Search

We propose that the non-observation of WIMPs may be explained by dark matter primarily annihilating to the darker concealed sector while coupling to the standard model with only minimal strength. To demonstrate this scenario, we focus on the WIMP dark matter candidate from a $U(1)_x$ hidden sector, which couples more strongly to another concealed $U(1)_c$ sector than to the standard model. We explore two possible cases for the evolution of dark particles among hidden sectors: (1) The WIMP annihilates efficiently and achieves the observed relic density with the assistance of the concealed sector. (2) The WIMP transforms into another type of dark matter within the concealed sector and attains the observed relic density. Annihilating to the darker explains why WIMPs have remained undetected, and all WIMP models will continue to hold interest., Comment: 5 pages, 3 figures

The galactic 511~keV photon signal can be fully explained by the decaying dark matter generated through the freeze-in mechanism. The explanation of the 511~keV signal requires an extremely tiny coupling between the decaying dark matter and $e^+e^-$ pair and thus cannot be generated via direct freeze-in from standard model particles. We construct models involving two $U(1)$ hidden sectors, one of which couples directly to the standard model, the other couples directly to the first hidden sector while couples indirectly to the standard model. The decaying dark photon dark matter, which explains the 511~keV signal, is generated via a two-step freeze-in process. In the models we study, the freeze-in mechanism generates the entire dark matter relic density, and thus any types of additional dark matter components produced from other sources are unnecessary. The two-$U(1)$ model remains a strong candidate for explaining the 511 keV signal consistent with various dark matter density profiles., Comment: 26 pages, 3 figures

We conduct a comprehensive study on the sub-GeV millicharge dark matter produced through the freeze-in mechanism. We discuss in general the mixing mechanism, encompassing both kinetic mixing and mass mixing, between the $U(1)_X$ hidden sector and the standard model, which can generate millicharge carried by the dark fermions from the hidden sector. We discuss in depth how such millicharge is generated, and clarify several misunderstandings regarding this subject in the literature. Without employing an effective field theory approach, where the photon field directly mixed with the additional $U(1)$, we analyze a general renormalizable model and investigate the complete evolution of the hidden sector particles. Due to the substantial self-interactions among hidden sector particles, the evolution of the hidden sector temperature plays a crucial role, which is addressed concurrently with the number densities of hidden sector particles by solving a set of coupled Boltzmann equations. We thoroughly examine eight benchmark models from six distinct cases. Some of our key findings from the analysis of these benchmark models may be generalizable and applicable to broader freeze-in scenarios. We also explore the possibility that the $\mathcal{O}$(keV) $U(1)_X$ dark photon is a viable dark matter candidate, even though it can contribute at most $\sim 5\%$ to the total observed dark matter relic density., Comment: 55 pages, 13 figures

The undoped BAlN electron-blocking layer (EBL) is investigated to replace the conventional AlGaN EBL in light-emitting diodes (LEDs). Numerical studies of the impact of variously doped EBLs on the output characteristics of LEDs demonstrate that the LED performance shows heavy dependence on the p-doping level in the case of the AlGaN EBL, while it shows less dependence on the p-doping level for the BAlN EBL. As a result, we propose an undoped BAlN EBL for LEDs to avoid the p-doping issues, which a major technical challenge in the AlGaN EBL. Without doping, the proposed BAlN EBL structure still possesses a superior capacity in blocking electrons and improving hole injection compared with the AlGaN EBL having high doping. This study provides a feasible route to addressing electron leakage and insufficient hole injection issues when designing UV LED structures.

In this work, performance and characteristics of AlGaN/AlGaN deep-ultraviolet light-emitting diodes (DUV LEDs) with varied number of quantum-well (QW) are investigated numerically. From our simulation, 1-QW structure give the best performance at low injection current. However, at higher injection current, 2-QWs structure give the largest power output due to its higher total radiative recombination rate and internal quantum efficiency (IQE) compared to other structures. The 2-QWs structure also has less serious efficiency droop at high current than 1-QW, which makes it an optimum structure for high-power LEDs., Comment: 11 pages, 4 figures, and one table