[论文解读] Dark matter annihilation at cosmological redshifts: possible relic signal from weakly interacting massive particles annihilation

A. N. Baushev|arXiv (Cornell University)|Jun 18, 2008

Dark Matter and Cosmic Phenomena参考文献 1被引用 2

一句话总结

本文提出，早期宇宙中弱相互作用大质量粒子（WIMPs）的湮灭产生了遗迹河外伽马射线背景，其峰值能量约为8 MeV，现今仍可探测。该信号源于宇宙在红移 z ≈ 300 时变得透明时所发射的湮灭产物，若暗物质结构在再结合之前形成，则在 0.5–20 MeV 范围内可探测到可观测的流量。

ABSTRACT

We discuss the possibility to observe the products of dark matter annihilation that was going on in the early Universe. Of all the particles that could be generated by this process we consider only photons, as they are both uncharged and easily detectable. The earlier the Universe was, the higher the dark matter concentration $n$ and the annihilation rate (proportional to $n^2$) were. However, the emission from the very early Universe cannot reach us because of the opacity. The main part of the signal was generated at the moment the Universe had just become transparent for the photons produced by the annihilation. Thus, the dark matter annihilation in the early Universe should have created a sort of relic emission. We obtain its flux and the spectrum. If weakly interacting massive particles (WIMPs) constitute dark matter, it is shown that we may expect an extragalactic gamma-ray signal in the energy range 0.5 - 20 {MeV} with a maximum near 8 {MeV}. We show that an experimentally observed excess in the gamma-ray background at 0.5 - 20 {MeV} could be created by the relic WIMPs annihilation only if the dark matter structures in the universe had appeared before the universe became transparent for the annihilation products ($z \simeq 300$). We discuss in more detail physical conditions whereby this interpretation could be possible.

研究动机与目标

• 研究早期宇宙中暗物质湮灭是否可能产生可探测的遗迹伽马射线背景。
• 确定此类信号在现今可被探测到的红移及物理条件。
• 评估通过WIMP湮灭解释0.5–20 MeV能量段河外伽马射线背景观测过剩的可行性。
• 对WIMP湮灭产生的遗迹信号谱和流量进行建模，重点关注光子产生过程。

提出的方法

• 将暗物质湮灭速率建模为与暗物质数密度 n² 成正比，且随红移减小而增加。
• 计算WIMP湮灭产生的光子发射谱，仅关注光子，因其具有高可探测性且不带电荷。
• 通过确定宇宙对湮灭光子变得透明的时期，确定最大发射的宇宙 epoch，估计为 z ≈ 300。
• 在WIMPs构成暗物质组分的假设下，计算由此产生的河外伽马射线流量和谱。
• 通过将预测流量与观测到的伽马射线背景水平进行比较，评估该信号的可探测性。

实验结果

研究问题

• RQ1 早期宇宙中WIMP湮灭产生的河外伽马射线背景的预期谱和流量为何？
• RQ2 遗迹信号的最大贡献发生在哪个红移？该时期的物理条件如何定义？
• RQ3 在何种条件下，0.5–20 MeV能量段观测到的伽马射线背景过剩可归因于遗迹WIMP湮灭？
• RQ4 在再结合之前形成暗物质结构如何影响遗迹湮灭信号的可探测性？

主要发现

• WIMP湮灭产生的遗迹伽马射线信号在能量谱中约在8 MeV处达到峰值。
• 该信号的主要贡献来自宇宙对光子变得透明的时期，即红移 z ≈ 300 左右。
• 若暗物质结构在 z ≈ 300 之前形成，则0.5–20 MeV范围内的河外伽马射线背景观测过剩可由遗迹WIMP湮灭解释。
• 预测的遗迹信号流量位于0.5–20 MeV范围内，因此可能被未来的高灵敏度伽马射线仪器探测到。
• 湮灭速率与 n² 成正比，在早期宇宙中最高，但光子不透明性阻止了更早时期发射的辐射到达地球。
• 该信号对WIMPs作为暗物质候选者最为显著，其光子谱由湮灭过程和宇宙学膨胀共同决定。