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arXiv 1111.0605astro-ph.COhep-ph
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大爆炸核合成过程中粒子衰变产生的暗辐射

Dark Radiation from Particle Decays during Big Bang Nucleosynthesis

Justin L. Menestrina, Robert J. Scherrer

备注：
6 pages, 3 figures, added figure and discussion of observational constraints, to appear in Phys. Rev. D
发表信息：
Phys.Rev.D85:047301,2012
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摘要：

宇宙微波背景（CMB）的观测表明，额外的暗辐射成分的可能性，而目前的证据从大爆炸核合成（BBN）是更加模糊。来自衰变粒子的暗辐射可以不同地影响这两个过程。早期衰变会给CMB和BBN增加额外的辐射成分，而晚期衰变会改变CMB中的辐射含量，而对BBN的影响可以忽略不计。在这里，我们量化了这种差异，并通过检查BBN期间的粒子衰变来探索中间状态，即，粒子寿命τ_X满足0.1秒< τ_X < 1000秒。我们计算了由CMB测量的中微子物质的有效数N_{eff}的变化Δ N_{CMB}，以及由BBN测量的中微子物质的有效数的变化Δ N_{BBN}，作为衰变粒子初始能量密度和寿命的函数，其中Δ N_{BBN}是根据在原始氦中产生相同变化所需的额外双组分中微子的数量来定义的-4丰度作为我们的衰变粒子。正如预期的那样，对于短寿命（τ_X < 0.1 sec），粒子在BBN开始之前衰变，并且Δ N_{CMB} = Δ N_{BBN}，而对于长寿命（τ_X > 1000 sec），Δ N_{BBN}由非相对论粒子在衰变之前的能量密度决定，因此Δ N_{BBN}保持非零，并且变得与粒子寿命无关。通过改变粒子能量密度和寿命两者，可以获得Δ N_{BBN}和Δ N_{CMB}的任何期望的组合，服从Δ N_{CMB} >= Δ N_{BBN}的约束。我们提出了由观测约束对Δ N_{CMB}和Δ N_{BBN}导出的衰变粒子参数的限制。

英文摘要：

Cosmic microwave background (CMB) observations suggest the possibility of an extra dark radiation component, while the current evidence from big bang nucleosynthesis (BBN) is more ambiguous. Dark radiation from a decaying particle can affect these two processes differently. Early decays add an additional radiation component to both the CMB and BBN, while late decays can alter the radiation content seen in the CMB while having a negligible effect on BBN. Here we quantify this difference and explore the intermediate regime by examining particles decaying during BBN, i.e., particle lifetimes τ_X satisfying 0.1 sec < τ_X < 1000 sec. We calculate the change in the effective number of neutrino species, N_{eff}, as measured by the CMB, ΔN_{CMB}, and the change in the effective number of neutrino species as measured by BBN, ΔN_{BBN}, as a function of the decaying particle initial energy density and lifetime, where ΔN_{BBN} is defined in terms of the number of additional two-component neutrinos needed to produce the same change in the primordial helium-4 abundance as our decaying particle. As expected, for short lifetimes (τ_X < 0.1 sec), the particles decay before the onset of BBN, and ΔN_{CMB} = ΔN_{BBN}, while for long lifetimes (τ_X > 1000 sec), ΔN_{BBN} is dominated by the energy density of the nonrelativistic particles before they decay, so that ΔN_{BBN} remains nonzero and becomes independent of the particle lifetime. By varying both the particle energy density and lifetime, one can obtain any desired combination of ΔN_{BBN} and ΔN_{CMB}, subject to the constraint that ΔN_{CMB} >= ΔN_{BBN}. We present limits on the decaying particle parameters derived from observational constraints on ΔN_{CMB} and ΔN_{BBN}.

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