Neff from an Excited Dark Matter State

For a cold dark matter (DM) originating from the coannihilation 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, Neff, 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 a collision term induced by the decay of the excited state. The numerical result shows that the maximum value of j Delta Neffj can reach the order Oo0.1 thorn with suitable mass and lifetime of the excited DM state, which may be probed by future CMB-S4 and SPT/SO projects.