arXiv 2108.027852021-08-05hep-phastro-ph.COastro-ph.GAastro-ph.HE

宇宙学简并费米子

Cosmologically Degenerate Fermions

Marcela Carena, Nina M. Coyle, Ying-Ying Li, Samuel D. McDermott, Yuhsin Tsai

备注：: 7 pages, 4 figures

摘要：

质量低于几keV的暗物质（DM）的相空间分布必须与标准模型粒子的热相空间有很大的不同：否则，它将在宇宙结构形成时自由流出。我们观察到，即使在完全没有热动能的情况下，在这个质量范围内的费米DM psi在早期宇宙中也将具有不可忽略的动量。这是因为费米子在更高的红移下不可避免地更密集，因此经历了泡利简并压力。它们填满了最低动量态，这样一个典型的费米子获得的动量~ O（p_F）可以超过它的质量m_psi。我们发现，在轻费米子中冷DM能量密度的电流分数f_psi与它们是相对论性的红移之间有一个简单的关系。考虑到DNeff和物质功率谱测量所揭示的非相对论性和相对论性行为之间的转变的影响，我们在f_psi-m_psi平面上定性地导出了新的边界。我们还改进了现有的边界为f_psi = 1的数量级为m_psi=2 keV。我们评论直接检测的影响，并建议模型的暗部门，可能会引起宇宙学退化费米子。

英文摘要：

Dark matter (DM) with a mass below a few keV must have a phase space distribution that differs substantially from the Standard Model particle thermal phase space: otherwise, it will free stream out of cosmic structures as they form. We observe that fermionic DM psi in this mass range will have a non-negligible momentum in the early Universe, even in the total absence of thermal kinetic energy. This is because the fermions were inevitably more dense at higher redshifts, and thus experienced Pauli degeneracy pressure. They fill up the lowest-momentum states, such that a typical fermion gains a momentum ~ O(p_F) that can exceed its mass m_psi. We find a simple relation between m_psi, the current fraction f_psi of the cold DM energy density in light fermions, and the redshift at which they were relativistic. Considering the impacts of the transition between nonrelativistic and relativistic behavior as revealed by measurements of DNeff and the matter power spectrum, we derive qualitatively new bounds in the f_psi-m_psi plane. We also improve existing bounds for f_psi = 1 by an order of magnitude to m_psi=2 keV. We remark on implications for direct detection and suggest models of dark sectors that may give rise to cosmologically degenerate fermions.

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