$\mathbf{{N}_{eff}}$ from Excited DM state

TL;DR

This paper studies how the decay of excited dark matter states in a pseudo-Dirac model affects the cosmological parameter $N_{ m eff}$, with potential implications for future CMB observations.

Contribution

It introduces a detailed analysis of the impact of excited dark matter decay on $N_{ m eff}$ within a Higgs portal pseudo-Dirac DM framework, including relic density and direct detection calculations.

Findings

$|N_{ m eff}|$ can be of order $10^{-4}$ depending on DM parameters.

Decay of excited states influences neutrino decoupling.

Results are relevant for future CMB measurements.

Abstract

For a cold dark matter (DM) originating from the co-annihilation processes, there will be excited state(s) in the dark sector, that may decay or annihilate away shortly after their freeze-out. In this paper we investigate the impact of the decay of these excited states on the effective number of neutrino species, $N_{\rm eff}^{}$, which is an important cosmological parameter and will be tested by the future CMB-S4 project. We work in the framework of pseudo-Dirac DM mode via the Higgs portal. The relic density and the direct detection signal of the DM are calculated, which gives the available parameter space. Impacts of the excited state (the heavy component of the pseudo-Dirac fermion) on the decoupling of active neutrinos are investigated by solving Boltzmann equations of photon and neutrinos with collision term induced by the decay of the excited state. The numerical result shows that $|N_{\rm eff}^{}|$ can be of the order $10^{-4}$, which depends on the mass and lifetime of excited DM state.