Imprints of MeV Scale Secluded Dark Sector at Planck Data

New light species can contribute to the number of effective relativistic degrees of freedom (Neff) at Cosmic Microwave Background (CMB) which is precisely measured by Planck.  We first discuss the cosmological constraints on MeV scale thermal dark matter from the current Planck data. Next we consider an MeV scale *thermally decoupled* non-minimal dark sector and study the imprint of the dark sector dynamics on the  Neff 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 Neff and dark matter relic density by Planck. It turns out that  the impact of the dark sector dynamics on Neff is significant in  case of a non-hierarchical mass spectrum of the dark sector particles.