我们用Volterra积分方程解了暖暗物质（WDM）密度涨落的宇宙学演化。在没有中微子的情况下，各向异性应力消失，Volterra方程减少到一个单一的积分方程。我们数值求解这个方程的DM费米子去耦在平衡和DM不育中微子去耦出平衡。给出了零波数下密度涨落和引力势的精确解析解。我们计算的密度对比度作为一个函数的比例因子a的波数k的范围很广。当a固定时，密度衬度随k的增大而增大，< k_c while it decreases for k >其中k k_c为1.6/Mpc。密度对比度取决于k和a，主要是通过乘积k a表现出自相似行为。我们的数值密度对比小k轻轻接近我们的解析解k = 0。当k < 1/（60 kpc）时，密度衬度一般随时间增长，而当k > 1/（60 kpc）时，密度衬度从RD期开始出现振荡，并随着k的增大而变强。我们计算了热费米子和无菌中微子的密度对比度的传递函数：a）Dodelson-Widrow（DW）模型和b）由标量粒子衰变产生的无菌中微子模型。传递函数随k增大，在k = k_c时达到最大值后减小，反映了密度衬度随时间的变化。Volterra方程中的积分核在时间上是非局部的，它们的衰减决定了自解耦以来过去演化的记忆。当DM在平衡时比在平衡之外时下降得更快。尽管中微子和光子在MD时代可以被忽视，但它们通过RD时代的记忆在MD时代做出了贡献。
英文摘要：
We solve the cosmological evolution of warm dark matter (WDM) density fluctuations with the Volterra integral equations of paper I. In the absence of neutrinos, the anisotropic stress vanishes and the Volterra equations reduce to a single integral equation. We solve numerically this equation both for DM fermions decoupling at equilibrium and DM sterile neutrinos decoupling out of equilibrium. We give the exact analytic solution for the density fluctuations and gravitational potential at zero wavenumber. We compute the density contrast as a function of the scale factor a for a wide range of wavenumbers k. At fixed a, the density contrast grows with k for k < k_c while it decreases for k > k_c, (k_c ~ 1.6/Mpc). The density contrast depends on k and a mainly through the product k a exhibiting a self-similar behavior. Our numerical density contrast for small k gently approaches our analytic solution for k = 0. For fixed k < 1/(60 kpc), the density contrast generically grows with a while for k > 1/(60 kpc) it exhibits oscillations since the RD era which become stronger as k grows. We compute the transfer function of the density contrast for thermal fermions and for sterile neutrinos in: a) the Dodelson-Widrow (DW) model and b) in a model with sterile neutrinos produced by a scalar particle decay. The transfer function grows with k for small k and then decreases after reaching a maximum at k = k_c reflecting the time evolution of the density contrast. The integral kernels in the Volterra equations are nonlocal in time and their falloff determine the memory of the past evolution since decoupling. This falloff is faster when DM decouples at equilibrium than when it decouples out of equilibrium. Although neutrinos and photons can be neglected in the MD era, they contribute in the MD era through their memory from the RD era.
Considering the gauge model SU(2)_L x U(1) with the extension of the lepton sector by right-handed sterile Majorana neutrinos, we distinguish a "minimal parametric mixing scenario" for a complete matrix of 6 x 6 light (active) and sterile neutrinos, which depends, in addition to experimentally measured physical parameters, only on the masses of sterile neutrinos. For such a scenario, improved cosmological bounds are provided, resulting from the lifetime of sterile neutrinos and the fraction of energy carried by sterile neutrino dark matter.
One of possible explanations of a faint narrow emission line at 3.5 keV reported in our Galaxy, Andromeda galaxy and a number of galaxy clusters is the dark matter made of 7 keV sterile neutrinos. Another signature of such sterile neutrino dark matter could be fewer ionizing sources in the early Universe (compared to the standard "cold dark matter" (CDM) scenario), which should affect the reionization of the Universe. By using a semi-analytical model of reionization, we compare the model predictions for CDM and two different models of 7 keV sterile neutrino dark matter (consistent with the 3.5…
We investigate how hypothetical particles - sterile neutrinos - can be produced in the interior of exploding supernovae via the resonant conversion of $\barν_μ$ and $\bar ν_τ$. The novelty of our treatment lies in the proper account of the resulting lepton number diffusion. We compute the yield of sterile neutrinos and find that even after taking into account back reaction, sterile neutrinos can carry out a sizeable fraction of the total energy of the explosion comparable to that of active neutrinos. The production is, however, exponentially sensitive to the temperature in the inner supernovae…
We analyze two dedicated NuSTAR observations with exposure ${\sim}190$ ks located ${\sim}10^\circ$ from the Galactic plane, one above and the other below, to search for x-ray lines from the radiative decay of sterile-neutrino dark matter. These fields were chosen to minimize astrophysical x-ray backgrounds while remaining near the densest region of the dark matter halo. We find no evidence of anomalous x-ray lines in the energy range 5--20 keV, corresponding to sterile neutrino masses 10--40 keV. Interpreted in the context of sterile neutrinos produced via neutrino mixing, these observations p…
Entering the Era of Measuring Sub-Galactic Dark Matter Structure: Accurate Transfer Functions for Axino, Gravitino & Sterile Neutrino Thermal Warm Dark Matter
In this short letter, we find that a magnetic transition dipole moment between tau and sterile neutrinos can account for the XENON1T excess events. Unlike the ordinary neutrino dipole moment, the introduction of the new sterile mass scale allows for astrophysical bounds to be suppressed. Interestingly, the best-fit regions that are compatible with the SN1987A imply either boron-8 or CNO neutrinos as the source flux. We find that sterile neutrinos of either $\sim$ 260 keV or in the $\sim$(500 - 800) keV mass range are capable of evading astrophysical constraints while being able to successfully…
Dark matter might be made of "warm" particles, such as sterile neutrinos in the keV mass range, which can decay into photons through mixing and are consequently detectable by X-ray telescopes. Axionlike particles (ALPs) are detectable by X-ray telescopes too when coupled to standard model particles and decay into photons in the keV range. Both particles could explain the unidentified 3.5 keV line and, interestingly, XENON1T observed an excess of electron recoil events most prominent at 2-3 keV. One explanation could be an ALPs origin, which is not yet excluded by X-ray constraints in an anomal…
We discuss numerous mechanisms for production of sterile neutrinos, which can account for all or a fraction of dark matter, and which can range from warm to effectively cold dark matter, depending on the cosmological scenario. We investigate production by Higgs boson decay, $(B-L)$ gauge boson production at high temperature, as well as production via resonant and nonresonant neutrino oscillations. We calculate the effects on structure formation in these models, some for the first time. If two populations of sterile neutrinos, one warm and one cold, were produced by different mechanisms, or if …
Neutrino-neutrino interactions, as mediated by standard model Z exchange, can drastically enhance the Dodelson-Widrow mechanism for the production of sterile neutrinos in the region of temperature $5-15$MeV. For a sterile mass in the KeV region with the usual type of active-sterile neutrino coupling, the production can be copious enough to provide ample dark matter, even when the active-sterile coupling is small enough to avoid the bounds set by laboratory results.
We investigate the prospects of the SRG mission in searches for the keV-scale mass sterile neutrino dark matter radiatively decaying into active neutrino and photon. The ongoing all-sky X-ray survey of the SRG space observatory with data acquired by the ART-XC and eROSITA telescopes can provide a possibility to fully explore the resonant production mechanism of the dark matter sterile neutrino, which exploits the lepton asymmetry in the primordial plasma consistent with cosmological limits from the Big Bang Nucleosynthesis. In particular, it is shown that at the end of the four year all-sky su…
We study a model of a keV-scale sterile neutrino with a relatively large mixing with the Standard Model sector. Usual considerations predict active generation of such particles in the early Universe, which leads to constraints from the total Dark Matter density and absence of X-ray signal from sterile neutrino decay. These bounds together may deem any attempt of creation of the keV scale sterile neutrino in the laboratory unfeasible. We argue that for models with a hidden sector coupled to the sterile neutrino these bounds can be evaded, opening new perspectives for the direct studies at neutr…
We describe two new generation mechanisms for Dark Matter composed of sterile neutrinos with ${\cal O}(1)$ keV mass. The model contains a light scalar field which coherently oscillates in the early Universe and modulates the Majorana mass of the sterile neutrino. In a region of model parameter space, the oscillations between active and sterile neutrinos are resonantly enhanced. This mechanism allows us to produce sterile neutrino DM with small mixing angle with active neutrinos, thus evading the X-ray constraints. At the same time the spectrum of produced DM is much cooler, than in the case of…
Unexplored interactions of neutrinos could be the key to understanding the nature of the dark matter (DM). In particular, active neutrinos with new self-interactions can produce keV-mass sterile neutrinos that account for the whole of the DM through the Dodelson-Widrow mechanism for a large range of active-sterile mixing values. This production typically occurs before Big-Bang Nucleosynthesis (BBN) in a yet uncharted era of the Universe. We assess how the mixing range for keV-mass sterile neutrino DM is affected by the uncertainty in the early Universe pre-BBN cosmology. This is particularly r…
