixing influence primordial perturbations, revealing regimes with distinct features and isocurvature effects that impact CMB observations.

Contribution

It provides a numerical analysis of multifield Higgs-$R^2$ inflation, identifying how non-minimal kinetic mixing affects primordial spectra and constrains inflation models.

Findings

Transient trajectory turning causes localized features in the curvature spectrum.

Weak coupling regime results in residual isocurvature perturbations.

The study computes CMB spectra to assess observational signatures.

Abstract

We study primordial perturbations in Higgs--$R^2$ inflation in the presence of non-minimal kinetic mixing between the Higgs field and the scalaron. By numerically solving the multifield background and linear perturbation equations, we identify distinct dynamical regimes controlled by the Higgs non-minimal coupling $\xi_h$. For $\xi_h \sim \mathcal{O}(0.1)$, transient turning of the inflationary trajectory leads to a transfer between adiabatic and isocurvature modes, generating localized features in the primordial curvature power spectrum. In contrast, in the weak-coupling regime $\xi_h \ll 1$, the curvature spectrum remains nearly featureless while isocurvature perturbations do not fully decay, resulting in a residual isocurvature component at the end of inflation. We compute the associated CMB angular power spectra and discuss the observational implications of these regimes. Our results highlight the role of multifield dynamics in shaping primordial perturbations and provide constraints on viable realizations of Higgs--$R^2$ inflation.