arXiv subjects

Zi-Hui Zhang

Publications and source records attributed to Zi-Hui Zhang.

8 recordsLinked to original sources

arXiv2026

Electroweak right-handed neutrino portal dark matter

We study dark matter coupled to the Standard Model via electroweak scale right-handed neutrinos in a Type-I seesaw framework. We consider a minimal dark sector containing a fermion $χ$ and a complex scalar $ϕ$ whose only connection to the Standard Model is through renormalizable Yukawa interactions with right-handed Majorana neutrinos, thus realizing a neutrino portal after seesaw mixing. We discuss three representative realizations of electroweak right-handed neutrinos arising from the Type-I seesaw mechanism, spanning small, tiny, and ultraweak couplings to the Standard Model sector, so that the dark particles can either undergo secluded freeze-out or be produced via freeze-in. Instead of merely estimating the order of magnitude of the seesaw couplings, we use the Particle Swarm Optimization algorithm to obtain viable seesaw parameter sets consistent with neutrino data and other constraints, and then compute the coupled evolution of the dark particles and right-handed neutrinos, reproducing the observed dark matter relic abundance in representative benchmark scenarios. For freeze-out, dark matter depletion is controlled by coupled dark sector dynamics, requiring a full Boltzmann treatment for a reliable relic abundance. For freeze-in, internal dark interactions also alter the relic density: treating hidden particles as independent components with late decays added afterward can misestimate the abundance by $30\%$ or even $95\%$, depending on the interaction structure. Electroweak right-handed neutrino portal dark matter thus provides a robust and predictive framework that tightly connects neutrino physics, heavy neutral lepton phenomenology, and the cosmological dark matter relic density, offering a well-motivated benchmark for complementary collider, neutrino, and cosmological probes at the high energy frontier.

hep-ph↗

arXiv2026

Supercool with PPO: Exploring Supercooled Phase Transitions via Reinforcement Learning

Gravitational waves from cosmological first-order phase transitions provide a powerful probe of hidden sectors and beyond the Standard Model physics. However, identifying phenomenologically relevant benchmark points remains computationally challenging, since viable and detectable signals typically occupy only a small fraction of the scanned parameter space. In this work, we introduce a reinforcement learning strategy based on Proximal Policy Optimization (PPO) to accelerate the search for gravitational wave signals from supercooled phase transitions in a minimal dark $U(1)_x$ sector. We construct a numerical reinforcement learning environment that maps the microscopic model parameters to the corresponding phase transition and gravitational wave observables, using a gauge-independent low-temperature formulation of the effective action. Several reward designs are developed to guide the agent toward parameter regions producing large gravitational wave amplitudes, broad frequency coverage, and detector sensitive benchmark points. We compare the PPO scans with conventional Monte Carlo scans in both narrow and broad windows of the $U(1)_x$ vacuum expectation value. Our results demonstrate that PPO provides an efficient goal-directed search strategy for gravitational wave phenomenology and offers a broadly applicable framework for learning-assisted exploration of high-dimensional scientific parameter spaces.

hep-ph↗

arXiv2026

Annihilating to the Darker: Thermal Relic Dark Matter with an Ultraweak Portal to the Standard Model

Thermal relic dark matter has been severely constrained in recent years by direct and indirect dark matter searches, as well as multi-messenger probes of dark sectors. At the current level of experimental precision, it has become difficult for many thermal dark matter models to deplete their abundance sufficiently through freeze-out to reproduce the observed relic density. We study the possibility that thermal dark matter couples only ultraweakly to the Standard Model (SM), and therefore remains effectively undetectable in current experiments, while interacting much more strongly with a darker sector that controls its freeze-out history. Hence, the dominant annihilation channels of a thermal relic may proceed primarily into the darker sector rather than into SM particles. We first summarize the general classes of portal interactions that may connect the SM, a hidden sector, and a darker concealed sector, together with the corresponding experimental constraints. We then illustrate the mechanism in two representative realizations. The first is a prototype $U(1)_x \times U(1)_c$ setup with kinetic and mass mixing between the hidden and concealed gauge sectors. The second is a more motivated $U(1)_{B-L}\times U(1)_c$ construction, in which the $U(1)_{B-L}$ gauge interaction is strongly constrained and the hidden--concealed connection is mediated primarily by a real scalar. In both frameworks, we identify two qualitatively distinct scenarios: assisted depletion and darker conversion. By solving the full set of coupled Boltzmann equations and presenting benchmark models for dark matter masses in the 1--200~GeV range, we show that electroweak scale thermal relic dark matter may remain viable even when its direct portal to the SM is ultraweak, provided that sufficiently strong hidden--concealed interactions govern the cosmological evolution.

hep-ph↗

arXiv2026

Gauge-independent gravitational waves from a minimal dark $U(1)$ sector with viable dark matter candidates

Searches for stochastic gravitational wave backgrounds generated by first-order phase transitions offer a powerful probe of hidden sectors, but quantitative predictions in gauge theories are obstructed by the gauge dependence of the finite-temperature effective potential and the associated tunneling action. We study a minimal gauged $U(1)$ dark sector containing a dark Higgs and a dark photon, optionally supplemented by a vectorlike dark fermion, coupled to the Standard Model through the Higgs portal or kinetic mixing. Using the Nielsen identity together with a controlled derivative expansion and power counting, we construct a gauge-independent effective action in the high- and low-temperature limits, enabling model-intrinsic nucleation dynamics and robust gravitational wave predictions. We perform dedicated Monte Carlo scans in both limits and map viable microscopic parameters to detector-facing peak frequencies and amplitudes, spanning bands relevant to pulsar timing arrays and planned space-based interferometers. In our scans, supercooled phase transitions typically produce much stronger signals and are more likely to fall within the sensitivity range of current and future gravitational wave detectors, whereas parametrically high-temperature phase transitions generally yield weaker signals. We further connect the phase transition phenomenology to viable dark matter candidates within the same minimal field content, providing benchmark targets for dark photon dark matter and dark fermion dark matter, and highlighting their complementarity with gravitational wave observables. Overall, our results provide an end-to-end, gauge-independent pipeline from a minimal hidden sector Lagrangian to gravitational wave spectra and cosmologically viable dark matter benchmarks, yielding the most reliable and concrete predictions to date for a minimal gauged $U(1)$ dark sector.

hep-ph↗

arXiv2025

Darker matter generating from the dark

The non-detection of dark matter may be attributed to the dark matter residing in a darker hidden sector. We explore the possibility that a hidden sector produced through the freeze-in mechanism, can further generate an even more hidden sector via an additional freeze-in process. Such a two-step freeze-in process produces dark matter coupled weaker-than-ultraweakly to the standard model particles, and is thus referred to as the "darker matter". To illustrate the two-step freeze-in process, we study a model featuring two $U(1)$ hidden sectors. The first $U(1)$ sector is directly coupled to the standard model with feeble interactions, while the second $U(1)$ sector is directly coupled to the first $U(1)$ sector and thus only indirectly to the standard model, rendering it darker. Remarkably, darker matter candidates residing in the second darker $U(1)$ sector, generated from the two-step freeze-in process, can account for almost the entire observed dark matter relic density. The darker matter, interacted with standard model particles through ultraweak couplings, can exhibit velocity-dependent self-interacting cross-sections, which potentially provides an explanation for addressing problems associated with cosmic small-scale structures. Additionally, the dark photon darker matter residing in the darker hidden sector can be responsible for the galactic 511 keV photon signal, consistent with various dark matter density profiles.

hep-ph↗

arXiv2024

Sub-GeV millicharge dark matter from the $U(1)_X$ hidden sector

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.

hep-ph↗

arXiv2020

BAlN for III-nitride UV light emitting diodes: undoped electron blocking layer

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.

physics.app-ph↗

arXiv2018

Optimization of quantum well number of AlGaN/AlGaN deep-ultraviolet light-emitting diodes

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.

physics.app-ph↗