FIMP Dark Matter from Leptogenesis in Fast Expanding Universe

TL;DR

This paper explores how a faster-than-standard universe expansion affects leptogenesis and FIMP dark matter production, revealing significant impacts on baryon asymmetry and dark matter abundance, especially in strong washout scenarios.

Contribution

It introduces a model with an additional species causing faster universe expansion and analyzes its effects on leptogenesis and FIMP dark matter, providing new insights into cosmological impacts.

Findings

Fast expansion significantly alters baryon asymmetry in strong washout.

FIMP dark matter relic density is affected by the faster expansion rate.

Distinct cosmological scenarios can be distinguished in strong washout cases.

Abstract

Within the framework of canonical type-I seesaw, a feebly interacting massive particle (FIMP) $\chi$ is introduced as a dark matter candidate. The leptogenesis mechanism and dark matter relic density share a common origin via decays of Majorana neutrinos $N$. Provided an additional species $\varphi$ whose energy density red-shifts as $\rho_{\varphi}\propto a^{-(4+n)}$, the Hubble expansion rate is larger than the standard scenario, i.e., the Universe expands faster. The consequences of such a fast expanding Universe on leptogenesis as well as FIMP dark matter are investigated in detail. We demonstrate a significant impact on the final baryon asymmetry and dark matter abundance due to the existence of $\varphi$ for the strong washout scenario. While for the weak washout scenario, the effects of FEU are relatively small. We introduce scale factors $F_L$ and $F_\chi$ to describe the corresponding effects of FEU. A semi-analytical approach to derive the efficiency factors $\eta_L$ and $\eta_\chi$ in FEU is also discussed. The viable parameter space for success thermal leptogenesis and correct FIMP DM relic density is obtained for standard cosmology and FEU. Our results show that it is possible to distinguish different cosmology scenarios for strong washout cases.