Secluded Dark Sector and Muon $(g-2)$ in the Light of Fast Expanding Universe

Abstract

The lack of information before Big Bang Neucleosynthesis (BBN) allow us to assume the presence of a new species $\varphi$ whose energy density redshifts as $a^{-(4+n)}$ where $n>0$ and $a$ is the scale factor. This non-standard cosmological setup facilitates a larger portal coupling $(ϵ)$ between the dark and the visible sectors even when the two sectors are not in thermal equilibrium. Here, we have considered $U(1{)}_{L_{\mu }-L_{\tau }}\otimes U(1{)}_X$ gauge extension of the Standard Model (SM) and studied different phases of the cosmological evolution of a thermally decoupled dark sector such as leak-in, freeze-in, reannihilation, and late-time annihilation in the presence of fast expansion. Due to the tree level kinetic mixing between $U(1{)}_X$ and $U(1{)}_{L_{\mu }-L_{\tau }}$ gauge bosons, the dark sector couples with the $\mu$ and $\tau$ flavored leptons of the SM. We show that in our scenario it is possible to reconcile the dark matter relic density and muon $(g-2)$ anomaly. In particular, we show that for $2\times 10^{-4}\lesssim ϵ\lesssim 10^{-3}$, $5.5\mathrm{MeV}\lesssim m_{Z^{\prime }}\lesssim 200\mathrm{MeV}$, $n=4$, and $1\mathrm{TeV}\lesssim m_{\chi }\lesssim 10\mathrm{TeV}$ relic density constraint of dark matter, constraint from muon $(g-2)$ anomaly, and other cosmological, astrophysical constraints are satisfied.

Comments: Fig.9 modified, minor changes in conclusion, reference list updated, published in JCAP