arXiv subjects

Hyun-Chul Kim

Publications and source records attributed to Hyun-Chul Kim.

At least 19 recordsLinked to original sources

arXiv2026

Transverse distributions of the energy-momentum tensor for a spin-$3/2$ baryon

We develop a multipole description of transverse distributions of the energy-momentum tensor for a spin-$3/2$ baryon in frames connected by a longitudinal boost. In the transverse Breit frame, the $T^{00}$, $T^{03}$, and $T^{33}$ matrix elements are expressed through seven multipole form factors for energy, angular momentum, and stress. At finite longitudinal momentum, we factorize the Lorentz mixing of these three components from the spin-$3/2$ Wigner rotations of the external states. The resulting elastic-frame matrix elements contain six transverse multipoles, whose Fourier transforms define the distributions of energy, longitudinal momentum, and longitudinal momentum flux. We also calculate $T^{++}$, $T^{+-}$, and $T^{--}$ directly with light-front Rarita-Schwinger spinors. The elastic-frame construction provides a continuous interpolation from the transverse Breit frame to the infinite-momentum frame. In this limit, the Wigner rotation becomes the Melosh rotation, and the leading elastic-frame matrix elements reproduce the corresponding light-front results. Using the $\Delta$-baryon gravitational form factors obtained in the Skyrme model as a representative numerical input, we find that the energy distribution is dominated by the energy monopole defined in the transverse Breit frame and changes only weakly under longitudinal boosts. Through boost mixing, this monopole provides the dominant contribution to the longitudinal momentum distribution and its flux at finite $P_z$. For a longitudinally polarized spin-$3/2$ target, the distributions contain only the monopole contributions, whereas those of a transversely polarized target exhibit spin-dependent quadrupole and octupole deformations and a dipole that shifts their maxima.

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arXiv2026

Multipole structure of the $N \to \Delta$ Transition Generalized Parton Distributions

We establish the multipole structure of the $N \to \Delta$ transition at the level of the generalized parton distributions (GPDs). We decompose the four transition GPDs into one monopole, two dipole, and one quadrupole components in the transverse plane by a multipole expansion of the covariant transition matrix element in terms of the three-dimensional spin-transition tensors and the transverse momentum transfer. These multipole components are in one-to-one correspondence with the light-front helicity amplitudes. In the zero-skewness limit, the multipole GPDs define impact-parameter transition densities, which generalize the transverse transition charge densities to the $x$-dependent level. These transition densities arise from non-diagonal matrix elements between two distinct hadronic states and must therefore be distinguished from the diagonal densities of the nucleon and the $\Delta$. The multipole transition densities visualize the monopole, dipole, and quadrupole structures of the partonic $N \to \Delta$ transition in the transverse plane.

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arXiv2026

Flavor decomposition of the gravitational form factors and mechanical structure of the proton

We review in the present talk a series of recent works on the flavor decomposition of the gravitational form factors of the proton and its mechanical properties within the framework of the chiral quark-soliton model. Starting from the energy-momentum tensor operator derived from the QCD instanton vacuum, we carry out the twist projection of the energy-momentum tensor operator into its twist-2 and twist-4 components, which enables us to isolate the $\overline{c}$ form factor originating from the twist-4 operator. We present the flavor-decomposed mass, spin, pressure, and shear-force distributions of the proton, together with the corresponding form factors. While the up quark dominates both the mass and the spin of the proton, the strange quark is found to contribute sizably to the $D$-term form factor. We also discuss the mechanical stability of the proton governed by the pressure and shear-force distributions.

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arXiv2026

Heavy mesons from the QCD instanton vacuum beyond the static limit

We study pseudoscalar heavy mesons in the QCD instanton vacuum beyond the static limit. Finite-mass effects in the heavy-light loop are encoded in a separable effective vertex built from a profile function $\phi(\vec{p})$, kept distinct from the static Wilson-line form factor $F_Q^{(\infty)}(\vec{q})$ of the $m_Q\to\infty$ limit. The pseudoscalar two-point function fixes the residual mass $\Lambda$ and the residue-normalized meson-quark coupling, from which we evaluate the decay constant, the spin-independent kinetic matrix element, and the zero-recoil slope of the Isgur-Wise function at order $1/m_Q$. The subleading calculation is restricted to the kinetic (derivative) part of the HQET operators. For a representative vertex calibrated to the $B$-meson decay constant and the spin-averaged $B$-meson mass, we obtain $f_B = 186.8$~MeV, $\Lambda = 184.5$~MeV, $m_b^{\mathrm{eff}} = 5.04$~GeV, $\lambda_1^{(\partial)} = -0.922~\mathrm{GeV}^2$, and $\rho_{\mathrm{IW}}^2 = 1.105$. The kinetic contribution yields a mass shift of order $\Lambda/2$ and a sizable $1/m_Q$ current correction, indicating that the spin-independent nonperturbative $1/m_Q$ sector is a sensitive probe of the finite-mass heavy-light vertex.

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arXiv2026

Nonexistence of hidden-charm pentaquarks in $J/\psi$ photoproduction

We investigate $J/\psi$ photoproduction off the proton, $\gamma p \to J/\psi p$, to elucidate the nonexistence of hidden-charm pentaquark signals reported by the GlueX and CLAS12 experiments. Within a coupled-channel rescattering mechanism, we employ the transition amplitudes from a previous coupled-channel analysis that dynamically generates the $P_{c\bar{c}}$ states. The kernel amplitudes for the transition to the $J/\psi N$ channel include both $t$-channel heavy-meson exchange and $u$-channel heavy-baryon exchange. We find that the rescattering contributions from the $\bar{D}^{(*)}\Sigma_c$ channels -- indispensable for the formation of the $P_{c\bar{c}}$ resonances -- are about one order of magnitude smaller than those from $\bar{D}^{(*)}\Lambda_c$, since $g_{\bar{D}^{(*)}N\Sigma_c}$ is roughly five times smaller than $g_{\bar{D}^{(*)}N\Lambda_c}$. Since the $P_{c\bar{c}}$ resonances couple to the $J/\psi N$ channel predominantly through the $\bar{D}^{(*)}\Sigma_c$ intermediate states, their suppression prevents the pentaquark signal from appearing in photoproduction. With only a single parameter controlling the overall normalization, the present work describes the GlueX and CLAS12 cross sections well. These results suggest that the null result from photoproduction need not be in conflict with the pentaquark signals observed by the LHCb Collaboration.

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arXiv2026

Probing hidden-charm pentaquarks from the $\pi N\rightarrow J/\psi N$ reaction

We investigate the dynamical generation of hidden-charm pentaquark resonances in the $\pi N \to J/\psi N$ reaction utilizing an off-shell coupled-channel formalism. Motivated by the absence of pentaquark signals in $J/\psi$ photoproduction, we evaluate rescattering effects with two-body kernel amplitudes constructed from effective Lagrangians that explicitly incorporate $t$-channel meson and $u$-channel baryon exchanges. We demonstrate that the $u$-channel $\Lambda_c$ exchange, of which an analogous contribution is absent in the photoproduction kernel, greatly enhances the rescattering contributions through the $\bar{D}^{(*)}\Sigma_c$ intermediate states. Consequently, the $\bar{D}^{(*)}\Sigma_c^{(*)}$ channels yield contributions of comparable magnitude to the $\bar{D}^{(*)}\Lambda_c$ channels, directly leading to prominent pentaquark signals. The partial-wave analysis reveals that the $P_{c\bar{c}}(4312)$ and $P_{c\bar{c}}(4457)$ states emerge as clear peak structures with $J^P=1/2^-$ and $3/2^-$, respectively. In contrast, the $P_{c\bar{c}}(4380)$ and $J^P=5/2^-$ states are strongly suppressed because the $\Lambda_c$ exchange does not provide the required tensor interactions. The result for the total cross section reaches the microbarn level at the peak positions.

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arXiv2026

Electromagnetic form factors of the nucleon from the instanton vacuum

We investigate the electromagnetic form factors of the nucleon within an effective chiral theory derived from the QCD instanton vacuum, taking into account the finite current quark mass. The momentum-dependent dynamical quark mass, generated by the instanton-antiinstanton medium, naturally plays the role of a regulator, so that no additional regularization is required to tame the divergences arising from quark loops. The instanton parameters, the average instanton size $\bar{\rho}=0.35$ fm and the average interdistance $\bar{R}=0.86$ fm, together with the dynamical quark mass at zero virtuality $M_0=385$ MeV, are all fixed by the saddle-point equation beyond the chiral limit, leaving no adjustable free parameter in the present calculation. We compute the Sachs electric and magnetic form factors of the proton and neutron, the nucleon charge and magnetization radii, the magnetic moments, and the ratios $\mu_{p,n} G_E^{p,n}(Q^2)/G_M^{p,n}(Q^2)$. The present results are compared with the experimental data, the chiral quark-soliton model ($\chi$QSM), and the Kelly parametrization. The proton charge radius, $\sqrt{\langle r^2 \rangle_\mathrm{ch}^p}=0.841$ fm, is in remarkable agreement with the recent muonic-hydrogen value, and the $Q^2$ dependence of the proton form-factor ratio $\mu_p G_E^p/G_M^p$ is reproduced very well, in clear contrast to the $\chi$QSM. The overall agreement with the experimental data confirms that the effective chiral theory derived from the QCD instanton vacuum provides a consistent and predictive framework for describing the electromagnetic structure of the nucleon.

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arXiv2026

Multipole structure of the nucleon tensor form factors

We investigate the multipole structure of the nucleon tensor form factors within the chiral quark-soliton model based on the $1/N_c$ expansion. Extending the previous leading-order analysis~\cite{Ghim:2025gqo}, we include the rotational $1/N_c$ corrections. These corrections provide the leading nonvanishing contributions to the flavor components that are absent at leading order, thereby completing the flavor decomposition of the tensor multipole form factors at the present order. We numerically evaluate the isoscalar tensor charge, the isovector anomalous tensor magnetic moment, and the isoscalar tensor quadrupole moment, obtaining $g_T^{u+d}=0.81$, $\kappa_T^{u-d}=1.97$, and $E_T^{u+d}(0)=5.98$, respectively. The isoscalar tensor charge and quadrupole moment are mainly governed by the valence-quark contribution, whereas the isovector anomalous tensor magnetic moment receives a sizable Dirac-sea contribution. We also examine the momentum-transfer dependence of the corresponding form factors. They decrease monotonically with increasing $-t$. In particular, the isovector anomalous tensor magnetic form factor shows a pronounced falloff in the small-$|t|$ region, reflecting the importance of the Dirac sea in the tensor dipole structure.

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arXiv2026

Dynamical generation of charmonium-like tetraquarks in an off-shell coupled-channel formalism

We investigate the dynamical generation of charmonium-like ($I=0$) with spin-parity $J^{PC}=0^{++}, 1^{++}, 2^{++}$, and $3^{--}$ in the mass range of $3.6$ to $4.3$ GeV. We employ the off-shell coupled-channel formalism, constructing kernel amplitudes from effective Lagrangians that respect heavy-quark spin-flavor and chiral symmetries. To focus solely on dynamically generated states, we explicitly exclude $s$-channel pole diagrams and include only $t$- and $u$-channel meson exchanges. Solving the integral equations, we identify six poles in the complex energy plane. In the scalar ($0^{++}$) sector, we find a bound state below the $D\bar{D}$ threshold and a resonance at $\sqrt{s_R}=(3861-i\,23)\,\mathrm{MeV}$. For the axial-vector ($1^{++}$) sector, the experimentally observed $\chi_{c1}(3872)$ is reproduced as a bound state near the $D\bar{D}^*$ threshold, alongside a broader resonance at $(3961-i\,32)\,\mathrm{MeV}$, which is a plausible candidate for the $X(3940)$. Furthermore, we find a narrow tensor ($2^{++}$) state at $4005\,\mathrm{MeV}$ and a vector ($3^{--}$) state at $4030\,\mathrm{MeV}$. The present results demonstrate that coupled-channel dynamics, particularly involving the $D^*\bar{D}^*$ channel, play a crucial role in the formation of these charmonium-like exotic states.

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arXiv2025

Quadrupole forces between quark/gluon subsystems inside higher-spin particles

We generalize the mechanical interpretation of the forces between quark and gluon subsystems, previously studied for the nucleon, to arbitrary higher-spin particles. For spin-0 and spin-1/2 particles, this force is characterized by the non-conserved $\bar{c}(t)$ form factor. However, such an interpretation has not yet been established for higher-spin particles due to the intricate structure of the non-conserved energy-momentum tensor (EMT) form factors. By performing a multipole expansion, we identify the physically meaningful combinations of the non-conserved covariant EMT form factors and provide them with a clear mechanical interpretation.

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arXiv2025

Double-strangeness hidden-charm pentaquarks

We investigate the possible existence of double-strangeness hidden-charm pentaquark states, denoted as $P_{c\bar{c}ss}$, within an off-shell coupled-channel formalism. Eleven meson-baryon channels with total strangeness $S = -2$ are constructed by combining charmed mesons and singly charmed baryons. The two-body scattering amplitudes are derived from an effective Lagrangian that respects heavy-quark spin symmetry, hidden local symmetry, and flavor SU(3) symmetry. The Bethe-Salpeter equation is solved using the Blankenbecler-Sugar reduction scheme, and resonances are identified as poles in the scattering amplitudes on the complex energy plane. We find five negative-parity $P_{c\bar{c}ss}$ states with spins $J = 1/2$, $3/2$, and $5/2$, all located below their relevant thresholds. Three positive-parity states are also found: two with $J = 1/2$ and one with $J = 3/2$, lying above the thresholds with substantial widths. The coupling strengths of each resonance to relevant meson-baryon channels are extracted. The sensitivity of the results to the cutoff parameter $Λ_0 = Λ- m$ is examined. These results provide theoretical predictions that may assist future experimental searches for $P_{c\bar{c}ss}$ states in the $J/ψ\,Ξ$ channel.

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arXiv2025

Triple-strangeness hidden-charm pentaquarks

We investigate the possible existence of triple-strangeness hidden-charm pentaquark states in the off-shell coupled-channel formalism. The open-charm meson-baryon $\bar{D}_s\Omega_c$, $\bar{D}_s\Omega_c^*$, $\bar{D}_s^*\Omega_c$, and $\bar{D}_s^*\Omega_c^*$ channels are considered, together with the hidden-charm $J/\psi\Omega$ channel. The two-body kernel Feynman amplitudes are constructed from an effective Lagrangian based on hidden local symmetry and heavy-quark spin symmetry. The coupled Blankenbecler-Sugar equation is solved in the partial-wave helicity basis. We observe two triple-strangeness hidden-charm pentaquark states: $P_{c\bar{c}sss}(4787)$ and $P_{c\bar{c}sss}(4841)$, both with $J^P=1/2^-$. The $P_{c\bar{c}sss}(4787)$ couples dominantly to the $\bar{D}_s^*\Omega_c$ and $\bar{D}_s^*\Omega_c^*$ channels, while the $P_{c\bar{c}sss}(4841)$ couples almost exclusively to the $\bar{D}_s^*\Omega_c^*$ channel. The total transition cross sections of $\bar{D}_s^{(\ast)}\Omega_c^{(\ast)}\to J/\psi\,\Omega$ indicate that the $P_{c\bar{c}sss}(4787)$ is clearly visible in the $J/\psi\,\Omega$ invariant mass spectrum, whereas the $P_{c\bar{c}sss}(4841)$ is obscured by cusp structures and background contributions.

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arXiv2025

Properties of neutron stars and strangeness-mixed stars from a pion mean-field approach

We investigate the properties of the static neutron stars and strangeness-mixed stars, based on the equations of state derived from a pion mean-field approach. Using the empirical data on the pion-nucleus scattering and bulk properties of nuclear matter, we have already fixed all the parameters in a previous work, where the nucleons and hyperons were shown to be modified in various nuclear medium. In the current work, we first examine the energy and pressure inside a neutron star. We show that the central densities in various neutron stars vary within the range of $(3-4)ρ_0$, where $ρ_0$ is the normal nuclear matter density. The mass-radius relations are obtained and discussed. As the slope parameter for neutron matter increases, the radii of the neutron stars increase with their masses fixed. We also study the strangeness-mixed stars or the hyperon stars using the same sets of the parameters. As the strangeness content of strange matter increases, the binding energy per nucleon is saturated and the corresponding equation of state becomes softened. Consequently, the central densities of the strangeness-mixed stars increase. Assuming that recently observed neutron stars are the strangeness-mixed ones, we find that the central densities increase.

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arXiv2025

Production mechanism of doubly charmed exotic mesons $T_{cc}$

We investigate the production mechanism for doubly charmed tetraquark mesons within a coupled-channel formalism. The two-body Feynman kernel amplitudes are constructed using effective Lagrangians that respect heavy quark symmetry, chiral symmetry, SU(3) flavor symmetry, and hidden local symmetry. The fully off-shell coupled scattering equations are solved within the Blankenbecler-Sugar (BbS) reduction scheme. We find three positive-parity and one negative-parity tetraquark states with total spin $J=1$. Among them, two positive-parity states appear as bound states in the isoscalar and isovector $DD^*$ channels, while another appears as a resonance in the $D^*D^*$ channel. A negative-parity resonance is also predicted in the isoscalar channel. We analyze the coupling strengths of these tetraquark states to various channels. The dependence of the results on the reduced cutoff mass $Λ_0$ is examined. The most significant tetraquark state remains stable within the range of $Λ_0=(600-700)$ MeV.

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arXiv2025

Production mechanism of hidden-charm pentaquark states $P_{c\bar{c}s}$ with strangeness $S=-1$

We investigate the hidden-charm pentaquark states with strangeness $S=-1$ ($P_{c\bar{c}s}$) within an off-shell coupled-channel approach based on effective Lagrangians that respect heavy-quark spin symmetry, SU(3) flavor symmetry, and hidden local symmetry. All relevant meson-baryon two-body channels composed of low-lying anti-charmed mesons and singly-charmed baryons with $S=-1$, as well as the $J/\psi \Lambda$ channel, are included. We find a total of eleven negative-parity states and three positive-parity states. Among the negative-parity states, the $P_{c\bar{c}s}(4338)$ and $P_{c\bar{c}s}(4459)$ can possibly be interpreted as $\bar{D}\Xi_c$ and $\bar{D}^* \Xi_c$ molecular states, respectively. We identify a second state, $P_{c\bar{c}s}(4472)$, located close to the $P_{c\bar{c}s}(4459)$ but with different spin and width, which may correspond to the structure observed by the Belle Collaboration. Both states are generated from the $\bar{D}^* \Xi_c$ channel and can be interpreted as spin partners. Their properties are consistent with recent experimental observations, providing strong support for the molecular interpretation of the $P_{c\bar{c}s}$ states. We also observe a two-pole structure near the $\bar{D}_s^* \Lambda_c$ and $\bar{D}\Xi_c^{'}$ thresholds, and find virtual and resonance states in the $\bar{D}^* \Xi_{c}^{'}$ channel depending on spin-parity.

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arXiv2025

Electromagnetic and axial-vector structure of singly heavy baryons in a pion mean-field approach

In this talk, we present a series of recent works on the electromagnetic and axial-vector structures of low-lying singly heavy baryons. We first explain the pion mean-field approach, in which light and singly heavy baryons can be considered on an equal footing. We then discuss the results for the electromagnetic and radiative transition form factors of the singly heavy baryons. We also demonstrate the results for the strong decay rates and quark spin content of the singly heavy baryons. Finally, we propose a consistent way of dealing with the $1/m_Q$ corrections in the pion mean-field approach.

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arXiv2025

Gravitational form factors of the nucleon and their mechanical structure: Twist-2 case

We present a series of recent works on the gravitational form factors (GFFs) of the nucleon within a pion mean-field approach, which is also called the chiral quark-soliton model. We investigate the flavor structure of the mass, angular momentum, and $D$-term form factors of the nucleon. The main findings of the present work are given as follows: the contribution of the strange quark is rather small for the mass and angular momentum form factors, it plays an essential role in the $D$-term form factors. It indicates that the $D$-term form factor is sensitive to the outer part of the nucleon. The flavor blindness, i.e, $D^{u-d}\simeq 0$, is valid only if the strange quark is considered. We also discuss the effects of twist-4 operators. Though the gluonic contributions are suppressed by the packing fraction of the instanton vacuum in the twist-2 case, contributions from twist-4 operators are significant.

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arXiv2025

Heavy-light quark systems from the QCD instanton vacuum: $N_f=1$ light flavor case

We investigate heavy-light quark systems within the framework of the QCD instanton vacuum, focusing on the $N_f = 1$ light flavor case. We derive an effective heavy-light quark interaction from the low-energy QCD partition function and construct a heavy-meson effective Lagrangian. The physical residual mass of heavy mesons, $Λ$, is determined by employing compositeness and normalization conditions. We calculate the masses of $D$ and $B$ mesons and their weak decay constants to the leading order and next-to-leading order in the $1/m_Q$ expansion. The current results for $f_D$ and $f_B$ are in good agreement with recent lattice QCD data and PDG average values.

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