arXiv subjects

Qin Chang

Publications and source records attributed to Qin Chang.

At least 19 recordsLinked to original sources

arXiv2026

Holographic Schwinger effect with Translational Symmetry Breaking

We investigate the holographic Schwinger effect in a background with translational symmetry breaking (TSB) at finite chemical potential. The gravitational background is characterized by two independent parameters: the TSB parameter $α$, which controls momentum relaxation, and the chemical potential $μ$, which determines the finite density of the dual field theory. Using the potential analysis method, we derive the total potential governing the pair production process and examine its dependence on $α$, $μ$, the external magnetic field, and the ratio $β=E/E_c$. Our results show that the effects of $α$ and $μ$ on the Schwinger process strongly depend on the dynamical regime. In the subcritical regime, increasing either $α$ or $μ$ lowers the potential barrier and facilitates pair production. However, near and above the critical electric field, the roles of these two parameters become qualitatively different. While increasing the chemical potential lowers the total potential and enhances the Schwinger pair production process, increasing the translational symmetry breaking parameter shifts the potential upward and suppresses the production process. We further show that, at fixed $β=E/E_c$, a perpendicular external magnetic field lowers the effective potential barrier and thereby facilitates the Schwinger process, while the physical electric field changes accordingly through the magnetic-field dependence of $E_c$. The corresponding pair production rate is not independently calculated; instead, its qualitative behavior is characterized through a proxy derived from the total potential. Overall, our analysis provides a comprehensive picture of how translational symmetry breaking, finite density, and external magnetic fields influence holographic non-perturbative pair production.

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arXiv2026

Semileptonic $\bar{B}^*_{c} \to (P, V) \ell^- \barν_{\ell}$ decays within the self-consistent light-front quark model

Motivated by the recent observation of the $B_c^*$ meson by the ATLAS Collaboration and its promising discovery potential at future high-luminosity colliders, we investigate the semileptonic decays $\bar{B}^*_{c} \to (P, V) \ell^- \barν_{\ell}$ ($P=D$, $B_{d,s}$, $η_c(1S,\,2S,\,3S)$; $V=D^*$, $B^*_{d,s}$, $ψ(1S,\,2S,\,3S)$) in the Standard Model. Using the relevant form factors obtained in the self-consistent covariant light-front quark model, we present theoretical predictions for several physical observables, including the branching fraction, the ratios of branching fractions $R_{(L)}$, the longitudinal polarization fraction of the daughter vector meson, the $τ$ lepton spin asymmetry and the forward-backward asymmetry. It is found that the branching fractions for the Cabibbo-favored $\bar{B}^*_{c}$ $\to$ $\bar{B}^*_{s} e^- \barν_e$ and $\bar{B}^*_{s} μ^- \barν_μ$ decays reach the order of ${\cal O}(10^{-6})$, which might be firstly observed at the future high-luminosity Large Hadron Collider.

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arXiv2026

Investigation of low-lying $Ω_b(1P)$ states in an unquenched coupled-channel framework

In this work, we study the unquenched effects on the low-lying $Ω_b(1P)$ states with a coupled-channel equations. We reveal how the unquenched effects affect the $Ω_b(1P)$ spectroscopy and component mixing. The numerical results indicate that the $J^P=1/2^-$ $Ω_b(1P)$ state dominated by the $j_\ell=0$ configuration exhibits significant coupled-channel effects due to its $S$-wave coupling to the $Ξ_b\bar{K}$ channel, where $j_\ell$ denotes the total angular momentum of the light flavor degrees-of-freedom. In this scenario, the mass of this state may be shifted close to or below the $Ξ_b\bar{K}$ threshold, making the radiative and isospin-breaking channels kinematically allowed decay processes. The present analysis provides a coupled-channel perspective on the low-lying $Ω_b(1P)$ spectrum and offers guidance for future experimental studies of excited bottom baryons.

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arXiv2026

Probing strange-sector flavor-changing neutral currents at DUNE-like facilities

We investigate the sensitivity of DUNE-like facilities to flavor-changing neutral-current interactions between down and strange quarks through inclusive deep-inelastic neutrino scattering. In the framework of a general low-energy effective Lagrangian for Dirac and Majorana neutrinos, we evaluate projected sensitivities for CP- and $τ$-optimized beam configurations. The strongest bounds on the dimensionless Wilson coefficients, defined relative to the Fermi constant $G_F$, reach $\mathcal{O}(10^{-3})\text{--}\mathcal{O}(10^{-2})$ at the near detector, whereas far-detector limits are roughly one order of magnitude weaker. When systematic uncertainties are included, the near-detector sensitivity is dominated by the background-rate uncertainty, while the far-detector sensitivity is limited primarily by the signal-rate uncertainty. At the far detector, the dependence of the projected bounds on the neutrino mass ordering and the Dirac CP-violating phase $δ_{\mathrm{CP}}$ is largely confined to coefficients involving the electron flavor.

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arXiv2026

Radiative generation of chiral vector operators in $b\to s ν\barν$ transition

The recent Belle II evidence for $B^+ \to K^+ ν\barν$, combined with a suppressed branching fraction ratio $R \equiv \mathcal{B}(B^0 \to K^{*0} ν\barν) / \mathcal{B}(B^+ \to K^+ ν\barν)$, necessitates new physics contributing to both left- and right-handed vector operators. We perform a systematic topological classification of one-loop completions that radiatively generate both operators without tree-level mediation, and construct two minimal benchmark scenarios: a scalar-rich model and a fermion-rich model. Evaluating these frameworks under specific benchmark mass schemes and four distinct flavor structures, we find a generic anti-correlation where enhancing one decay channel typically suppresses the other. A notable exception is a decoupled flavor configuration within the scalar-rich model, which yields simultaneous constructive interference, reducing $R$ by $4.22\%$ while satisfying all complementary flavor bounds. Under a universal coupling assumption, the scalar-rich model yields exact cancellation in the charged mode, highlighting the non-trivial interplay between flavor structure and loop-generated Wilson coefficients.

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arXiv2026

Interpreting the 95 GeV di-photon and di-tau Excesses in the Georgi-Machacek Model

We revisit the 95~GeV $γγ$ and $ττ$ excesses in the Georgi--Machacek model using a combined fit to a single light $CP$-even custodial singlet $H$. Using the one-loop renormalization group-improved effective potential and positive-definiteness conditions for vacuum stability, together with perturbative unitarity, electroweak precision tests, $B$-physics, and Higgs data, we identify a narrow but viable parameter region. In the allowed region $μ_{γγ}$ approaches the experimental central value, while $μ_{ττ}$ reaches only $\sim0.5$ (about $1.4σ$ below the CMS central value) and remains within the $2σ$ interval - a good description of the di-photon excess but only a partial accommodation of the di-tau excess. The scenario predicts characteristic patterns in the singlet mixing and triplet vacuum expectation values and is highly testable at the HL-LHC and future lepton colliders via precision $κ_V$ measurements and direct exotic searches.

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arXiv2026

Searching for Ultralight Dark Matter with M{ö}ssbauer Resonance

We investigate the feasibility of probing the interactions between ultralight scalar dark matter and atomic nuclei using a stationary Mössbauer spectroscopy scheme. The exceptional energy resolution of the Mössbauer resonance enables searches for tiny nuclear energy shifts induced by the local dark matter field. The dark matter mass range considered in this work is $10^{-18}$--$10^{-8}~\mathrm{eV}$. We present projected constraints for two candidate Mössbauer isotopes, $^{109}\mathrm{Ag}$ and $^{45}\mathrm{Sc}$, with $^{109}\mathrm{Ag}$ providing the strongest sensitivity. For $^{109}\mathrm{Ag}$, projected sensitivities as low as approximately $10^{-19}$, $10^{-22}$, and $10^{-21}~\mathrm{GeV^{-1}}$ can be achieved for the scalar DM--photon, DM--gluon, and DM--quark couplings $f_γ^{-1}$, $f_{g}^{-1}$, and $f_{\hat{m}}^{-1}$, respectively. In the low-mass region, the projected sensitivity to the scalar DM--photon coupling approaches the current constraints from equivalence-principle (EP) tests. These results demonstrate that Mössbauer-based techniques provide a promising and competitive approach for probing ultralight dark matter interactions with Standard Model particles.

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arXiv2026

Light-cone Distribution Amplitudes of Vector Mesons within the Self-Consistent Light-front Quark Model

In this paper, we investigate the twist-2 and twist-3 light-cone distribution amplitudes (LCDAs) of vector mesons within the self-consistent light-front quark model, and the $n$th Gegenbauer moment $a_n(μ)$, $ξ$-moment $\langle ξ^n\rangle^{\parallel(\perp)}$ and transverse moment $\langle \mathbf{k}^n_\perp\rangle^{\parallel(\perp)}$ are also carried out. Adopting the parameter set fixed by confinements from the mesonic decay constants, we perform the numerical analysis and the results reveal several key insights: (i) The flavor symmetry breaking effects are more pronounced in the twist-3 LCDAs of vector mesons, which leads to the establishment of $a_1^\perp>a_1^\parallel$ and $\langle ξ^n\rangle^\perp>\langle ξ^n\rangle^\parallel$. This is consistent with previous findings in research for the LCDAs of pseudoscalar mesons. (ii) For vector mesons, the twist dependence decreases in the heavy quark limit which lead to $ϕ_{2}^\parallel(x)\simeqϕ_{3}^\perp(x)$. For pseudoscalar and vector mesons composed of the same quark constituents, their LCDAs with the same twist exhibit similarity and gradually converge as the increasing of quark mass, i.e., $ϕ_{2}^A(x)\simeqϕ_{2}^\parallel(x)$ and $ϕ_{3}^P(x)\simeqϕ_{3}^\perp(x)$ in $m_q\rightarrow\infty$. In the heavy-quark limit within the self-consistent LFQM framework, we find an approximate correlated spin-twist independence pattern, with $ϕ_2^A(x) \simeq ϕ_2^{\parallel}(x) \approxϕ_3^P(x) \simeq ϕ_3^{\perp}(x)$, resulting from the suppression of twist dependence and the approximate spin independence between pseudoscalar and vector mesons.

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arXiv2026

Semileptonic decay of $Λ_b^0 \to Λ_c (2860)^+/Λ_c(2625)^+\ell^-\overlineν_\ell$ within QCD light-cone sum rules

In this work, we calculate the transition form factors for the weak decays $Λ_b^0 \to Λ_c(2860)^+$ and $Λ_b^0 \to Λ_c(2625)^+$ using QCD light-cone sum rules, and compute the branching fractions of the corresponding semileptonic decays $Λ_b^0 \to Λ_c(2860)^+ \ell^- \barν_\ell$ and $Λ_b^0 \to Λ_c(2625)^+ \ell^- \barν_\ell$. Our predicted branching fraction for $Λ_b^0 \to Λ_c(2625)^+ \ell^- \barν_\ell$ is consistent with experimental data and other theoretical predictions, validating the reliability of our method. On this basis, we also present the branching fraction of $Λ_b^0 \to Λ_c(2860)^+ \ell^- \barν_\ell$. These results may serve as a theoretical reference for future experimental measurements of this decay channel.

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arXiv2026

Holographic Schwinger Effect In a Step Dilaton Background

We investigate the holographic Schwinger effect in a confining background with a step dilaton profile, which induces a sharp transition between ultraviolet and infrared regimes and provides a qualitatively distinct realization of confinement. Within this framework, the quark--antiquark potential is extracted from the classical configuration of a fundamental string, allowing for a direct analysis of vacuum instability and pair production. In the absence of a magnetic field, the step dilaton leads to a significantly sharper suppression of the potential barrier as the electric field increases, implying an enhanced sensitivity of the critical electric field compared to smooth soft-wall models and demonstrating that the abrupt geometric transition qualitatively enhances the onset of vacuum decay. Incorporating an external magnetic field through the Dirac--Born--Infeld action, we find a nontrivial and amplified deformation of the potential barrier, resulting in a pronounced shift of the critical electric field that depends on both the magnitude and orientation of the magnetic field. Overall, the step dilaton background exhibits a substantially stronger response of the Schwinger effect to external electromagnetic fields than conventional soft-wall models, providing a novel mechanism for controlling pair production and highlighting the crucial role of dilaton structure in non-perturbative dynamics of holographic QCD.

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arXiv2026

Quarkonium light-cone distribution amplitudes: twist structure and mass dependence

We present a systematic study of the leading- and next-to-leading-twist light-cone distribution amplitudes (LCDAs) of ground-state pseudoscalar and vector quarkonium within the light-front quark model (LFQM). By implementing the replacement $M \to M_0$, we analyze the longitudinal and transverse structures of the LCDAs, together with their Gegenbauer moments, $ξ$-moments, and transverse momentum moments. We show that charge-conjugation symmetry enforces the exact vanishing of all odd Gegenbauer moments and odd $ξ$-moments. For pseudoscalar quarkonium, the twist-2 and twist-3 LCDAs become identical, which leads to the same Gegenbauer moments, $ξ$-moments, and transverse momentum moments. For vector quarkonium, although the twist-2 and twist-3 LCDAs differ in the case of finite quark masses, they progressively converge as the quark mass increases. In the heavy-quark limit, all quarkonium LCDAs satisfy $ϕ^A_{2} = ϕ^P_{3} \simeq ϕ^{\parallel}_{2} \simeq ϕ^{\perp}_{3}$, demonstrating an emergent twist-independence of quarkonium distribution amplitudes. We further find that the LCDAs become increasingly peaked and narrower with increasing quark mass, indicating that the meson system becomes increasingly close to a nonrelativistic bound state, with a more uniform and stable distribution of internal longitudinal momentum. For all quarkonium, the peak value exhibits a simple phenomenological scaling behavior governed by the ratio $m/β$. The transverse momentum moments increase with the meson mass, indicating a progressively more compact bound-state structure. These results reveal a universal and systematic evolution of quarkonium LCDAs driven by the quark mass.

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arXiv2026

Form Factors for $B_c^*\to η_c+lν_l$ at NLO in QCD

We present the Non-Relativistic QCD (NRQCD) calculations at the next-to-leading order (NLO) of $α_s$ for $B_c^*\to η_c$ vector, axial-vector, tensor and axial-tensor form factors, and obtain complete analytical expressions for the form factors, along with their asymptotic forms in the hierarchical heavy quark limit. Our results show that the NLO corrections are both sizable and well-behaved in the low squared transfer momentum $(q^2)$ region. Using the NRQCD + lattice + $z$-series method, we further provide theoretical predictions for $B_c^*\to η_c$ form factors across the full physical $q^2$ range. Based on these predicted form factors, we finally compute the decay widths and branching fractions for the semileptonic decays $B_c^*\to η_c+l{ν_l}$.

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arXiv2026

The $Λ_{b} \to Λ$ transition form factors in perturbative QCD approach

In this work, we investigate the $Λ_b \to Λ$ transition form factors in the perturbative QCD (PQCD) approach, incorporating higher-twist light-cone distribution amplitudes (LCDAs). The resulted form factors show that higher-twist LCDAs are dominant numerically. By combining our PQCD predictions at low-$q^2$ with lattice QCD results at high-$q^2$, $z$-series expansion fits are performed to parametrize the form factors over the full kinematic range. We also provide the prediction for physical observables in the rare decay $Λ_b \to Λμ^+ μ^-$, including the differential branching fraction, dilepton longitudinal polarization fraction, and forward-backward asymmetries (lepton-side, hadron-side, and combined lepton-hadron). Our obtained form factors are consistent with those in other theoretical methods within the uncertainties.

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arXiv2026

Distribution Amplitudes of Pseudoscalar Mesons within the Light-Front Quark Model

In this paper, we investigate the distribution amplitudes (DAs) of pseudoscalar mesons within the light-front quark model (LFQM), and the $n$-th Gegenbauer moment $a_n(μ)$, $ξ$-moment $\langle ξ^n\rangle$ and transverse moment $\langle \mathbf{k}^n_\perp\rangle$ are also studied. Two parameter sets are adopted by fitting to mesonic decay constants or mass spectra, leading to distinct schemes for numerical analysis. Our results reveal several key insights: (1) Under scheme-I, the second Gegenbauer moment for the pion, $a_{2,π} = 0.054$, is in excellent agreement with predictions from the platykurtic model and the nonlocal chiral quark model (NL$χ$QM). Meanwhile, scheme-II yields $a_{2,π} = 0.126$, which is consistent with lattice QCD (LQCD), Dyson-Schwinger equation (DSE) and data-driven light-cone sum rules (LCSR) analyses. Furthermore, we observe that $ϕ_{2,π} = ϕ^P_{3,π}$ with the replacement $M \to M_0$. (2) Flavor symmetry breaking effects are found to be more significant in higher-twist DAs, with a notable relation $a_1^P\approx 2 a_1$ emerging in heavy meson systems. (3) Interestingly, numerical analysis indicates that the transverse moment $\langle \mathbf{k}^n_\perp\rangle$ is twist-independent with the replacement $M\to M_0$. Additionally, we propose an empirical scaling relation $\frac{Δm^{0.61}}{β^{1.10}}x_p=0.67$ to effectively describe the shape of DAs for heavy mesons. Collectively, these results demonstrate that mass asymmetry remarkably influences DAs and kinds of moments, these theoretical predictions will be tested in future experiments.

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arXiv2026

Search for Light Dark Matter in Rare Meson Decays

Current dark matter direct detection experiments have low sensitivity to sub-GeV dark matter. In this work, we demonstrate that rare $B$ and $K$ meson decays with missing energy in the final state can serve as efficient probes in this mass range. We analyze a generic $Z^{\prime}$ portal dark matter model and derive upper limits on its parameters from experimental bounds on the rare $B$ and $K$ meson decays. Our results show that such meson decay processes provide complementary constraints to current direct detection experiments for sub-GeV dark matter, particularly for interaction forms mediated by dark matter momentum-dependent operators.

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arXiv2025

Semileptonic decay of double strangeness heavy flavor baryons

This paper investigates the double strangeness heavy flavor baryons $Ω_c^0$ and $Ω_b^-$, which contain two strange quarks. Using QCD light-cone sum rules (LCSRs), we calculate the form factors for the Cabibbo-suppressed processes $Ω_c^0\toΞ^-$ and $Ω_b^-\toΞ^0$, corresponding to the heavy-quark transitions $c\to d$ and $b\to u$, respectively. Combining these with the helicity amplitude formalism for semileptonic decay differential widths, we computed the branching fractions of their corresponding semileptonic decay processes. Our analysis reveals significant discrepancies between two versions of QCD LCSRs: one using the light-cone distribution amplitudes (LCDAs) of the final-state $Ξ$ baryon and the other using the LCDAs of the initial-state double strangeness heavy flavor baryons. The results obtained with the $Ξ$ baryon's LCDAs show excellent agreement with other theoretical calculations. However, when using the LCDAs of the double strangeness heavy flavor baryons, the results differ by orders of magnitude, warranting further investigation.

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arXiv2025

Interpretation of 95 GeV Excess within the Georgi-Machacek Model in Light of Positive Definiteness Constraints

The recent observation of a di-photon excess around 95 GeV by the CMS and ATLAS Collaborations, along with the $b\bar{b}$ excess reported by the LEP Collaboration in the same mass region, has drawn significant interest in the possibility of new physics beyond the Standard Model (SM). The Georgi-Machacek (GM) model, which extends the Higgs sector of the SM by introducing additional triplet scalars while preserving custodial symmetry at tree level, provides a compelling framework to explain both excesses simultaneously via a light custodial singlet Higgs. In this work, we investigate whether the GM model can still accommodate these excesses when taking into account newly proposed vacuum stability constraints, particularly the positive definiteness conditions. Our numerical analysis not only confirms the existence of a viable parameter space capable of explaining the 95 GeV excesses, but also demonstrates that, compared to traditional tree-level constraints at the electroweak scale, the positive definiteness conditions further expand the allowed parameter space, thereby enhancing the viability of the GM model. Furthermore, we emphasize that future collider experiments will play a crucial role in testing this interpretation by refining Higgs coupling measurements and searching for additional Higgs bosons.

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arXiv2025

Polarization Probes of New Physics in Lepton-Flavor-Violating Hyperon Production from $e^- N \to τ^- Y$ Scattering

While charged lepton-flavor-violating tau decays to strange mesons provide powerful probes of the underlying $τ\to\ell d \bar{s}$ transition, the corresponding decay modes involving hyperons are kinematically forbidden. To address this gap, we propose the quasi-elastic scattering processes $e^- N \to τ^- Y$. Within a general low-energy effective Lagrangian, we perform a comprehensive analysis of the final-state polarizations of both the $τ$ lepton and the hyperon $Y$, systematically addressing the theoretical uncertainty driven by the model dependence of form factors. Our analysis demonstrates that while this uncertainty leads to large variations in predicted rates, the polarization observables can potentially distinguish between the form factor models, and help to determine the chiral nature of the vectorial new physics interaction. Finally, we forecast the event rates for future facilities, revealing a strong dependence on the form factor models, with predicted yields ranging from potentially observable levels to rates far below current detection thresholds.

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