arXiv subjects

Zhen-Hua Zhao

Publications and source records attributed to Zhen-Hua Zhao.

At least 19 recordsLinked to original sources

arXiv2026

Localization of Vector and Fermion Fields on a Thick Brane with a Lump-type Scalar Background

We study a thick braneworld model in asymptotically anti-de Sitter spacetime, supported by a non-topological lump-type scalar field. Unlike in boson stars, which are also non-topological solutions, the complex scalar field in our setup must have zero oscillation frequency. Consequently, the background scalar field is static and real, which breaks the global $U(1)$ symmetry. This is similar to spontaneous symmetry breaking, but it is not the same. We then investigate the localization of vector and fermion fields within this framework. By introducing a specific non-minimal coupling, we demonstrate that the zero modes of both fields can be successfully localized. Notably, this mechanism allows for the simultaneous localization of both left- and right-handed fermion zero modes.

gr-qc↗

arXiv2026

Static stable timelike circular orbits and Aschenbach effect in horizonless solutions of Einsteinian cubic gravity

In modified gravity theories, horizonless compact objects serve as a compelling alternative to black holes for testing strong-field gravity. Einsteinian cubic gravity (ECG) provides a gravitational framework for constructing these viable astrophysical models. We investigate the existence, stability, and observable signatures of static stable timelike circular orbits (SSTCOs) in static spherically symmetric ECG horizonless spacetimes. We derive timelike geodesic equations, construct the effective potential for circular orbits, and perform a numerical integration to verify orbital stability. We confirm that SSTCOs exist in both solution branches of ECG horizonless objects and that their radii coincide with the innermost stable circular orbit (ISCO). The Aschenbach effect manifests as a non-monotonic radial dependence of a zero angular momentum observer (ZAMO) measured velocity. Furthermore, we find that the stability of circular orbits exhibits a 'double stable region' structure. As the specific energy $E$ of a test particle transitions from the outer edge to the inner edge (i.e., the ISCO) of the inner stable region, its variation can exceed $1$ (i.e., $ΔE > 1$), implying that during this process, the gravitational system can release an amount of energy exceeding the rest mass of the particle itself.

gr-qc↗

arXiv2026

Light Rings, Accretion Disks and Shadows of Hayward Boson Stars

In this paper, we investigate the Einstein-Hayward gravity coupled to a complex scalar field without self-interaction. Using numerical methods, we construct a class of Hayward boson star solutions and examine their fundamental properties as well as the optical appearance of the accretion disk. Our results show that in the frozen state, both the quasi-horizon radius and the light ring radii increase with the magnetic monopole charge. Furthermore, using ray-tracing method, we find that for non-frozen states, the absence of an quasi-horizon results in the appearance of multiple photon rings within the shadow region of the accretion disks. In contrast, for frozen states, the presence of a quasi-horizon causes their images to resemble those of Schwarzschild black holes, with no additional photon rings appearing.

gr-qc↗

arXiv2025

Frozen solitonic Hayward-boson stars in Anti-de Sitter Spacetime

We construct solitonic Hayward-boson stars (SHBSs) in Anti-de Sitter (AdS) spacetime, which consists of the Einstein-Hayward model and a complex scalar field with a soliton potential. Our results reveal a critical magnetic charge $q_c$. For $q\geq q_c$ in the limit of $ω\rightarrow 0$, the matter field is primarily distributed within the critical radius $r_c$, beyond which it decays rapidly, while the metric components $-g_{tt}$ and $1/g_{rr}$ become very small at $r_c$. These solutions are termed ``frozen solitonic Hayward-boson stars" (FSHBSs). Continuously decreasing $Λ$ disrupts the frozen state. However, we did not find a frozen solution when $q<q_c$. The value of $q_c$ depends both on the cosmological constant $Λ$ and the self-interaction coupling $η$. We also found that for high frequency solutions, increasing $η$ can yield a pure Hayward solution. However, for low frequency solutions, increasing $η$ reduces both $1/g_{rr}$ and $-g_{tt}$. Furthermore, we analyzed the effective potential of SHBSs and identified an extra pair of light rings in the second solution branch.

gr-qc↗

arXiv2025

Non-Topological Soliton Bardeen Boson Stars and Their Frozen States

We investigate a Bardeen model coupling Einstein gravity with nonlinear electromagnetic fields and non-topological soliton complex scalar fields, governed by the magnetic charge $\tilde{q}$, the complex scalar field frequency $\tildeω$, and the self-interaction parameter $\tildeη$. Our results reveal that the magnetic charge $\tilde{q}$ exhibits $\tildeη$-dependent critical values $\tilde{q}_c$, beyond which ($\tilde{q} > \tilde{q}_c$) Bardeen boson stars (BBSs) may transition into frozen states ($\tildeω \to 0$). These frozen states are characterized by a critical horizon whose radius $\tilde r^\mathrm{H}_{c}$ satisfies $\tilde r_{\text{inner}}^{\text{H,RN}} < \tilde r_{c} < \tilde r_{\text{outer}}^{\text{H,RN}}$, where $\tilde r_{\text{inner}}^{\text{H,RN}}$ and $\tilde r_{\text{outer}}^{\text{RN}}$ denote the inner and outer horizons of magnetic Reissner-Nordström (RN) black holes with equivalent mass and magnetic charge. Notably, the ADM mass of frozen BBSs is independent of $\tildeη$. Furthermore, light ring (LR) solutions exist universally across all tested combinations of $\tilde{q}$ and $\tildeη$, with all frozen BBSs exhibiting LRs whose outer radius $\tilde r_{\text{outer}}^{\text{LR}}$ is independent of $\tildeη$. Compared to magnetic RN black holes, frozen BBSs possess a smaller outer LR radius ($\tilde r_{\text{outer}}^{\text{LR}} < \tilde r_{\text{outer}}^{\text{LR, RN}}$).

gr-qc↗

arXiv2025

Orbits of photon in Bardeen-boson stars and their frozen states

In a recent study [1], the Bardeen-boson star (BBS) model involving a scalar field minimally coupled to Einstein gravity and a Bardeen's nonlinear electromagnetic field was investigated. It was found that when the magnetic charge $q$ of the electromagnetic field exceeds a certain critical value $q_c$, a frozen Bardeen-boson star (FBBS) can be obtained with the frequency approaching zero. In this paper, we study the null orbits in the background of the general BBS and FBBS. We find that similar to the boson star (BS), all BBSs do not have the event horizon and possess complete null geodesics, allowing photons to move throughout the entire spacetime of BBS. Among these BBSs, the FBBSs whose spacetime is very similar to that of black holes are particularly special. The null orbits around the FBBSs exhibit sharp deflections near the critical horizon while becoming nearly straight inside the critical horizon. Furthermore, the photon in the background of FBBSs moves for a very long time inside the critical horizon from the perspective of an infinity viewer.

gr-qc↗

arXiv2024

Bardeen Spacetime with Charged Dirac Field

In this article, we investigate soliton solutions in a system involving a charged Dirac field minimally coupled to Einstein gravity and the Bardeen field. We analyze the impact of two key parameters on the properties of the solution family: the magnetic charge $p$ of the Bardeen field and the electric charge $q$ of the Dirac field. We discover that the introduction of the Bardeen field alters the critical charge of the charged Dirac field. In reference [1], solutions named frozen stars are obtained when the magnetic charge is sufficiently large and the frequency approaches zero. In this paper, we define an effective frequency and find that, when the magnetic charge is sufficiently large, a frozen star solution can also be obtained, at which point the effective frequency approaches zero rather than the frequency itself.

gr-qc↗

arXiv2023

Localization of $U(1)$ gauge field by non-minimal coupling with gravity

In this paper, we investigate the localization of the $U(1)$ gauge field on Randall-Sundrum-like braneworld models. The localization of the $U(1)$ gauge field is important because it plays a fundamental role in the branworld theories. To achieve the localization, we propose a novel action with a non-minimal coupling between the $U(1)$ gauge field and gravity. We find that the mass spectrum of the gauge field is continuous, without any gap between the zero-mass mode and the massive modes, and except for the zero-mass mode all the massive modes are not localized on the brane. Furthermore, the massive modes have negative squared masses, indicating they are tachyonic. Our analysis can be applied to a wide range of thin and thick braneworld scenarios, provided that the five-dimensional spacetime is asymptotically anti-de Sitter.

hep-th↗

arXiv2021

Particular textures of the minimal seesaw model

Following the Occam's razor principle, we explore the particular textures (featuring texture zeros and equalities) of the Dirac neutrino mass matrix in the minimal seesaw model (where only two right-handed neutrinos $N^{}_1$ and $N^{}_2$ are responsible for the generation of the light neutrino masses), in light of current neutrino experimental results and leptogenesis. The study will be performed for two particular patterns of the Majorana mass matrix $M^{}_{\rm R}$ for $N^{}_1$ and $N^{}_2$: (A) $M^{}_{\rm R}$ being diagonal $ \left( \begin{array}{cc} M^{}_1 & 0 \cr 0 & M^{}_2 \cr \end{array} \right)$; (B) $M^{}_{\rm R}$ being of the form $\left( \begin{array}{cc} 0 & M \cr M & 0 \cr \end{array} \right)$.

hep-ph↗

arXiv2019

Fermion Localization and Degenerate Resonances on Brane Array

In this work, we consider the multi-wall braneworld arisen from multi-scalar fields, and investigate the localization and resonances of spin-1/2 fermion on the multi-walls. We build two analytic multi-wall solutions with a polynomial superpotential and a modified sine-Gordon superpotential respectively. The massless fermion is the only bound state and localized between the two outermost sub-branes. The factors affecting the number of massive resonant fermions are analyzed. What interesting is that all the fermion resonant states are non-degenerate for the cases of single- and two-walls, however, doubly-degenerate fermion resonant states emerge for the cases of three- and four-walls. This novel phenomenon could be potentially interesting in phenomenology.

hep-th↗

arXiv2019

Localization of five-dimensional Elko Spinors on dS/AdS Thick Branes

Different from the Dirac spinor, the localization of a five-dimensional Elko spinor on a brane with codimension one is very difficult because of the special structure of the Elko spinor. By introducing the coupling between the Elko spinor and the scalar field generating the brane, we have two mechanisms for localization of the zero mode of a five-dimensional Elko spinor on a brane. One is the Yukawa-type coupling and the other is the non-minimal coupling. In this paper, we investigate the localization of the Elko zero mode on de Sitter and Anti-de Sitter thick branes with the two localization mechanisms, respectively. It shows that both the mechanisms can realize the localization. The forms of the coupling functions of the scalar field in the two mechanisms have similar properties respectively and they play similar role for the localization.

hep-th↗

arXiv2018

Localization of $U(1)$ gauge vector field on flat branes with five-dimension (asymptotic) AdS$_{5}$ spacetime

In order to localize $U(1)$ gauge vector field on Randall-Sundrum-like braneworld model with infinite extra dimension, we propose a new kind of non-minimal coupling between the $U(1)$ gauge field and the gravity. We propose three kinds of coupling methods and they all support the localization of zero mode. In addition, one of them can support the localization of massive modes. Moreover, the massive tachyonic modes can be excluded. And our method can be used not only in the thin braneword models but also in the thick ones.

hep-th↗

arXiv2017

Majorana neutrino signals at Belle-II and ILC

For some theoretical and experimental considerations, the relatively light Majorana neutrinos at the GeV scale have been attracting some interest. In this article we consider a scenario with only one Majorana neutrino $N$, negligible mixing with the active neutrinos $ν_{L}$, where the Majorana neutrino interactions could be described in a model independent approach based on an effective theory. Under such a framework, we particularly study the feasibility of observing the $N$ with mass in the range 0$-$30 GeV via the process $e^+ e^- \to νN \toγ+ \slashed E$ in the future Belle-II and ILC experiments. The results show that it is unpromising for Belle-II to observe the signal, while ILC may easily make a discovery for the Majorana neutrino.

hep-ph↗

arXiv2017

Spectrum Structure of Fermion on Bloch Branes with Two Scalar-fermion Couplings

It is known that the Bloch brane is generated by an odd scalar field $ϕ$ and an even one $χ$. In order to localize a bulk fermion on the Bloch brane, the coupling between the fermion and scalars should be introduced. { There are two localization mechanisms in the literature, the Yukawa coupling $-η\barΨ F_1(ϕ,χ) Ψ$ and non-Yukawa coupling $λ\bar ΨΓ^M \partial_M F_2(ϕ,χ) γ^5 Ψ$. The Yukawa coupling has been considered.} In this paper, we consider { both couplings between the fermion and the scalars with $F_1=χ^mϕ^{2p+1}$ and $F_2=χ^nϕ^{2q}$}, and investigate the localization and spectrum structure of the fermion on the Bloch brane. { It is found that the} left-handed fermion zero mode can be localized on the Bloch brane under some conditions, and the effective potentials have { rich} structure and may be volcano-like, finite square well-like, and infinite potentials. As a result, the spectrum { consists of} a series of resonant Kaluza-Klein fermions, finite or infinite numbers of bound Kaluza-Klein fermions. { Especially, we find a new feature of the introduction of both couplings: the spectrum for the case of finite square well-like potentials contains discrete quasi-localized and localized massive KK modes simultaneously.

hep-th↗

arXiv2015

Localization and mass spectra of various matter fields on scalar-tensor brane

Recently, a new scalar-tensor braneworld model was presented in [Phys. Rev. \textbf{D 86} (2012) 127502]. It not only solves the gauge hierarchy problem but also reproduces a correct Friedmann-like equation on the brane. In this new model, there are two different brane solutions, for which the mass spectra of gravity on the brane are the same. In this paper, we investigate localization and mass spectra of various bulk matter fields (i.e., scalar, vector, Kalb-Ramond, and fermion fields) on the brane. It is shown that the zero modes of all the matter fields can be localized on the positive tension brane under some conditions, and the mass spectra of each kind of bulk matter field for the two brane solutions are different, which implies that the two brane solutions are not physically equivalent.

hep-th↗

arXiv2014

New localization method of U(1) gauge vector field on flat branes in (asymptotic) $AdS_{5}$ spacetime

It is well known that the U(1) gauge vector field, with the standard five-dimensional (5D) action, cannot be localized on Randall-Sundrum-like braneworlds with an infinite extra dimension. In this paper, we propose a modified 5D action to localize U(1) gauge vector field on flat branes with an infinite or finite extra dimension. The localization method is realized by adding a dynamical mass term into the standard 5D action of the vector field, which is proportional to the 5D scalar curvature. It is shown that the vector zero mode is localizable if the 5D spacetime is (asymptotic) $AdS_{5}$. Moreover, the massive tachyonic modes can be excluded.

hep-th↗

arXiv2014

$U(1)$ Gauge Field Localization on Bloch Brane with Chumbes-Holf da Silva-Hott Mechanism

We follow the Chumbes-Holf da Silva-Hott (CHH) mechanism to study the (qusi-)localization of $U(1)$ gauge field on the Bloch brane. The localization of $U(1)$ gauge field is discussed for four kinds of Bloch brane solutions: the original Bloch brane solution, the generalized Bloch brane solution, and the degenerate Bloch brane solutions I and II. With the CHH mechanism, we find that the mass spectrum of the gauge field Kaluza-Klein modes is continuous and only the zero mode of the gauge field is localized on the brane. For the massive modes, there exist resonant modes only on the degenerate Bloch branes.

hep-th↗

arXiv2014

Brane structure and metastable graviton in five-dimensional model with (non)canonical scalar field

The appearance of inner brane structure is an interesting issue in domain wall {brane model}. Because such structure usually leads to quasilocalized modes of various kinds of bulk fields. In this paper, we construct a domain wall brane model by using a scalar field $ϕ$, which couples to its kinetic term. The inner brane structure emerges as the scalar-kinetic coupling increases. With such brane structure, we show that it is possible to obtain gravity resonant modes in both tensor and scalar sectors. The number of the resonant modes depends on the vacuum expectation value of $ϕ$ and the form of scalar-kinetic coupling. The correspondence between our model and the canonical one is also discussed. The noncanonical and canonical background scalar fields are connected by an integral equation, while the warp factor remains the same. Via this correspondence, the canonical and noncanonical models share the same linear perturbation spectrum. So the gravity resonances {obtained} in the noncanonical frame can also be obtained in the standard model. However, due to the inequivalence between the corresponding background scalar solutions, the localization condition for the left-chiral fermion zero mode can be largely different in different frames. Our estimate shows that the magnitude of the Yukawa coupling in the noncanonical frame might be hundreds times larger than the one in the canonical frame, if one demands the localization of the left-chiral fermion zero mode as well as the appearance of a few gravity resonance modes.

hep-th↗