arXiv subjects

Zhaofeng Kang

Publications and source records attributed to Zhaofeng Kang.

At least 19 recordsLinked to original sources

arXiv2026

A MeV-Scale Dark QCD Solution to the Axion Domain Wall Problem

PQ solution to the strong CP problem probably encounters the axion domain wall problem. In this article, we propose a simple and testable solution, assuming that the $U(1)_{\rm PQ}$ possesses mixed anomaly to a hidden $SU(N_c)$ color. Then, the axion field receives a new cosine potential from the hidden instantons, which breaks the $Z_{N_{\rm DW}}$ subgroup explicitly. The new potential lifts the vacua degeneracy, but also drives the effective $\theta$ angle away from the origin, re-incuring the strong CP problem. However, we find that the dark QCD scale within the 0.1 to 3 MeV window survives, maintaining a delicate balance. Two observational signatures are explored: gravitational waves from domain wall collapse, already probed by current PTAs, and di-photon signals from axion-dark-glueball mixing, which require next-generation MeV telescopes. The scenario favors a cold dark QCD sector consistent with dark glueball relic constraints.

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arXiv2025

Primordial Black Hole Formation and Spin in Matter Domination Revisited

In this article, we calculate the mass distribution of primordial black holes (PBHs) formed in the matter-dominated (MD) era by the peak theory. We apply the Zel'dovich approximation to track the nonlinear evolution of overdensities and compute the PBH abundance and mass function by incorporating a PBH formation criterion based on the hoop conjecture. We find that the PBH abundance $\beta$ follows the scaling law $\beta \simeq A_\gamma \sigma_h^{*5}$ for $\sigma_h^*\ll 1$. Here, $\sigma_h^*$ is the quantity that characterizes the variance of the density fluctuation at the horizon entry. We also find that, in contrast to the previous estimates, the PBH spin is very small for $\sigma_h^*\ll 1$ but could be larger for larger $\sigma_h^*$ and broader power spectra. Finally, specializing to a monochromatic power spectrum, we prove analytically that the PBH mass distribution becomes effectively monochromatic and reveal that the resultant PBH abundance is approximately 19 times the previous prediction.

gr-qc↗

arXiv2025

Gravitational wave cosmology

Gravitational waves (GWs) originating from cosmological sources offer direct insights into the physics of the primordial Universe, the fundamental nature of gravity, and the cosmic expansion of the Universe. In this review paper, we present a comprehensive overview of our recent advances in GW cosmology, supported by the national key research and development program of China, focusing on cosmological GW sources and their implications for fundamental physics and cosmology. We first discuss the generation mechanisms and characteristics of stochastic gravitational wave backgrounds generated by physical processes occurred in the early Universe, including those from inflation, phase transitions, and topological defects, and summarize current and possible future constraints from pulsar timing array and space-based detectors. Next, we explore the formation and observational prospects of primordial black holes as GW sources and their potential connection to dark matter. We then analyze how GWs are affected by large-scale structure, cosmological perturbations, and possible modifications of gravity on GW propagation, and how these effects can be used to test fundamental symmetry of gravity. Finally, we discuss the application of GW standard sirens in measuring the Hubble constant, the expansion history, and dark energy parameters, including their combination with electromagnetic observations. These topics together show how GW observations, especially with upcoming space-based detectors, such as LISA, Taiji, and Tianqin, can provide new information about the physics of the early Universe, cosmological evolution, and the nature of gravity.

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arXiv2025

A Light Lepton-flavor-violating Flavon: the Messenger of Neutrino Mixing and Muon $g-2$

In neutrino physics, a class of models with local or global family symmetries may be invoked, and then a flavon field is needed to realize the full neutrino mixing. This flavon may shed light on the long-standing muon $g-2$ puzzle. In this work, we explore this idea in the $({B-L})_{13}$ gauge extension to the standard model (SM), in which realistic neutrino mixing requires both a SM singlet flavon $s$ and a vector-like lepton (VLL) doublet. The dominant coupling between the flavon and leptons is in the manner of lepton-flavor-violation (LFV). Through an analytical analysis of the SM lepton-VLL mixing matrix, we find that the parameter space of the $s\bar\mu e$-type flavon to explain the muon $g-2$ has been completely excluded by the specific LFV process, muonium-antimuonium oscillation. But the $s\bar\mu \tau$-type flavon still has the opportunity; however, it confronts the strong constraint from $\tau\to \mu$ conservation and, in particular, the lepton flavor universality test of $Z$ boson decay, which arises due to our way to realize the LFV flavon. The surviving flavon is highly predictable, with mass in the narrow window $m_\tau\lesssim m_s\lesssim 1.5~ m_\tau$ and LFV coupling strength $\sim 10^{-2}$. Besides, it leaves a TeV scale VLL with a multi-lepton signature at the LHC.

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arXiv2025

Dark Chiral Phase Transition Driven by Chemical Potential and its Gravitational Wave Test

In this article, for the first time, we explore the scenario that the dark-QCD sector has a large chemical potential $\mu$ (on the order of magnitude of temperature) of dark quarks. It leads to a complex-valued Polyakov loop and tilts the partial confinement effect, driving the dark-QCD phase transition to a first-order one in the early universe. We present a toy model via the Affleck-Dine mechanism that could generate degenerate dark quarks. Our study, in the framework of PNJL, focuses on the dynamical impacts of a large chemical potential on the chiral phase transition without turning on the KMT instanton term. We plot the phase diagram of the dark-QCD in the chiral limit. The resulting first-order phase transition actually refers to a chiral phase transition, with the transition to the confinement vacuum being a cross-over. Following the phase diagram, we find that increasing $\mu$ can considerably prolong the duration of the phase transition and also the release of latent heat, which together make the cosmic dark-QCD phase transition at the critical temperature above 1 GeV and below 100 GeV produce gravitational wave signal in the intermediate frequency band, which is well probable in space detectors such as BBO

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arXiv2024

Matter Asymmetries in the $Z_N$ Dark matter -companion Models

A class of $Z_{N\geq 3}$-symmetric WIMP dark matter models that are characterized by the semi-annihilation into the companion of dark matter has been proposed in Ref.~\cite{Guo:2021rre}, providing a mechanism to evade the stringent direct detection constraint. In this work, we point out that such models naturally provide the three Sakharov elements necessary for dark matter asymmetry, and moreover this asymmetry can be transferred to the visible sector with a proper link to the leptonic or quark sector. In our minimal $Z_3$ example, the migration to the leptonic sector is via the asymmetric companion decay into neutrinos, and the lepton asymmetry can be further transferred to the quark sector. The CP violation parameter is restrained in this model. Thus, we explore the thermal motion effect of dark matter and find that it gives an enhancement to the CP violation parameter, which is studied for the first time. A preliminary numerical analysis based on the Boltzmann equations shows that both correct relic density of dark matter and baryon asymmetry can be accommodated.

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arXiv2024

The Stochastic Gravitational Wave Background from Primordial Gravitational Atoms

We propose a scenario of primordial gravitational atoms (PGAs), which may exist in the current and past universe due to spinning primordial black holes (PBHs) and very light bosonic fields. In a monochromatic mass scenario with a sizable dimensionless spin, which may arise in a short matter dominated (MD) era, we analyze the resulting stochastic gravitational wave background (SGWB) signal. Its spectrum is approximately characterized by a rising $\propto f^3$ followed by a falling $\propto f^{-1}$ where $f$ is the frequency. Then, we investigate the constraints and prospects of such a SGWB, and find that PGAs with a core mass $M_{\rm BH}\sim {\cal O}(10)~M_{\odot}$ and a cloud of light scalar with mass $\mu \sim {\cal O} (10^{-13})$ eV could yield constraints even stronger than those from bare PBHs. Future detectors such as LISA, Taiji and TianQin are able to explore PGAs over a narrow and elongated strap in the $(\mu,M_{\rm BH})$ plane, spanning over 10 orders of magnitude for the maximum spin, $10^{-8}~M_{\odot}\lesssim M_{\rm BH}\lesssim 10^4~M_{\odot}$, $10^{-16}~{\rm eV}\lesssim \mu\lesssim 10^{-3}~\rm eV$. If the PGA is dressed with a vector cloud, the SGWB signal has a much better opportunity to be probed.

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arXiv2024

Gamma-ray Signal from $Z_{N\geq 3}$ Dark Matter-Companion Models

In Ref.~\cite{Guo:2021rre}, we proposed to replace the final dark matter (DM) particle in the semi-annihilation mode $\rm DM+DM\to antiDM+Higgs~boson$ with its $Z_{N\geq 3}$ companion, thus reducing DM number density without DM-nucleon scattering. In this work, we study the indirect detection signals from DM annihilation, the Higgs boson pair with one of them from the companion decay being on- or off- shell, depending on the DM-companion mass splitting. We generate the photon spectrum by using PYTHIA8 and study the properties of the spectrum, to find that the hard part of the spectrum in our model is mainly shaped by the direct Higgs boson and thus does not differ much from that of the conventional semi-annihilation mode. Using the Fermi-LAT data of white dwarfs, we derive the current limit of the DM annihilation cross section for ${\rm DM+DM\to companion^*+Higgs~ boson}$, and for the relatively light DM, it reaches the typical thermal cross section. However, for the TeV scale DM, we have to rely on the Cherenkov Telescope Array, which is able to rule out the whole parameter space except for the coannihilation region.

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arXiv2024

Interplay between Vector-like Lepton and Seesaw Mechanism:Oblique Corrections

The non-vanishing neutrino mass strongly hints the existence of right-handed neutrinos (RHNs), singlets of the standard model (SM). However, they are highly decoupled from the SM and difficult to probe. In this work, we consider the Majorana RHNs from the type-I seesaw mechanism may well mix with the heavy neutral lepton dwelling in certain vector-like lepton (VLL), thus acquiring a sizable electroweak charge. Such a simple scenario yields many interesting consequences, and the imprint on oblique corrections, well expected from the mass splitting between components of VLL by virtue of VLL-RHN mixing, is our focus here. We analytically calculate the Peskin-Takeuchi parameters S, T and U with full details, carefully treating the Majorana loop to obtain the self consistent expressions free of divergence. Then, we constrain on the VLL-RHN system which only gives a sizable $T$ parameter using the PDG-2021 data and CDF-II data, separately, by imposing $T\lesssim{\cal O}(0.1)$. It is found that for the RHN and VLL below the TeV scale, with a properly large mixing, stands in the frontier of the electroweak precision test such as W-boson mass.

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arXiv2024

Dark Confinement-Deconfinement Phase Transition: A Roadmap from Polyakov Loop Models to Gravitational Waves

We explore the confinement-deconfinement phase transition (PT) of the first order (FO) arising in $SU(N)$ pure Yang-Mills theory, based on Polyakov loop models (PLMs), in light of the induced gravitational wave (GW) spectra. We demonstrate that the PLMs with the Haar measure term, involving models successful in QCD with $N=3$, are potentially incompatible with the large $N$ scaling for the thermodynamical quantities and the latent heat at around the criticality of the FOPT reported from the lattice simulations. We then propose a couple of models of polynomial form, which we call the 4-6 PLM (with four- and six-point interactions among the basic PL fields which have center charge 1) and 4-8 PLM (with four- and eight-point interactions), and discuss how such models can naturally arise in the presence of a heavy PL with charge 2. We show that those models give the consistent thermodynamical and large $N$ properties at around the criticality. The predicted GW spectra are shown to have high enough sensitivity to be probed in the future prospected interferometers such as LISA, BBO, DICIGO, and TianQin.

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arXiv2024

Massive Gauge Theory with Quasigluon for Hot $SU(N)$: Phase Transition and Thermodynamics

It is challenging to build a model that can correctly and unifiedly account for the deconfinement phase transition and thermodynamics of the hot $SU(N)$ pure Yang-Mills (PYM) system, for any $N$. In this article, we slightly generalize the massive PYM model to the situation with a quasigluon mass $M_g(T)$ varying with temperature, inspired by the quasigluon model. In such a framework, we can acquire an effective potential for the temporal gauge field background by perturbative calculation, rather than adding by hand. The resulting potential works well to describe the behavior of the hot PYM system for all $N$, via the single parameter $M_g(T)$. Moreover, under the assumption of unified eigenvalue distribution, the $M_g(T)$ fitted by machine learning is found to follow $N$-universality.

hep-ph↗

arXiv2024

Confinement Bubble Wall Velocity via Quasiparticle Determination

Lattice simulations reveal that the deconfinement-confinement (D-C) phase transition (PT) of the hot pure $SU(N>2)$ Yang-Mills system is first order. This system can be described by a pool of quasigluons moving in the Polyakov loop background, and in this picture, we establish an effective distribution function for quasigluons, which encodes interactions among quasigluons and in particular the confinement effect. With it, we made the first attempt to calculate the confinement bubble wall velocity $v_w$ at the microscopical level, and we obtained a small velocity $v_w\sim 0.04$ using two different approaches, which is qualitatively consistent with others results like holography.

hep-ph↗

arXiv2021

WIMP Dark Matter Hidden behind its Companion

The WIMP dark matter (DM) hypothesis now is in an awkward position, owing to the stronger and stronger exclusion from DM direct detection. In this article we design a mechanism to evade this constraint.The idea is simple. DM has a companion, and they are both charged under the DM protecting symmetry G; they admit the trilinear coupling DM-DM-companion, so the latter provides a portal to the standard model (SM) via, for instance, the coupling to Higgs doublet.Then, DM semi-annihilates into the companion to arrive correct relic density, without leaving DM-nucleon scattering signal. The idea can be realized for ZN symmetric models with N >2.We stress that this mechanism has the characteristics of co-annihilation, and as a matter of fact its effect becomes necessary near or above the TeV region. This means that it may be difficult to detect our dark matter directly or indirectly.

hep-ph↗

arXiv2021

$(g-2)_μ$ Versus $K \to π+E_{miss}$ Induced by the $(B-L)_{23}$ Boson

To address the long-standing $(g-2)_μ$ anomaly via a light boson, in Ref. [1] we proposed to extend the standard model (SM) by the local $(B-L)_{23}$, under which only the second and third generations of fermions are charged. It predicts an invisible $Z'$ with mass ${\cal O}(100)$ MeV, and moreover it has flavor-changing neutral current (FCNC) couplings to the up-type quarks at tree level. Such a $Z'$, via $K_L \to π^0 + Z'(\to ν\barν)$ at loop level, may be a natural candidate to account for the recent KOTO anomaly. In this article, we investigate this possibility, to find that $Z'$ can readily do this job if it is no longer responsible for the $(g-2)_μ$ anomaly. We further find that both anomalies can be explained with moderate tuning of the CP violation, but may contradict the $B$ meson decays.

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arXiv2020

Signatures of a Flavor Changing $Z'$ Boson in $B_q \to γZ'$

Rare $B$ meson decays offer an opportunity to probe a light hidden $Z'$ boson. In this work we explore a new channel $B_q \to γZ'$ ($q = d, s$) followed by a cascade decay of $Z'$ into an invisible (neutrino or dark matter) or charged lepton pair $\ell^+ \ell^-$ ($\ell=e ,μ)$. The study is based on a simplified effective model where the down quark sector has tiny flavor-changing neutral current couplings with $Z'$. For the first time, we calculate ${\rm BR}(B_q \to γZ')$ at the leading power of $1/m_b$ and $1/E_γ$. Confronting with the strong constraints from semi-invisible decays of $B$ meson, we find that the branching ratio for $B_d \to {\rm invisible} + γ$ can be larger than its Standard Model prediction, leaving a large room for new physics, in particular for light dark matter. Additionally, the branching ratio for $B_d \to e^+ e^- γ$ can also be sizable when the corresponding flavor violating $Z'$ coupling to quarks is of the axial-vector type. On the other hand, the predicted branching ratios of $B_d \to μ^+ μ^- γ$ and $B_s \to \ell^+ \ell^- γ$ are severely constrained by the experimental measurements.

hep-ph↗

arXiv2020

Interpretation of the cosmic ray positron and electron excesses with an annihilating-decaying dark matter scenario

The precise measurements of energy spectra of cosmic ray positrons and/or electrons by recent experiments show clear excesses above 10 GeV. Moreover, a potential sharp spectral feature was suggested by the Dark Matter Particle Explorer (DAMPE) data. These results inspire quite a number of discussions on the connection with either the annihilation/decay of dark matter (DM) or the astrophysical origins. Here we discuss a DM scenario in which DM particles could annihilate and decay into standard model particle pairs simultaneously. In this model, the peak structure is due to the DM annihilation in a nearby subhalo and the broad positron/electron excesses are due to the decay of DM in the Milky Way. This model can reasonably explain the DAMPE and AMS-02 data of the total $e^+e^-$ spectra and the positron fraction, with model parameters being consistent with existing constraints. A simple realization of such a DM model is the spin-1 vector DM model.

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arXiv2020

Scale-genesis by Dark Matter and Its Gravitational Wave Signal

Classical scale invariance (CSI) may shed light on the weak scale origin, but the realistic CSI extension to the standard model requires a bosonic trigger. We propose a scalar Dark Matter(DM) field $X$ as the trigger, establishing a strong connection between the successful radiative breaking of CSI and DM phenomenologies. The latter forces the breaking scale to $\sim {\cal O}(\rm TeV)$. It brightens the test prospect of this scenario via gravitational wave, a sharp prediction of CSI phase transition (CSIPT), which is first order and has strong supercooling. Moreover, we carefully deal with some techniques which are commonly used to analyze CSIPT but maybe missed. In particular, we clarify the imprecision of Witten's formula used in the single field case to calculate the bubble nucleation rate and stress that the essence of Witten's approximation is the validity of high temperature expansion.

hep-ph↗

arXiv2019

$(g-2)_μ$ Versus Flavor Changing Neutral Current Induced by the Light $(B-L)_{μτ}$ Boson

We propose the local $(B-L)_{μτ}$ model, which minimally retains the local $B-L$ extension for the sake of neutrino phenomenologies, and at the same time presents an invisible gauge boson $Z'$ with mass $\sim {\cal O}(10)$ MeV to account for the discrepancy of the muon anomalous magnetic moment. However such a scenario is challenged by flavor physics. To accommodate the correct pattern of Cabibbo-Kobayashi-Maskawa matrix, we have to introduce either a $SU(2)_L$ doublet flavon or vector-like quarks plus a singlet flavon. In either case $Z'$ induces flavor changing neutral current (FCNC) in the quark sector at tree-level. We find that the former scheme cannot naturally suppress the FCNC from the down-type quark sector and thus requires a large fine-tuning to avoid the stringent $K \to πν\bar ν$ bound. Whereas the latter scheme, in which FCNC merely arises in the up-type quark sector, is still free of strong constraint. In particular, it opens a new window to test our scenario by searching for flavor-changing top quark decay mode $t \rightarrow u/c+$(invisible), and the typical branching ratio $\sim\mathcal{O}(10^{-4})$.

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