在QCD相变温度以下，宇宙中任何显著的轻子数只能存在于中微子领域。在本工作中，我们系统地探讨了非零轻子数对大爆炸核合成（BBN）和宇宙微波背景（CMB）的影响。基于我们最近开发的基于动量平均量子动力学方程的中微子密度矩阵框架，我们解出了完整的BBN反应网络以获得原始元素的丰度。我们发现，由BBN和CMB允许的最大原始总轻子数L为-0.12(-0.10) ≤ L ≤ 0.13(0.12)（对于NH和IH），而特定味方向可能受到更严格的限制。这个限制与通过避免电弱相变处质子过量生产所获得的限制互补，尽管数值上较弱，但适用于更低的温度，并且是完全独立获得的。我们公开发布了在GitHub上解决中微子量子动力学方程演化的C++代码COFLASY-C。
英文摘要：
At temperatures below the QCD phase transition, any substantial lepton number in the Universe can only be present within the neutrino sector. In this work, we systematically explore the impact of a non-vanishing lepton number on Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB). Relying on our recently developed framework based on momentum averaged quantum kinetic equations for the neutrino density matrix, we solve the full BBN reaction network to obtain the abundances of primordial elements. We find that the maximal primordial total lepton number $L$ allowed by BBN and the CMB is $-0.12 \,(-0.10) \leq L \leq 0.13\,(0.12) $ for NH (IH), while specific flavor directions can be even more constrained. This bound is complementary to the limits obtained from avoiding baryon overproduction through sphaleron processes at the electroweak phase transition since, although numerically weaker, it applies at lower temperatures and is obtained completely independently. We publicly release the C++ code COFLASY-C on GitHub which solves for the evolution of the neutrino quantum kinetic equations numerically.
Very recently, the LUNA collaboration has reported a new measurement of the $d+p\to {}^{3}\text{He}+γ$ reaction rate, which plays an important role in the prediction of the primordial deuterium abundance at the time of BBN. This new measurement has triggered a new set of global BBN analyses within the context of the Standard Model. In this addendum to JCAP 01 (2020) 004 (arXiv:1910.01649), we consider the implications of these new results for our constraints on MeV-scale dark sectors. Importantly, we find that our bounds in the BBN-only and Planck-only analyses are insensitive to these updates…
New light states thermally coupled to the Standard Model plasma alter the expansion history of the Universe and impact the synthesis of the primordial light elements. In this work, we carry out an exhaustive and precise analysis of the implications of MeV-scale BSM particles in Big Bang Nucleosynthesis (BBN) and for Cosmic Microwave Background (CMB) observations. We find that, BBN observations set a lower bound on the thermal dark matter mass of $m_χ> 0.4\,\text{MeV}$ at $2σ$. This bound is independent of the spin and number of internal degrees of freedom of the particle, of the annihilation b…
We present new Big Bang Nucleosynthesis (BBN) limits on the cosmic expansion rate or relativistic energy density, quantified via the number $N_ν$ of equivalent neutrino species. We use the latest light element observations, neutron mean lifetime, and update our evaluation for the nuclear rates $d+d \rightarrow He3 + n$ and $d+d \rightarrow H3 + p$. Combining this result with the independent constraints from the cosmic microwave background (CMB) yields tight limits on new physics that perturbs $N_ν$ and $η$ prior to cosmic nucleosynthesis: a joint BBN+CMB analysis gives $N_ν= 2.898 \pm 0.141$, …
We derive new CMB anisotropy power spectrum and BBN constraints for evaporating primordial black holes by explicitly solving the electromagnetic particle cascades of emitted particles and the deposition of this emitted energy to the background baryon-photon plasma. We show that the CMB anisotropies can provide stronger constraints compared to BBN and CMB spectral distortions on black holes with masses as small as $M_{\rm BH}=1.1\times 10^{13}$g, a slightly smaller mass than what has been considered in literature until now. We also show that, with more up-to-date data on abundances of deuterium…
Dark sectors provide a compelling theoretical framework for thermally producing sub-GeV dark matter, and motivate an expansive new accelerator and direct-detection experimental program. We demonstrate the power of constraining such dark sectors using the measured effective number of neutrino species, $N_\text{eff}$, from the Cosmic Microwave Background (CMB) and primordial elemental abundances from Big Bang Nucleosynthesis (BBN). As a concrete example, we consider a dark matter particle of arbitrary spin that interacts with the Standard Model via a massive dark photon, accounting for an arbitr…
The Role of Big Bang Nucleosynthesis in Joint Cosmological Analyses
Boston University（波士顿大学）；University of California, Santa Barbara（加州大学圣塔芭芭拉分校）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；University of California, Berkeley（加州大学伯克利分校）
Using a new sample of extremely metal poor systems, the EMPRESS survey has recently reported a primordial helium abundance that is $3σ$ smaller than the prediction from the standard big bang nucleosynthesis (BBN) scenario. This measurement could be interpreted as a hint for a primordial lepton asymmetry in the electron neutrino flavor. Motivated by the EMPRESS results, we present a comprehensive analysis of the lepton asymmetry using measurements of the abundances of primordial elements, along with cosmic microwave background (CMB) data from Planck. Assuming that there is no dark radiation in …
Injection of high energy electromagnetic particles around the recombination epoch can modify the standard recombination history and therefore the CMB anisotropy power spectrum. Previous studies have put strong constraints on the amount of electromagnetic energy injection around the recombination era (redshifts $z\lesssim 4500$). However, energy injected in the form of energetic ($>$ keV) visible standard model particles is not deposited instantaneously. The considerable delay between the time of energy injection and the time when all energy is deposited to background baryonic gas and CMB photo…
We constrain the lifetime of thermally produced Heavy Neutral Leptons (HNLs) from primordial nucleosynthesis. We show that even a small fraction of mesons present in the primordial plasma leads to the over-production of the primordial helium. This puts an upper bound on the lifetime of HNLs $τ_{N}<0.02$ s for masses $m_{N}>m_π$ (as compared to 0.1 s reported previously). In combination with accelerator searches, this allows us to put a new lower bound on the HNLs masses and defining the "bottom line" for HNL searches at the future Intensity Frontier experiments.
In this article, we study the implications of the coupling between Axion-Like-Particles (ALPs) and Leptons to cosmology, in particular, the Big Bang Nucleosynthesis (BBN). We show that the BBN, through the constraint on the effective number of relativistic neutrino species, provides the most stringent bound on the ALP-electron interaction strength for the mass of axion between 20 keV and 1 MeV. For other values of the mass, the BBN bound complements the stellar-evolution and laboratory bounds.
We consider the effect on Big Bang Nucleosynthesis (BBN) of new measurements of the $d(p,γ){}^3$He cross section by the LUNA Collaboration. These have an important effect on the primordial abundance of D/H which is also sensitive to the baryon density at the time of BBN. We have re-evaluated the thermal rate for this reaction, using a world average of cross section data, which we describe with model-independent polynomials; our results are in good agreement with a similar analysis by LUNA. We then perform a full likelihood analysis combining BBN and Planck cosmic microwave background (CMB) lik…
Many attempts to solve the Hubble tension with extended cosmological models combine an enhanced relic radiation density, acting at the level of background cosmology, with new physical ingredients affecting the evolution of cosmological perturbations. Several authors have pointed out the ability of combined Baryon Acoustic Oscillation (BAO) and Big Bang Nucleosynthesis (BBN) data to probe the background cosmological history independently of both CMB maps and supernovae data. Using state-of-the-art assumptions on BBN, we confirm that combined BAO, deuterium, and helium data are in tension with t…
Big Bang Nucleosynthesis (BBN) is very sensitive to the cosmological expansion rate. If the gravitational constant $G$ took a different value during the nucleosynthesis epoch than today, the primordial abundances of light elements would be affected. In this work, we improve the bounds on this variation using recent determinations of the primordial element abundances, updated nuclear and weak reaction rates and observations of the Cosmic Microwave Background (CMB). When combining the measured abundances and the baryon density from CMB observations by Planck, we find $G_\mathrm{BBN}/G_0 = 0.99^{…
We study the fit of cosmological models with two additional free parameters $N_\mathrm{eff}$ and $ξ_e$ in addition to the parameters of $Λ$CDM. We introduce extra radiation components such as hot axions or sterile neutrinos in addition to the energy density of neutrinos with large neutrino degeneracy. Then, a larger $N_\mathrm{eff}$ is allowed without spoiling Big Bang Nucleosynthesis (BBN), as positive neutrino degeneracy $ξ_e$ could improve BBN fit. By analysing the data from Planck, baryon acoustic oscillation (BAO), BBN and type-Ia supernovae (SNeIa), it can be seen that the Hubble tension…
Heavy Neutral Leptons (HNLs) are strongly motivated by theory due to their capability of simultaneously explaining the observed phenomena of dark matter, neutrino oscillations and the baryon asymmetry of the Universe. The existence of such particles would affect the expansion history of the Universe and the synthesis of primordial abundances of light elements. In this work we review, revise and extend the phenomenology of HNLs during the Big Bang Nucleosynthesis (BBN) epoch for masses up to 1 GeV. This is of great importance, as BBN is able to provide complementary bounds to those from upcomin…
The LBT $Y_{\rm p}$ Project V: Cosmological Implications of a New Determination of Primordial $^4$He
TRIUMF（特利安夫实验室）；William I. Fine Theoretical Physics Institute, School of Physics and Astronomy, University of Minnesota（明尼苏达大学物理与天文学院威廉·I·芬理论物理研究所）；University of Illinois（伊利诺伊大学）；Gonzaga University（冈萨加大学）；The Ohio State University（俄亥俄州立大学）；Northwestern University（西北大学）；University of Minnesota（明尼苏达大学）
We study the type-I seesaw model with three right-handed neutrinos and Majorana masses below the pion mass. In this mass range, the model parameter space is not only strongly constrained by the requirement to explain the light neutrino masses, but also by experimental searches and cosmological considerations. In the existing literature, three disjoint regions of potentially viable parameter space have been identified. In one of them, all heavy neutrinos decay shortly before big bang nucleosynthesis. In the other two regions, one of the heavy neutrinos either decays between BBN and the CMB deco…
BBN with Late Electron-Sterile Neutrino Oscillations - The Finest Leptometer
比重子不对称性大几个数量级的遗留轻子不对称性可能隐藏在遗留中微子背景中。没有直接的理论或实验限制其大小和符号是已知的。间接宇宙学约束的范围从$|L|<0.01$到$L<10$。我们讨论了一个具有晚期无电子中微子振荡的BBN模型，它是一个很好的轻子振荡器--它能够感觉到极小的遗留轻子不对称性-- $|L|>10^{-8}$。这种灵敏度是通过这样小的$L$对中微子振荡的影响来实现的，抑制或增强它们，从而改变最初产生的$^4He$。在模型和$ L \ge 10^{-10}$的整个振荡参数范围内，数值分析了$L$对前BBN时期核子冻结的影响。本文还简要讨论了晚期电子不育振荡产生的振荡不对称性及其对原始$^4He$的影响。
No Hiding in the Dark: Cosmological Bounds on Heavy Neutral Leptons with Dark Decay Channels
Washington University（华盛顿大学）；Johannes Gutenberg-Universität Mainz（美因茨约翰内斯古腾堡大学）；Institute of High Energy Physics, Chinese Academy of Sciences（中国科学院高能物理研究所）；Kaiping Neutrino Research Center（开平中微子研究中心）
We assess the status of big-bang nucleosynthesis (BBN) in light of the final Planck data release and other recent developments, and in anticipation of future measurements. Planck data fix the cosmic baryon density to 0.9% precision, and determine the helium abundance and effective number of neutrinos with precision approaching that of astronomical and BBN determinations respectively. In addition, new high-redshift measurements give D/H to better precision than theoretical predictions, and new Li/H data reconfirm the lithium problem. We present new ${}^{7}{\rm Be}(n,p){}^{7}{\rm Li}$ rates usin…
