再电离和河外γ背景对轻子湮灭暗物质的限制
Constraints on leptonically annihilating Dark Matter from reionization and extragalactic gamma background
摘要：
PAMELA、Fermi和HESS实验（PFH）已经显示出宇宙正电子和电子通量的异常过剩。一个非常令人兴奋的可能性是，这些过剩是由于湮灭暗物质（DM）。本文利用河外弥散γ射线背景的观测资料和末次散射面的光学厚度测量值，计算了轻子湮灭DM的约束条件，并与m_{DM} - <σ_AV>平面上PFH有利区的约束条件进行了比较。使用PYTHIA蒙特卡罗工具指定了能量输入的详细形式后，我们求解了辐射转移方程，这使我们能够确定宇宙介质吸收的能量以及剩余的漫伽玛背景能量。我们发现，对于τ^{-}+τ^{+}湮灭通道，来自光学深度测量的约束能够完全排除PFH有利区，而对于μ^{-}+μ^{+}湮灭通道，则几乎完全排除PFH有利区。事实证明，这些约束是相当强大的，几乎不依赖于低红移集群提升。来自伽马射线背景的约束对假设的晕浓度模型敏感，并且对于幂律模型，排除了PFH对所有轻子湮灭通道有利的区域。我们还发现，有可能有模型，完全符合宇宙在低红移。然而，这些模型在z > ~100处产生太大的自由电子分数，并且与光学深度测量相冲突。此外，在这些情况下的湮没截面的大小是大于PFH数据所建议的。
英文摘要：
The PAMELA, Fermi and HESS experiments (PFH) have shown anomalous excesses in the cosmic positron and electron fluxes. A very exciting possibility is that those excesses are due to annihilating dark matter (DM). In this paper we calculate constraints on leptonically annihilating DM using observational data on diffuse extragalactic gamma-ray background and measurements of the optical depth to the last-scattering surface, and compare those with the PFH favored region in the m_{DM} - <σ_A v> plane. Having specified the detailed form of the energy input with PYTHIA Monte Carlo tools we solve the radiative transfer equation which allows us to determine the amount of energy being absorbed by the cosmic medium and also the amount left over for the diffuse gamma background. We find that the constraints from the optical depth measurements are able to rule out the PFH favored region fully for the τ^{-}+τ^{+} annihilation channel and almost fully for the μ^{-}+μ^{+} annihilation channel. It turns out that those constraints are quite robust with almost no dependence on low redshift clustering boost. The constraints from the gamma-ray background are sensitive to the assumed halo concentration model and, for the power law model, rule out the PFH favored region for all leptonic annihilation channels. We also find that it is possible to have models that fully ionize the Universe at low redshifts. However, those models produce too large free electron fractions at z > ~100 and are in conflict with the optical depth measurements. Also, the magnitude of the annihilation cross-section in those cases is larger than suggested by the PFH data.
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