The CDM isocurvature perturbation in the curvaton scenario

TL;DR

This work analyzes residual isocurvature perturbations in the curvaton scenario, showing that when curvaton decay happens after the creation of a given quantity (CDM, baryon number, or lepton number), a fully correlated isocurvature perturbation arises, ${\cal S}_i = s_i \, {\\zeta}$. It derives a general method to compute these residual perturbations from the unperturbed background via the function $n_i(\\rho_r,\\rho_\\sigma)$ and the parameter $r$, and applies it to CDM candidates in the sudden-decay approximation. The authors conclude that CDM created before curvaton decay generically yields large residuals, with viability depending on the specific creation epoch and CDM type: WIMP and axion CDM are typically disfavored unless created after decay or in fine-tuned regimes, while CDM from a fixed-mass oscillating field can admit an observable residual in certain parameter ranges. The results highlight the importance of the creation history in constraining curvaton models and offer a concrete framework for linking particle-physics CDM scenarios to CMB/isocurvature observations.

Abstract

We discuss the residual isocurvature perturbations, fully-correlated with the curvature perturbation, that are automatic in the curvaton scenario if curvaton decay is sufficiently late. We contrast these residual isocurvature perturbations with the generally un-correlated `intrinsic' isocurvature perturbation generated by an additional field such as the axion. We present a general formula for the residual isocurvature perturbations, referring only to the generation of the relevant quantity (Cold Dark Matter, baryon number or lepton number) in an unperturbed universe. Specific formulas for the residual isocurvature CDM perturbation are given, for most of the commonly-considered CDM candidates.