分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We revisit dark-matter production through freeze-in and freeze-out by solving the Boltzmann equations at the level of the phase-space distribution $f(p,t)$. Using the $2\to2$ annihilation and the $1\to2$ decay processes for illustration, we compare the resulting dark-matter relic abundance with that from the number-density approach. In the transition regime between freeze-in and freeze-out, we find the difference can be quite significant, or even by orders of magnitude if the annihilation of dark-matter particles or the decaying mediator is neglected. The freeze-in production in the $2\to2$ and the $1\to 2$ processes can also result in non-thermal phase-space distributions, or even multi-modal ones with out-of-equilibrium decay, which can potentially affect structure formation at late times. We also investigate how elastic scatterings can distort such non-thermal distributions.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The Lyman-$\alpha$ absorption spectrum associated with photons traversing the intergalactic medium allows us to probe the linear matter power spectrum down to relatively small distance scales. Finding ways of accurately evaluating Lyman-$\alpha$ constraints across large classes of candidate models of dark-matter physics is thus of paramount importance. While such constraints have been evaluated for dark-matter models with relatively simple dark-matter velocity distributions, more complex models -- particularly those with dark-matter velocity distributions stretching across multiple scales -- are receiving increasing attention. In this paper, we undertake a study of the Lyman-$\alpha$ constraints associated with general dark-matter velocity distributions. Although these constraints are difficult to evaluate in principle, in practice there exist two ways of recasting them into forms which are easier to evaluate and which therefore allow a more rapid determination of whether a given dark-matter model is ruled in or out. We utilize both of these recasts in order to determine the Lyman-$\alpha$ bounds on different classes of dark-matter velocity distributions. We also develop a general method by which the results of these different recasts can be compared. For relatively simple dark-matter velocity distributions, we find that these two classes of recasts tend to align and give similar results. However, the situation is far more complex for distributions involving multiple velocity scales: while these two recasts continue to yield similar results within certain regions of parameter space, they nevertheless yield dramatically different results within precisely those regions of parameter space which are likely to be phenomenologically relevant. This, then, serves as a cautionary tale regarding the use of such recasts for complex dark-matter velocity distributions.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Increasing attention has recently focused on non-traditional dark-matter production mechanisms which result in primordial dark-matter velocity distributions with highly non-thermal shapes. In this paper, we undertake an assessment of how the detailed shape of a general dark-matter velocity distribution impacts structure formation in the non-linear regime. In particular, we investigate the impact on the halo-mass and subhalo-mass functions, as well as on astrophysical observables such as satellite and cluster-number counts. We find that many of the standard expectations no longer hold in situations in which this velocity distribution takes a highly non-trivial, even multi-modal shape. For example, we find that the nominal free-streaming scale alone becomes insufficient to characterize the effect of free-streaming on structure formation. In addition, we propose a simple one-line conjecture which can be used to "reconstruct" the primordial dark-matter velocity distribution directly from the shape of the halo-mass function. Although our conjecture is completely heuristic, we show that it successfully reproduces the salient features of the underlying dark-matter velocity distribution even for non-trivial distributions which are highly non-thermal and/or multi-modal, such as might occur for non-minimal dark sectors. Moreover, since our approach relies only on the halo-mass function, our conjecture provides a method of probing dark-matter properties even for scenarios in which the dark and visible sectors interact only gravitationally.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： A variety of scenarios for early-universe cosmology give rise to a population of primordial black holes (PBHs) with a broad spectrum of masses. In this paper we demonstrate that the evaporation of PBHs in such scenarios has the potential to place the universe into an extended period of "stasis" during which the abundances of matter and radiation remain absolutely constant despite cosmological expansion. This surprising phenomenon can give rise to new possibilities for early-universe dynamics and lead to distinctive signatures of the evaporation of such PBHs.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The measurement of gravitational waves produced by binary black-hole mergers at the Advanced LIGO has encouraged extensive studies on the stochastic gravitational wave background. Recent studies have focused on gravitational wave sources made of the same species, such as mergers from binary primordial black holes or those from binary astrophysical black holes. In this paper, we study a new possibility -- the stochastic gravitational wave background produced by mergers of one primordial black hole and one astrophysical black hole. Such systems are necessarily present if primordial black holes exist. We study the isotropic gravitational wave background produced through the history of the Universe. We find it is very challenging to detect such a signal. We also demonstrate that it is improper to treat the gravitational waves produced by such binaries in the Milky Way as a directional stochastic background, due to a very low binary formation rate.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
Yunzhou Deng
Feng Peng
Yao Lu
Xitong Zhu
Wangxiao Jin
Jing Qiu
Jiawei Dong
Yanlei Hao
Dawei Di
Yuan Gao
Tulai Sun
Linjun Wang
Lei Ying
Fei Huang
Yizheng Jin
摘要： Quantum-dot light-emitting diodes (QD-LEDs) promise a new generation of efficient, low-cost, large-area, and flexible electroluminescent devices. However, the inferior performance of green and blue QD-LEDs is hindering the commercialization of QD-LEDs in display and solid-state lighting. Here, we demonstrate best-performing green and blue QD-LEDs with ~100% conversion of the injected charge carriers into emissive excitons. Key to this success is eliminating electron leakage at the organic/inorganic interface by using hole-transport polymers with low electron affinity and reduced energetic disorder. Our devices exhibit record-high peak external quantum efficiencies (28.7% for green, 21.9% for blue), exceptionally high efficiencies in wide ranges of luminance, and unprecedented stability (T95 lifetime: 580,000 h for green, 4,400 h for blue). The overall performance surpasses previously reported solution-processed green and blue LEDs.
分类： 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间： 2017-11-10
摘要： The width of three-body single-pion decay process d∗ → NNπ0,± is calculated by using the d∗ wave function obtained from our chiral SU(3) constituent quark model calculation. The effect of the dynamical structure on the width of d∗ is taken into account in both the single ∆∆ channel and cou-pled ∆∆+CC two-channel approximations. Our numerical result shows that in the coupled-channel approximation, namely, the hidden-color configuration being considered, the obtained partial decay width of d∗ → NNπ is about several hundred KeV, while in the single ∆∆ channel it is just about 2 ∼ 3 MeV. We, therefore, conclude that the partial width in the single-pion decay process of d∗ is much smaller than the widths in its double-pion decay processes. Our prediction may provide a criterion for judging different interpretations of the d∗ structure, as different pictures for the d∗ may result quite different partial decay width.
分类： 物理学 >> 核物理学 提交时间： 2016-09-13
摘要： The decay widths of the $\ds\to d \pi^0\pi^0$ and $\ds\to d \pi^+\pi^-$ processes are explicitly calculated in terms of our chiral quark model. By using the experimental ratios of cross sections between various decay channels, the partial widths of the $\ds\to pn \pi^0\pi^0$, $\ds\to pn \pi^+\pi^-$, $\ds\to pp \pi^0\pi^-$, and $\ds\to nn \pi^+\pi^0$ channels are also extracted. Further including the estimated partial width for the $\ds\to pn$ process, the total width of the $\ds$resonance is obtained. In the first step of the practical calculation, the effect of the dynamical structure on the width of$\ds$ is studied in the single ΔΔ channel approximation. It is found that the width is reduced by few tens of MeV, in comparison with the one obtained by considering the effect of the kinematics only. This presents the importance of such effect from the dynamical structure. However, the obtained width with the single ΔΔ channel wave function is still too large to explain the data. It implies that the $\ds$ resonance will not consist of the ΔΔ structure only, and instead there should be enough room for other structure such as the hidden-color (CC) component. Thus, in the second step, the width of $\ds$ is further evaluated by using a wave function obtained in the coupled ΔΔ and CC channel calculation in the framework of the Resonating Group Method (RGM). It is shown that the resultant total width for $\ds$ is about 69 MeV, which is compatible with the experimental observation of about 75 MeV and justifies our assertion that the $\ds$ resonance is a hexaquark-dominated exotic state.
分类： 物理学 >> 核物理学 提交时间： 2016-09-01
摘要： The decay widths of four-body double-pion decays $\ds\to pn \pi^0\pi^0$, $\ds\to pn \pi^+\pi^-$, and iso-scalar parts of $\ds\to pp \pi^0\pi^-$ and $\ds\to nn \pi^+\pi^0$ are explicitly calculated with the help of the d∗ wave function obtained in a chiral SU(3) quark model calculation. The effect of the dynamical structure on $\ds$'s width is analyzed both in the single ΔΔ channel and coupled ΔΔ and CC channel approximations. It is found that in the coupled-channel approximation, the obtained partial decay widths of $\ds\to pn \pi^0\pi^0$, $\ds\to pn \pi^+\pi^-$, and those of d∗ to the iso-scalar parts of ppπ0π− and nnπ+π0 are about 7.4MeV, 16.4MeV, 3.5MeV and 3.5MeV, respectively As a consequence, the total width is about 64.5MeV. These widths are consistent with those estimated by using the corresponding cross section data in our previous investigation and also the observed data. But in the single ΔΔ channel approximation, the widths are still almost 2-times larger than the measured values. Apparently, the explicitly calculated width together with the evaluated mass of d∗ in the coupled ΔΔ and CC channel approximation can well explain the observed data, which again supports our assertion that the $\ds$resonance is a six-quark dominated exotic state.
点击量
待评议
点击量
下载量
评论
