Imprints of MeV Scale Hidden Dark Sector at Planck Data

TL;DR

This paper investigates how an MeV-scale hidden dark sector influences the effective number of relativistic species ($N_{ m eff}$) measured by Planck, revealing significant impacts especially with non-hierarchical dark particle masses.

Contribution

It introduces a detailed analysis of MeV-scale dark sectors' effects on $N_{ m eff}$ and constrains model parameters using Planck data on dark matter and relativistic degrees of freedom.

Findings

Dark sector dynamics significantly affect $N_{ m eff}$ for non-hierarchical mass spectra.

The allowed parameter space is constrained by Planck measurements of $N_{ m eff}$ and dark matter relic density.

The impact on $N_{ m eff}$ can be substantial, providing potential signatures for hidden dark sectors.

Abstract

New light species can contribute to the number of effective relativistic degrees of freedom ($N_{\rm eff}$) at Cosmic Microwave Background (CMB) which is precisely measured by Planck. In this work, we consider an MeV scale thermally decoupled non-minimal dark sector and study the imprint of the dark sector dynamics on the measurement of $N_{\rm eff}$ at the time of CMB formation. We have predicted the allowed region of model parameter space in the light of constraints arising from the measurements of both $N_{\rm eff}$ and dark matter relic density by Planck. It turns out that the impact of the dark sector dynamics on $N_{\rm eff}$ is significant in case of a non-hierarchical mass spectrum of the dark sector particles.