A new U(1) gauge symmetry is the simplest extension of the Standard Model and has various theoretical and phenomenological motivations. In this paper, we study the cosmological constraint on the MeV scale dark photon. After the neutrino decoupling era at $T = \mathcal{O}(1)\,$MeV, the decay and annihilation of the dark photon heats up the electron and photon plasma and accordingly decreases the effective number of neutrino $N_{\mathrm{eff}}$ in the recombination era. We derive a conservative lower-limit of the dark photon mass around 8.5 MeV from the current Planck data if the mixing between the dark photon and ordinary photon is larger than $\mathcal{O}(10^{-9})$. We also find that the future CMB stage-$\rm I\! V$ experiments can probe up to 17 MeV dark photon.

中文翻译：

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内夫暗光子的宇宙学约束

新的 U(1) 规范对称性是标准模型的最简单扩展，具有各种理论和现象学动机。在本文中，我们研究了 MeV 尺度暗光子的宇宙学约束。在 $T = \mathcal{O}(1)\,$MeV 的中微子解耦时代之后，暗光子的衰变和湮灭加热了电子和光子等离子体，从而减少了中微子的有效数量 $N_{\mathrm {eff}}$ 在重组时代。如果暗光子和普通光子之间的混合大于 $\mathcal{O}(10^{-9})$，我们可以从当前的普朗克数据中推导出一个保守的暗光子质量下限约为 8.5 MeV。我们还发现未来CMB阶段-$\rm I\! V$ 实验可以探测高达 17 MeV 的暗光子。