arXiv subjects

Wei Chao

Publications and source records attributed to Wei Chao.

At least 19 recordsLinked to original sources

arXiv2026

The Axion Helical Misalignment Mechanism

Understanding axion production in the early Universe remains a pivotal challenge, given the axion as a compelling cold dark matter candidate. Conventional misalignment scenarios often overlook the possibility that a large initial axion velocity can fundamentally reshape the subsequent evolution of the axion field. In this letter, we provide a comprehensive analysis of how primordial magnetic fields impact the axion relic abundance. By accounting for the axion coupling to the Chern-Simons term of the hypercharge gauge field, the equation of motion of the axion is recast as a driven oscillator equation. This modification effectively shifts the onset of axion oscillations, leading to a significant reevaluation of the final relic abundance, a novel effect we term the axion helical misalignment mechanism. Furthermore, in the presence of primordial chiral asymmetries, the chiral magnetic effect (CME) emerges as a critical driver of axion dynamics. The interplay between the axion field and the CME not only profoundly influences the evolution of Standard Model chiral fermions but also provides a viable pathway for generating the observed baryon asymmetry of the Universe.

hep-ph↗

arXiv2026

Graviton energy spectra arising from the KSVZ axion model

Axion, the goldstone boson arising from the spontaneous breaking of a global $U(1)$ Peccei-Quinn symmetry, provides a dynamical solution to the strong CP problem and is an excellent dark matter candidate. Various experiments are designed to search for the axion, however no confirmative signal has been observed. On the other hand, there are also hypothetical heavy particles in axion models, such as the heavy scalar $s$, which is the CP-even component of the complex scalar that carries $U(1)_{PQ}$ charge, and the vector-like heavy quark (VLQ) in the Kim-Shifman-Vainshtein-Zakharov~(KSVZ) axion model. Studying signals induced by them are helpful for axion searches. In this paper, we calculate the graviton bremsstrahlung energy spectrum arising from the decay of the heavy scalar or VLQ in the KSVZ model. The result shows that these heavy particles can emit ultrahigh-frequency gravitational waves (GWs), with the peak frequency depending on the model's parameter inputs. In addition, the graviton spectrum is distinguished from the thermal GW background at high frequencies if there is an early matter-dominated era induced by these heavy particles. Future measurements of ultrahigh-frequency GWs may provide indirect evidence for the KSVZ axion.

hep-ph↗

arXiv2025

Quantum field theory approach to neutrino oscillations in dark matter and implications at JUNO

Neutrino oscillation is a significant physical process worthy of in-depth exploration. In this paper, we investigate the matter effect of massive neutrinos in a scalar-type ultra-light dark matter and calculate the neutrino oscillation probability using the quantum field theory method. The result reveals that the neutrino oscillation probability derived from the quantum field theory approach exhibits no additional time dependence, which marks the most significant distinction from the oscillation result obtained through the quantum mechanics method. Furthermore, we discuss predictions of the Juno experiment regarding neutrino oscillation behavior in scalar-type ultra-light dark matter. This study extends the understanding of the interaction between neutrinos and dark matter, which warrants further exploration.

hep-ph↗

arXiv2025

Chiral Magnetic Effect induced Spectator Process for Leptogenesis

Conventional Leptogenesis mechanism, which provides compelling explanation to the origin of the baryon asymmetry of the universe (BAU), assumes the absence of hypermagnetic field in the early universe, thereby disregard the implications of hyper gauge field helicity, that have been thoroughly studied in the magnetogenesis mechanism. In this paper, we address impacts of a general U(1) gauge field on Leptogenesis by deriving equation of motions for the helicity and the energy density of a general magnetic field, to which the chiral magnetic effect (CME) is identified as essential, and studying their effects on the evolution chiral asymmetries. Notably, CME in the $U(1)_{\mathbf{L}_i-\mathbf{L}_j}$ framework, where $\mathbf{L}_{i,j}$ means specific lepton flavor, explicitly breaks the total lepton number and provides an efficient spectator process, that can wash out pre-existing lepton asymmetries. This establishes a natural connection to the wash-in Leptogenesis paradigm. We demonstrate that this spectator effect enables the generation of the BAU, eliminating the need for both an initial $\mathbf{B}-\mathbf{L}$ charge and primordial helicity.

hep-ph↗

arXiv2025

Testing the type-II seesaw mechanism with gravitational waves

Traditional seesaw mechanisms provide an elegant theoretical framework for explaining the small yet non-zero masses of neutrinos. Nevertheless, they face significant experimental challenges, primarily because the energy scale associated with the seesaw mechanism is too high to be directly probed in terrestrial experiments. In this paper, we explore the gravitational waves (GWs) generated via graviton bremsstrahlung during the decay of seesaw particles in the early Universe. Specifically, we compute the GW spectrum resulting from the decay of the Higgs triplet within the type-II seesaw model. Our results demonstrate that the resulting GW spectrum depends sensitively on the mass of the Higgs triplet and its couplings to the Standard Model Higgs doublet and the left-handed lepton doublet. The detection of such a high-frequency GW background could offer a unique experimental window into the seesaw mechanism and provide indirect evidence for its validity.

hep-ph↗

arXiv2025

Completing Axion Double Level Crossings

In this work, we present the refinement of axion double level crossings within the context of multi-axion mass mixing, specifically focusing on cases where the number of axions exceeds two. Our investigation reveals that double level crossings are a common phenomenon in the mass mixing of the $Z_{\mathcal N}$ axion and axion-like particles. Physically, these double level crossings involve a first level crossing at high temperatures followed by a second level crossing induced by the $Z_{\mathcal N}$ axion mass transition at $T_{\rm QCD}$. We introduce the general model for double level crossings, along with several toy examples, and redefine the light and heavy axion scenarios. In the light axion scenario, double level crossings can occur multiple times in the large ${\mathcal N}$ limit. However, excessively large values of ${\mathcal N}$ may also prevent the occurrence of double level crossings. Conversely, in the heavy axion scenario, excessively small ${\mathcal N}$ may similarly prevent their occurrence. Our findings also have some intriguing implications for axion cosmology.

hep-ph↗

arXiv2025

Gravitational Wave Spectrum from the Production of Dark Matter via the freeze-in Mechanism

Since the first detection of gravitational waves by ground-based interferometers, it has emerged as a novel probe for exploring physics in the early universe. The particle nature of cold dark matter (DM) and its underlying production mechanisms remain long-standing unresolved issues in the field. Notably, if DM is generated through the freeze-in mechanism in the early universe, direct laboratory detection becomes extraordinarily challenging due to its extremely weak coupling with standard model particles. In this study, we calculate the graviton bremsstrahlung process involved in the freeze-in production of dark matter, deriving the gravitational wave spectra for both the conventional freeze-in mechanism and ultraviolet freeze-in scenarios. Our analysis reveals that these spectra exhibit distinct characteristics, though they fall beyond the detection limits of currently proposed gravitational wave experiments. However, advancements in high-frequency gravitational wave detection technologies in the future may offer a means to indirectly probe the ultraviolet freeze-in mechanism.

hep-ph↗

arXiv2025

New Physics Search at the CEPC: a General Perspective

The Circular Electron-Positron Collider (CEPC), a proposed next-generation Higgs factory, provides new opportunities to explore physics beyond the Standard Model (SM). With its clean electron-positron collision environment and the ability to collect large samples of Higgs, W, and Z bosons, the CEPC enables precision measurements and searches for new physics. This white paper outlines the CEPC's discovery potential, including studies of exotic decays of the Higgs, Z, and top quarks, dark matter and dark sector phenomena, long-lived particles, supersymmetry, and neutrino-related signatures. Advanced detector technologies and reconstruction techniques, such as one-to-one correspondence reconstruction and jet origin identification, significantly improve sensitivity to rare and weakly interacting processes. The CEPC is particularly well suited to probe the electroweak phase transition and test models of electroweak baryogenesis and dark sector interactions. In addition, global fit analyses highlight the CEPC's complementary role in constraining a wide range of new physics scenarios. These features position the CEPC as a powerful tool for exploring the next frontier in fundamental particle physics in the post-Higgs discovery era.

hep-ex↗

arXiv2024

Probing Sub-eV Dark Photon, Scalar and Axion-like Particle Dark Matters with Transmon Qubits

In this paper, we investigate constraints of the transmon qubit, an improved version of the charge qubit, on bosonic light dark matters. Phonon excitations induced by the scattering or absorption of dark matter on a superconductor may destroy the Cooper pair, leading to the production of quasiparticles made by the electron. By measuring the production rate of the quasiparticle density, one may read out the coupling between dark matter and ordinary matter, assuming that these quasiparticles are solely induced by dark matter interactions. For the first time, we show constraints on the parameter space of the dark photon, light scalar dark matter, and axion-like particles from the measurement of quasiparticles in transmon qubit experiments. This study offers insights for the development of quantum qubit experiments aimed at the direct detection of dark matter in underground laboratories.

hep-ph↗

arXiv2024

Eogenesis via the High-scale Electroweak Symmetry Restoration

In this paper, we propose a novel electron-assisted Baryogenesis scenario that does not require explicit B-L violation, which is essential for the traditional Leptogenesis mechanism. This scenario is based on the assumption of high-scale electroweak symmetry restoration, which implies that the electron Yukawa interaction, crucial for the mechanism, does not reach thermal equilibrium before the electroweak sphaleron process is quenched in the early universe. Primordial charge asymmetries for chiral electrons, which can be generated through various mechanisms such as axion inflation, the evaporation of primordial black holes, or the CP-asymmetric decays of a heavy Higgs doublet, serve as the initial condition for the amplification of the baryon asymmetry through transport equations. Right-handed electron asymmetry is almost irrelevant to the baryon asymmetry due to high-scale electroweak symmetry restoration, leading to both a non-zero baryon asymmetry and the electron asymmetry. We dub this mechanism as the Eogenesis.

hep-ph↗

arXiv2024

Axion effects on gamma-ray spectral irregularities. II: Implications of EBL absorption

The extragalactic background light (EBL) plays a crucial role in the propagation of high-energy particles throughout the Universe. In this work, we explore the impact of the EBL absorption effect on photon to axionlike particle (ALP) conversions from the very-high-energy gamma-ray spectral irregularities. For our purpose, we select four BL Lac blazars: Markarian 501, 1ES 0229+200, PKS 0301-243, and PKS 0447-439 for analysis. Their redshifts range from approximately 0.03 to 0.34. We first discuss the EBL absorption effect on the gamma-ray spectral energy distributions (SEDs) using three common EBL spectral models: Finke-10, Franceschini-17, and Saldana-Lopez-21. Then we consider the photon-ALP conversions in astrophysical magnetic fields. The best-fit chi-square distributions of these EBL models under the ALP assumption in the ALP parameter $\{m_a, g_{a\gamma}\}$ plane are provided, showing similar distributions. For comparison, we define a new delta chi-square, $\chi_d^2$, to quantify the difference in chi-square values. The distributions of $\chi_d^2$ and the gamma-ray SEDs corresponding to the maximum delta chi-square, $\chi^2_{d, \rm max}$, are also presented for comparison. Our results indicate that the influence of these different EBL models is non-dominant at the low-redshift gamma-ray axionscope. In these cases, choosing the latest model, Saldana-Lopez-21, is sufficient. However, as the redshift of the sources increases, this influence becomes more significant.

hep-ph↗

arXiv2024

Axion-Inflation Baryogenesis via New U(1) gauge symmetries

We investigate axion-inflation baryogenesis models, embedded into U(1) gauge symmetric extensions of the Standard Model (SM), in which the new gauge field couples to the pseudo-scalar inflaton via the Chern-Simons coupling. The motion of the inflaton induces a techyonic instability for one of the two helicities of the gauge field, resulting in the production of the helical gauge field. It further leads to the generation of the SM particle number densities via anomalies during the reheating, which is sufficient to generate the matter-antimatter asymmetry of the universe. Our numerical results show that this mechanism works for the $U(1)_{\mathbf L}$, $U(1)_{\mathbf R}$ and $U(1)_{\mathbf{B-L}}$ gauge symmetry cases, where subscripts ${\mathbf L}$, $\mathbf{R}$, $\mathbf{B-L}$ indicate the lepton number, the right-handed fermion and the baryon number minus the lepton number respectively. The key point for these mechanisms to work is that the evolution of the number density for right-handed neutrinos is decoupled from those of the SM particles, which shares the same merit as the Dirac Leptogenesis mechanism.

hep-ph↗

arXiv2024

Upper limit on the axion-photon coupling from Markarian 421

Markarian 421 is a well-known nearby BL Lac blazar at the redshift $z=0.031$. Many previous works were investigated to constrain the axion-photon coupling from its TeV gamma-ray observations, showing the upper limit on the coupling constant $g_{a\gamma} \lesssim 2.0\times 10^{-11} \rm \, GeV^{-1}$ for the axion mass $[5.0\times10^{-10} \, {\rm eV} \lesssim m_a \lesssim 5.0\times10^{-7} \, {\rm eV}]$. While in this work, we obtain a more stringent upper limit on the axion-photon coupling from the 1038 days gamma-ray observations of the blazar Markarian 421. The long-term gamma-ray spectra are measured by the collaborations Large Area Telescope on board NASA's Fermi Gamma-ray Space Telescope (Fermi-LAT) and High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory from 2015 June to 2018 July. We show the best-fit spectral energy distributions (SEDs) of Markarian 421 under the null and axion hypotheses. Then we set the axion-photon limit in the $\{m_a, \, g_{a\gamma}\}$ plane. The 99% $\rm C.L.$ upper limit set by Markarian 421 is $g_{a\gamma} \lesssim 4.0\times 10^{-12} \rm \, GeV^{-1}$ for the axion mass $[1.0\times10^{-9} \, {\rm eV} \lesssim m_a \lesssim 1.0\times10^{-8} \, {\rm eV}]$. It is the most stringent upper limit in this axion mass region.

hep-ph↗

arXiv2024

Supermassive black holes triggered by QCD axion bubbles

The supermassive black holes (SMBHs) are ubiquitous in the center of galaxies, although the origin of their massive seeds is still unknown. In this paper, we investigate the SMBHs formation from the QCD axion bubbles. In this case, the primordial black holes (PBHs) are considered as the seeds of SMBHs, which are generated from the QCD axion bubbles due to an explicit Peccei-Quinn (PQ) symmetry breaking after inflation. The QCD axion bubbles are formed when the QCD axion starts to oscillate during the QCD phase transition. We consider a general case in which the axion bubbles are formed with the bubble effective angle $θ_{\rm eff}\in(0, \, π]$, leading to the minimum PBH mass $\sim\mathcal{O}(10^4-10^7)M_\odot$ for the axion decay constant $f_a\sim\mathcal{O}(10^{16})\, \rm GeV$. The PBHs at this mass region may account for the seeds of SMBHs.

hep-ph↗

arXiv2024

Axion limits from the 10-year gamma-ray emission 1ES 1215+303

We present the limits on photon to axionlike particle (ALP) coupling from the 10-year period observations of the TeV BL Lacertae blazar 1ES 1215+303 (with redshift $z=0.130$). The contemporaneous gamma-ray spectra are measured by the collaborations Fermi-LAT and VERITAS with five flux phases from 2008 to 2017, including four low states and one flare. Using these flux phases, we show the spectral energy distributions (SEDs) under the null/ALP hypotheses and set the combined limit on ALP. The 95% $\rm C.L.$ combined limit set by 1ES 1215+303 with the 10-year gamma-ray data is roughly at the photon-ALP coupling constant $g_{aγ} \gtrsim 1.5\times 10^{-11} \rm \, GeV^{-1}$ for the ALP mass $5.0\times10^{-10} \, {\rm eV} \lesssim m_a \lesssim 1.0\times10^{-7} \, {\rm eV}$.

astro-ph.HE↗

arXiv2024

Light QCD Axion Dark Matter from Double Level Crossings

The even light QCD axion called the $Z_{\mathcal N}$ axion can both solve the strong CP problem and account for the dark matter (DM). We point out that the single and double level crossings can naturally take place in the mass mixing between the $Z_{\mathcal N}$ axion and axionlike particle (ALP). The first level crossing occurs much earlier than the QCD phase transition, while the second level crossing occurs exactly during the QCD phase transition if it exists. We also find that the single level crossing can transform into the double level crossings, depending on the ALP mass $m_A$ versus the zero-temperature $Z_{\mathcal N}$ axion mass $m_{a,0}$. Compared with the no level crossing case, the $Z_{\mathcal N}$ axion relic density can be suppressed in the single level crossing, and enhanced or suppressed in the double level crossings.

hep-ph↗

arXiv2023

Oscillations of Ultralight Dark Photon into Gravitational Waves

The discovery of gravitational waves (GWs) opens a new window for exploring the physics of the early universe. Identifying the source of GWs and their spectra today turn out to be the important tasks so as to assist the experimental detection of stochastic GWs. In this paper, we investigate the oscillations of the ultralight dark photon (ULDP) into GWs in the dark halo. Assuming dark matter is composed of the ULDP and there are primordial dark magnetic fields (PDMFs) arising from the axion inflation and/or the dark phase transition, then the ULDP can oscillate into the GW when it passes through an environment of PDMFs. We derive the local energy density of GWs in the galaxy cluster induced by the instaneous oscillation of ULDP in the PDMFs. These stochastic local GWs exhibit a pulse-like spectrum, with frequency depending on the mass of the ULDP, and can be detected in Pulsar Timing Arrays (PTAs) or future space-based interferometers. We also find that the low-frequency GW signal observed by the NANOGrav collaboration and other PTA experiments can be explained by the oscillation of the ULDP in the PDMFs in the early universe.

hep-ph↗

arXiv2023

Axion and Dark Fermion Electromagnetic Form Factors in Superfluid He-4

Condensed matter materials have shown great potential in searching for light dark matter (DM) via detecting the phonon or magnon signals induced by the scattering of DMs off the materials. In this paper, we study the possibility of detecting electromagnetic form factors of fermionic DM and axion-like particles (ALPs) using superfluid Helium-4. The phonon induced by a sub-GeV fermionic DM scattering off the superfluid can be described using the effective field theory with the interaction between DM and the bulk ${}^4$He. Signals arising from the electromagnetic form factors of light DM in the presence of an external electric field are calculated. Projected constraints on the charge radius, the anapole moment, and the magnetic moment of the DM are derived with 1 kg$\cdot$year exposure. The phonon signal induced by the scattering of ALPs off the superfluid is also calculated, which can put competitive and the first direct detection bounds on ALP-photon-dark photon couplings in the projected experiments.

hep-ph↗