arXiv subjects

Kai-Yu Zhang

Publications and source records attributed to Kai-Yu Zhang.

5 recordsLinked to original sources

arXiv2026

Perturbations of a Schwarzschild black hole and Newman-Unti gauge

In this paper, we derive the complete transformations of a generic first order perturbative metric in Schwarzschild spacetime to the Newman-Unti (NU) gauge in series expansion near null infinity. This allows us to determine the asymptotic shear, the mass aspect and the angular momentum aspect of the perturbative fields. As a direct application, we derive the corresponding asymptotic NU data for quasinormal modes (QNMs) of a Schwarzschild black hole. The total energy and angular momentum of QNMs obtained from the asymptotic NU data vanish but classical supertranslation charges are non-trivial, which can be applied to fix the BMS frame at first perturbative order.

gr-qc↗

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↗

arXiv2024

Near horizon linearized gravity and soft theorem

In this paper, we study the linearized gravity theory in the near horizon region of the Schwarzschild black hole in four dimensional spacetime. Under the Newman-Unti gauge, we derive the most general near horizon symmetry and solution space without any near horizon fall-off condition. There are four towers of surface charges that are generic functions on the horizon associated to the near horizon symmetry. With suitable near horizon fall-off conditions, we reveal a soft graviton theorem from the Ward identity of the near horizon supertranslation in both coordinates space and momentum space.

hep-th↗

arXiv2024

Soft theorems in de Sitter spacetime

In this paper, we derive a soft photon theorem and a soft gluon theorem in the de Sitter spacetime from the Ward identity of the near cosmological horizon large gauge transformation. Taking the flat limit of the de Sitter spacetime, the soft theorems naturally recover the corresponding flat spacetime soft theorems.

hep-th↗

arXiv2023

Explaining the $W$ boson mass anomaly and dark matter with a $U(1)$ dark sector

The $W$ boson mass recently reported by the CDF collaboration shows a deviation from the standard model prediction with an excess at $7σ$ level. We investigate two simple extensions of the standard model with an extra $U(1)$ dark sector. One is the $U(1)_x$ extension, where the $U(1)_x$ gauge field mixes with the standard model through gauge kinetic terms. The other is a general $U(1)_{\mathbf{A} Y+\mathbf{B} q}$ extension of the standard model. Fitting various experimental constraints we find the $U(1)_x$ extension with only kinetic mixing can enhance the $W$ boson mass for at most 10~MeV. While the $U(1)_{\mathbf{A} Y+\mathbf{B} q}$ extension can easily generate 77~MeV enhancement of the $W$ boson mass and also offer a viable dark matter candidate with mass ranging from several hundred GeV to TeV, which may be detected by future dark matter direct detection experiments with improved sensitivities.

hep-ph↗