$\mathbf{{N}_{eff}}$从激发DM状态
$\mathbf{{N}_{eff}}$ from Excited DM state
摘要：
对于起源于共湮灭过程的冷暗物质（DM），在暗扇区中将存在激发态，这些激发态可能在冻结后不久衰减或湮灭。在本文中，我们调查的影响，这些激发态的衰变的中微子物种的有效数，$N_{\rm eff}^{}$，这是一个重要的宇宙学参数，并将测试未来的CMB-S4项目。我们在赝狄拉克DM模式的框架下通过希格斯门进行研究。计算了遗迹密度和直接探测信号，给出了有效的参数空间。通过求解光子与中微子的Boltzmann方程，研究了激发态（赝狄拉克费米子的重组分）对主动中微子退耦的影响，其中碰撞项是由激发态的衰变引起的.计算结果表明，$|N_{\rm eff}^{}|$可以是$10^{-4}$量级，其量级与DM激发态的质量和寿命有关.
英文摘要：
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.
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