CMB signature of non-thermal Dark Matter produced from self-interacting dark sector

Abstract

The basic idea of this work is to achieve the observed relic density of a non-thermal dark matter(DM) and its connection with Cosmic Microwave Background (CMB) via additional relativistic degrees of freedom which are simultaneously generated during the period $T_{\mathrm{BBN}}\mathrm{to}{T}_{\mathrm{CMB}}$ from a long-lived dark sector particle. To realize this phenomena we minimally extend the type-I seesaw scenario with a Dirac fermion singlet($\chi$) and a complex scalar singlet ($\phi$) which transform non-trivially under an unbroken symmetry ${\mathcal{Z}}_3$. $\chi$ being the lightest particle in the dark sector acts as a stable dark matter candidate while the next to lightest state $\phi$ operates like a long lived dark scalar particle. The initial density of $\phi$ can be thermally produced through either self-interacting number changing processes ($3\phi \to 2\phi$) within dark sector or the standard annihilation to SM particles ($2\phi \to 2\mathrm{SM}$). The late time (after neutrino decoupling) non-thermal decay of $\phi$ can produce dark matter in association with active neutrinos. The presence of extra relativistic neutrino degrees of freedom at the time of CMB can have a significant impact on $\Delta {\mathrm{N}}_{\mathrm{eff}}$. Thus the precise measurement of $\Delta {\mathrm{N}}_{\mathrm{eff}}$ by current PLANCK 2018 collaboration and future experiments like SPT-3G and CMB-S4 can indirectly probe this non-thermal dark matter scenario which is otherwise completely secluded due to its tiny coupling with the standard model.

Comments: Accepted for publication in JCAP