Pushing the Primordial Frontier: Cosmological Collider Signatures at Strong Mixing

Abstract

We develop an analytic treatment of primordial non-Gaussianity in multifield inflation that is nonperturbative in the constant curvature--isocurvature mixing strength $\lambda$. Using exact linear solutions for the coupled curvature perturbation $\zeta$ and isocurvature perturbation $\sigma$, we construct dressed propagators and derive exact integral representations for the tree-level bispectra generated by the interactions $\dot\zeta^2\sigma$, $\dot\zeta\,\sigma^2$, and $\sigma^3$. This formalism resums curvature--isocurvature transfer to all orders in $\lambda$ and applies for arbitrary values of the entropy mass $\mu$. We obtain closed-form expressions for the leading squeezed limit of the bispectrum contributions and recover the well-known cosmological-collider and quasi-single-field results in the weak-mixing limit. In the strong-mixing regime, where conventional transfer perturbation theory breaks down, we find that both the power spectrum and the bispectrum can be dramatically enhanced, giving rise to distinctive nonperturbative scaling laws for the reduced non-Gaussian amplitude. Together, these results open a new analytic window onto multifield inflation beyond the weak-coupling approximation and establish a framework for studying cosmological-collider signals, primordial-black-hole production, and loop corrections in strongly mixed inflationary dynamics.