arXiv subjects

Seung-il Nam

Publications and source records attributed to Seung-il Nam.

At least 19 recordsLinked to original sources

arXiv2026

Zero-locus diagnostic of the direct contact term in $ϕ\toπ^+π^-π^0$

We formulate, within a fixed FSI-improved amplitude convention, a zero-locus diagnostic for the direct contact term in $ϕ\toπ^+π^-π^0$. The resonant $ρπ$ contribution is dressed channel by channel with the complex Omnès function, the direct term is multiplied by the averaged Omnès factor, and a real constant background is retained as a smooth residual amplitude. In this convention, $F^{\rm FSI}=R+c e^{iδ_{\rm dir}}D$ with $c=g_{\phi3π}^{\rm eff}$, and the difference of two constrained invariant-mass gradients of the reduced Dalitz density gives a local quadratic identity for $c$ on the zero-contour. The construction provides a closure and stability diagnostic complementary to a global Dalitz-plot fit. For the benchmark input $g_{\phi3π}^{\rm eff}=-24.0~{\rm GeV}^{-3}$, we identify a numerically stable zero-contour branch around $M_{+0}=0.460\pm0.025~{\rm GeV}$ and $M_{+-}\le0.560~{\rm GeV}$, where the tight-contour median estimator reproduces $g_{\phi3π}^{\rm est}\simeq -24.0~{\rm GeV}^{-3}$, a closure recovery of the injected coupling within the chosen FSI-improved convention rather than an independent extraction. We further vary the mass-window selection of the branch. The median remains close to the injected value under shifts of the $M_{+0}$ window and moderate changes of its width, while the mean and standard deviation are more sensitive to outlier points near unstable quadratic branches. The method therefore identifies Dalitz-plane regions where the direct-term interference is locally well-conditioned and provides useful guidance for control-region choices and systematic variations in future global fits.

hep-ph↗

arXiv2026

Three-body decay $ϕ\toπ^+π^-π^0$ with Omnès-type final-state interactions

We investigate the decay $ϕ\toπ^+π^-π^0$ in an effective-Lagrangian framework that keeps the dominant $ρπ$ mechanism, the direct three-pion term, and a real constant background contribution explicitly separated at the amplitude level. The resonant contribution is described with the Gounaris-Sakurai propagator, the neutral channel includes $ρ^0$-$ω$ mixing, and the leading elastic $I=1$, $P$-wave $ππ$ final-state interaction is incorporated through the complex, $s$-dependent Omnès function $Ω_1(s+i0)$. The Omnès factor is applied channel by channel to the three two-pion subsystems, while the direct amplitude is dressed by the averaged factor $Ω_{\rm dir}=(Ω_0+Ω_++Ω_-)/3$. To avoid double-counting the elastic rescattering already contained in the physical $ρ\toππ$ coupling, we introduce $g_{ρππ}^{\rm bare}=g_{ρππ}^{\rm phys}/|Ω_1(m_ρ^2)|$ in the Omnès-dressed $ρ$-exchange amplitude. With the final parameter set used in this work, we obtain $Γ_{\rm th}=0.6915~{\rm MeV}$, slightly above the empirical estimate $Γ_{\rm exp}\approx0.660\pm0.020~{\rm MeV}$, while the direct integrated weight is reproduced at the KLOE scale, $I_{\rm dir}=8.393\times10^{-3}$. The corresponding resonant weight is $I_{ρπ}=0.9619$. The resulting Dalitz distribution retains the expected $ρπ$-dominated three-band structure and shows the localized neutral-channel deformation induced by $ρ^0$-$ω$ mixing. Remaining discrepancies in the $x$ and especially the $y$ projections indicate the limitations of the present minimal single-channel Omnès implementation and motivate a next-stage analysis based on a full dispersive crossed-channel treatment and a direct fit to the efficiency-corrected KLOE Dalitz-bin data.

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arXiv2026

Lesser Green's Function and Chirality-Reduced Entropy via the In-Medium NJL Model

We investigate chiral symmetry restoration in hot and dense quark matter using a correlator-based chirality-reduced entropy in the in-medium Nambu--Jona-Lasinio (NJL) model. Starting from the lesser Green's function $G^{<}(k)$ in the real-time formalism, we construct the equal-time correlation matrix $C(\bm{k})$ and define the left-handed reduced correlator $C_L(\bm{k}) = P_L C(\bm{k}) P_L$. The corresponding von Neumann entropy, $S_χ= -\mathrm{Tr}\left[C_L \ln C_L + (1-C_L)\ln(1-C_L)\right]$, characterizes the mixedness of the chirality-reduced subsystem. We show that the reduced correlator retains a nontrivial helicity structure and must therefore be described by its full eigenvalue spectrum rather than by a single scalar occupation probability. The self-consistent dynamical quark mass $M_q(T,μ_q)$ reproduces the expected QCD-like phase structure, with a second-order transition in the chiral limit and a smooth crossover for finite current quark mass. The chirality-reduced entropy correlates with chiral restoration but is not itself an order parameter; instead, it provides complementary information via the full spectrum of the reduced correlator. Our numerical results show that $S_χ$ exhibits characteristic nontrivial behavior across the chiral transition region and serves as an information-theoretic diagnostic of reduced chiral-sector mixedness.

hep-ph↗

arXiv2026

A Maximum-Entropy Method for Zero-Skewness Valence GPDs Constrained by Nucleon Electromagnetic Form Factors

We formulate a reduced-profile maximum-entropy method (MEM) framework for constructing constrained zero-skewness valence-quark generalized parton distribution (GPD) transverse profiles from the four nucleon electromagnetic form factors $F_1^p(t)$, $F_1^n(t)$, $F_2^p(t)$, and $F_2^n(t)$. The form-factor sum rules fix only $x$-integrated moments of the GPDs; the forward limit of $H_v^q$ is fixed separately by the valence parton distribution functions, and the normalization of $E_v^q$ by the flavor anomalous magnetic moments. These complementary constraints are combined through the ansatz $H_v^q(x,t)=q_v(x)\exp[t f_H^q(x)]$ and $E_v^q(x,t)=e_v^q(x)\exp[t f_E^q(x)]$, where the positive profile functions encode the $x$-dependent transverse structure. Rather than attempting an unrestricted functional inversion, we use the entropy functional as a regularizing criterion on a low-dimensional positive profile manifold. In the numerical proof-of-concept calculation, a smooth elastic form-factor input and analytic forward distributions are adopted, together with the reduced form $f(x)=0.05+(1-x)^2\exp(c_0+c_1x+c_2x^2)$, which suppresses local modes that elastic moments alone cannot constrain. Within this reduced ansatz, the resulting profiles reproduce the imposed elastic moment constraints, satisfy the forward normalizations after discrete-grid normalization, and give impact-parameter distributions with the expected transverse shrinkage at large $x$. The construction provides a controlled zero-skewness baseline for connecting elastic form-factor constraints to $x$-dependent transverse profiles, and it offers a stable starting point for future analyses incorporating empirical form-factor fits, modern PDF inputs, lattice-QCD generalized form factors, and hard exclusive observables.

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arXiv2026

Flow-Based Global Proposals for Monte Carlo Sampling in SU(2) Lattice Gauge Theory

We propose a formally valid machine-learning-assisted global proposal mechanism for Monte Carlo sampling in lattice gauge theory. The construction is based on a coupling-flow update on the SU(2) lattice-link manifold, in which active links are transformed conditionally on a frozen-link background. For fixed frozen links, the proposal is explicitly invertible and preserves the product Haar measure, so it can be embedded into a Metropolis-Hastings correction without requiring an explicit model of the full proposal density. We implement the method in two-dimensional pure SU(2) lattice gauge theory and benchmark it against a baseline local Metropolis algorithm used as a controlled reference kernel. In the present testbed, the learned proposal reproduces the target ensemble within statistical resolution across the tested configurations. In matched local-step comparisons, the learned proposal reproduces the target ensemble at a quality comparable to the baseline, but does not outperform the pure local baseline in the conservative matched-step case examined with seed-level statistics within this proof-of-principle setup. At the same time, a favorable mixed-step hybrid configuration yields a modest improvement in effective sample size per unit runtime. Because the learned transformation remains in a near-identity regime, the present results should be interpreted as a proof-of-principle demonstration of formal correctness and limited, configuration-dependent efficiency gain within a controlled comparison, rather than as evidence of superiority over optimized conventional update schemes. This work provides a concrete foundation for extending machine-learned nonlocal updates to larger lattices and non-Abelian gauge theories relevant to lattice QCD.

hep-lat↗

arXiv2026

Thermal modification of $K_1(1270)\to π^+π^-K^+$ in a hot hadronic medium

We study the thermal modification of the exclusive decay $K_1^+(1270)\to π^+π^-K^+$ in a hot hadronic medium. The decay amplitude is constructed from effective hadronic interactions dominated by the $ρ$- and $K^*$-pole contributions, which enables a Dalitz-level analysis of the three-body decay in medium. Thermal effects associated with partial chiral-symmetry restoration are incorporated through a phenomenological interpolation toward vector--axial-vector degeneracy near the chiral crossover. As the temperature increases, the reduction of the parent $K_1$ mass strongly compresses the available three-body phase space, leading to substantial deformation of the Dalitz distribution and invariant-mass spectra, as well as to a pronounced suppression of $Γ_{K_1\toπ^+π^-K^+}(T)$. As a further step toward future experimental comparison, we introduce normalized shape observables that quantify the thermal evolution of the $K^*$-dominated $πK$ region, the upper-edge weight of the $ππ$ spectrum, and the compactification of the Dalitz population. The dominant effect identified in the present framework is therefore kinematic: thermal phase-space reduction in the strange axial-vector channel. These results suggest that the exclusive $K_1(1270)$ channel may provide a useful qualitative probe of in-medium strange axial-vector dynamics near the pseudocritical region.

hep-ph↗

arXiv2025

The curvature of the pseudo-critical line in the QCD phase diagram from mesonic lattice correlation functions

In the QCD phase diagram, the dependence of the pseudo-critical temperature, $T_{\rm{pc}}$, on the baryon chemical potential, $μ_B$, is of fundamental interest. The variation of $T_{\rm{pc}}$ with $μ_B$ is normally captured by $κ$, the coefficient of the leading (quadratic) term of the polynomial expansion of $T_{\rm{pc}}$ with $μ_B$. In this work, we present the first calculation of $κ$ using hadronic quantities. Simulating $N_f=2+1$ flavours of Wilson fermions on {\sc Fastsum} ensembles, we calculate the ${\cal O}(μ_B^2)$ correction to mesonic correlation functions. By demanding degeneracy in the vector and axial-vector channels we obtain $T_{\rm{pc}}(μ_B)$ and hence $κ$. While lacking a continuum extrapolation and being away from the physical point, our results are consistent with previous works using thermodynamic observables (renormalised chiral condensate, strange quark number susceptibility) from lattice QCD simulations with staggered fermions.

hep-lat↗

arXiv2025

Entanglement entropy for $π^+ p$ elastic scattering using spin-density matrix

We study the $π^+ p$ elastic scattering process using an effective Lagrangian approach that incorporates the $s$-, $u$-, and $t$-channel amplitudes, including $Δ^{++}(1232)$, $Δ^{0}(1232)$, neutron, and $ρ^0$ contributions. By constructing the spin-density matrices from the scattering amplitudes, we derive the von Neumann (entanglement) entropy associated with the spin degrees of freedom of the initial and final state particles. We compute the entropy by performing a partial trace over the spin subsystems, and its behavior is analyzed as a function of the center-of-mass energy $W$ and scattering angle $θ$. We find that entropy exhibits nontrivial angular and energy dependences, reflecting the interplay among resonance contributions and background amplitudes. In particular, the $Δ^{++}$ region shows relatively low entanglement, suggesting spin coherence dominated by the specific resonant contribution, while the background contributions increase entropy due to mixed spin configurations. These results indicate that entanglement entropy can serve as a novel probe into the quantum structure of hadronic scattering amplitudes, providing complementary insights beyond conventional cross-section analyses.

hep-ph↗

arXiv2025

Double $ϕ$-meson production via $\bar{p}p$ scattering

We investigate double $ϕ$ production in $\bar{p}p$ reactions near threshold using an effective Lagrangian approach, highlighting a significant OZI rule violation through hadronic processes. Our model includes $t$- and $u$-channel contributions from the nucleon and $s$-wave $N^*$ resonances ($N^*(1535,1650,1895)$), as well as the pentaquark-like state $P_s(2071)$. In the $s$-channel, scalar ($f_0$), tensor ($f_2$), and pseudoscalar ($η(2225)$) mesons are incorporated. The $N$, $N^*$, $f_0$, and $f_2$ contributions enhance the cross-section and generate peak structures near $W=2.2$ GeV, consistent with JETSET data, while cusp structures arise from $\barΛΛ$ and $\barΣΣ$ threshold openings. Polarization observables, such as the spin density matrix element (SDME), further elucidate the underlying hadronic mechanisms.

hep-ph↗

arXiv2025

Double $ϕ$ Production in $\bar{p}p$ Reactions Near Threshold

We employ an effective Lagrangian approach to investigate double $ϕ$ production in the $\bar{p}p$ reaction near the threshold and describe a notable violation of the Okubo-Zweig-Iizuka (OZI) rule in this reaction process through hadronic degrees of freedom, using the currently available theoretical and experimental information. The ground-state nucleon and its $s$-wave resonances, i.e., $N^*(1535,1650,1895)$, are taken into account in the $t$- and $u$-channel tree-level diagrams. The pentaquark-like nucleon resonance $P_s(2071,3/2^-)$ is also considered. In the $s$-channel diagrams, scalar and tensor mesons are incorporated, specifically $f_0(2020,2100,2200)$ and $f_2(1950,2010,2150)$, respectively, along with the pseudoscalar meson $η(2225)$. Our calculations suggest that contributions from the $N$ and $N^*$ resonances significantly enhance the cross-section near the threshold. At the same time, those from the $f_0$ and $f_2$ mesons produce distinctive peak structures around $W=2.2$ GeV, qualitatively reproducing the JETSET experimental data. The openings of the $\barΛΛ$ and $\barΣΣ$ channels lead to the cusp structures, which can explain the nontrivial structures in the cross-section at $W\approx2M_Λ$ and $2M_Σ$ observed in the data. It turns out that contributions from the $η$ and $P_s$ resonances are almost negligible. Additionally, polarization observables such as the spin density matrix element (SDME) provide crucial insights into the individual hadronic processes involved in this reaction.

hep-ph↗

arXiv2025

Strangeness $+1$ light multiquark baryons

In view of the renewing experimental interest for searching strangeness $+1$ baryons at J-PARC, we study the existence of light baryon resonances with strangeness +1 generated in the $K$-$(N^*/Δ^*)$ system, where $N^*$ represents either $N^*(1535)$/$N^*(1650)$/$N^*(1700)$, and $Δ^*$ corresponds to $Δ(1620)$. The description of the properties of the aforementioned states requires considering the dynamics involved in the coupled pseudoscalar-baryon and vector-baryon systems with strangeness $S=0$ in the s-wave. For the purpose of our current study, we consider the pseudoscalar-baryon (PB) and vector-baryon channels (VB) to which the mentioned $N^*$ and $Δ^*$ resonances couple and solve the Faddeev equations for the coupled channel system $K$-$\text{PB}$, $K$-$\text{VB}$, with all interactions being in the s-wave. Despite some strong attraction present in two of the subsystems, we do not find clear evidence supporting the formation of strangeness +1 states, with spin-parity $J^P=1/2^+$, in the energy region $2000-2200$ MeV. However, the case of spin-parity $J^P=3/2^+$ seems more promising, showing the formation of a resonance with a mass around 2167 MeV, with a width of 90-100 MeV. We suggest that a signal of such a state could be found in processes with final states like $KN$, $K^*(892) N$.

hep-ph↗

arXiv2024

Bolstering up the existence of $P_s(2080)$

We present a detailed study of the partial decay widths of a spin-parity resonance $J^P=3/2^-$ $N^*$ with a mass of $\simeq$ 2070 MeV obtained from the coupled channel s wave vector-baryon $ρN$, $ωN$, $ϕN$, $K^*Λ$ and $K^*Σ$ dynamics. This state, which couples strongly to the $K^*Σ$ channel, corresponds to a nucleon with a hidden strange quark content, in analogy to the $P_c$ states discovered by the LHCb collaboration, and we denote it as $P_s(2080)$. A state with such a nature can decay to vector-baryon, pseudoscalar-baryon, and pseudoscalar-baryon resonance channels, involving triangular loops in the latter two cases. As we will show, the partial decay widths to pseudoscalar-baryon resonance channels, like $πN^*(1535)$, $πN^*(1650)$, $KΛ(1405)$, are comparable to those related to ground state baryons in the final state, like $πN$, $ηN$, $KΛ$. In this way, reactions involving such lighter baryon resonances in the final state can be used as an alternative source of information on the properties of a $N^*$ with hidden strangeness.

hep-ph↗

arXiv2024

Effect of neutrino electromagnetic properties on the quasielastic neutral-current neutrino-nucleus scattering

In the quasielastic region, we investigate the effect of neutrino electromagnetic properties constrained from the recent experiments on the electroweak neutral current reaction process of the neutrino-$^{12}$C scattering. For a relativistic description of the nuclear dynamics, we employ the relativistic mean-field model, which has been proven to describe the data nicely in the quasielastic region. In the present work, we analyze the influence beyond the Standard Model by considering the neutrino magnetic and electric dipole form factors and charge radius on the neutrino electroweak interactions within $^{12}$C. To this end, we use the values of the neutrino charge radius and the magnetic moment at the squared four momentum transfer $Q^2=0$ obtained from the recent experiments and calculate the neutrino differential cross section of the neutrino-$^{12}$C scattering. We find that the effect of the charge radius and the electric dipole form factor is very small, but the role of the magnetic dipole form factor is sensitive to $Q^2$ and becomes sizable at small momentum transfer.

nucl-th↗

arXiv2024

Intriguing aspects of light baryon resonances

We discuss that some light baryon resonances exhibit properties which cannot be described when attributing a three-valence quark structure to them. Besides pointing out the hadron resonances which clearly require description beyond the quark model, we focus on the third $s_{11},~ N^*$ state and its decay to final states consisting of the lightest hyperon resonances which have a partial width comparable to that for the decay to $πN$. Such properties of the mentioned nucleon resonance get manifested in the cross sections and other observables related to processes producing the lightest hyperon resonances. We show that all these findings arise from the strong association of the baryon resonances to the dynamics among the ground-state hadrons.

hep-ph↗

arXiv2024

Strangeness plus-one ($S=+1$) resonance-state $P^{+}_0$ via $K^+n\to K^{0}p$

In our current study, we delve into the peak-like structure observed during the reaction process of $K^+n\to K^{0}p$ at approximately $\sqrt{s}\sim2.5$ GeV. Our focus centers on exploring the potential $S=+1$ resonance $P^{+*}_0\equiv P^*_0$ as an excited state within the extended vector-meson and baryon ($VB$) antidecuplet. To achieve this aim, we employ the effective Lagrangian method in conjunction with the $(u,t)$-channel Regge approach, operating within the tree-level Born approximation. We thoroughly examine various spin-parity quantum numbers for the resonance, resulting in a compelling description of the data, where $M_{P^*_0}\approx2.5$ GeV and $Γ_{P^*_0}\approx100$ MeV. Furthermore, we propose an experimental technique to amplify the signal-to-noise ratio ($S/N$) for accurately measuring the resonance. Notably, our findings reveal that background interference diminishes significantly within the $K^*$ forward-scattering region in the center-of-mass frame when the $K^*$ is perpendicularly polarized to the reaction plane. Additionally, we explore the recoil-proton spin asymmetry to definitively determine the spin and parity of the resonance. This study stands to serve as a valuable reference for designing experimental setups aimed at investigating and comprehending exotic phenomena in QCD. Specifically, our insights will inform future J-PARC experiments, particularly those employing higher kaon beam energies.

hep-ph↗

arXiv2024

Pion-nucleus elastic scatterings incorporating medium effects within the Eikonal-Glauber mode

In this present investigation, we explore the elastic scattering of pions with nuclei ($π$-$A$), primarily influenced by the $Δ$(1232) resonance, within the Eikonal-Glauber model. The medium effects are incorporated by considering nuclear-density ($ρ_A$) dependent masses of baryons and strong coupling constants. These dependencies are computed and parameterized up to $\mathcal{O}(ρ_A^2)$ based on the quark-meson coupling (QMC) model. The Wood-Saxon type density profile is utilized for the bound nucleons within finite nuclei. The element $π^+$-$N$ scattering cross section for the Glauber approach is determined using the conventional effective Lagrangian method. Subsequently, we analyze the total cross sections for elastic scattering with $^4$He and $^{12}$C targets. Our numerical results demonstrate a favorable agreement with JINR data for the $^4$He target, accurately reproducing the total cross-section. However, when considering the $^{12}$C target, deviations of approximately $\lesssim10\%$. We also consider the multiple-scattering effects inside the nucleus approximately, using the single-channel meson-baryon Bethe-Salpeter equation, resulting in the effective width broadening of the $Δ$ resonance to reproduce the data better.

hep-ph↗

arXiv2024

Study on the $ϕ$-meson photoproduction off the proton target with the pentaquark-like $K^*Σ$ bound state $P_s$

We utilize the effective Lagrangian method within the tree-level Born approximation to explore $ϕ$-meson photoproduction, i.e., $γp \to ϕp$. Our analysis encompasses contributions from various sources, including the Pomeron, $f_1$-Regge, pseudoscalar particles ($π$, $η$), scalar particles ($a_0$, $f_0$), protons, and three-nucleon resonance states. In addition, we consider a possible pentaquark-like $K^* Σ$-bound state $P_s$. The findings indicate that, apart from the region near the threshold, contributions other than the Pomeron generally have a limited impact on the total cross section. However, at specific angles, alternative contributions become crucial, particularly at smaller values of ${\rm cos}\,θ$. The incorporation of $P_s$ and other nucleon resonances proves essential to elucidate the bump observed near $W \sim 2.15$ GeV at very forward angles and behaviors within the range of $W=(2.0-2.3)$ GeV. Furthermore, in the region with $W\ge 2.5$ GeV, where nucleon resonances become negligible, contributions from the t-channel mesons become pivotal. Our calculations for spin density matrix components, examined in various frames, exhibit improvement when considering all contributions. This comprehensive approach successfully reproduces the observed bump by including $P_s$. We also briefly estimate the $P_s$ production via $ϕ$-meson photoproduction in the future Electron-Ion Collider (EIC), resulting in the luminosity of 10 fb$^{-1}$ per month.

hep-ph↗

arXiv2023

Effects of Symmetry Energy on the Equation of State for Hybrid Neutron Stars

In this paper, the implications of the symmetry energy on the hadron and quark phase transitions in the compact star, including the properties of the possible configurations of the quark-hadron hybrid stars, are investigated in the frameworks of the energy-density functional (EDF) models and the flavor SU(2) Nambu--Jona-Lasinio (NJL) model with the help of the Schwinger's covariant proper-time regularization (PTR) scheme. In this {theoretical setup}, the equations of states (EoSs) of hadronic matter for various values of symmetry energies obtained from the EDF models are employed to describe the hadronic matter, and the {flavor} SU(2) NJL model with various repulsive-vector interaction strengths are used to describe the quark matter. We then observe the obtained EoS in the mass-radius properties of the hybrid star configurations for various vector interactions and nuclear symmetry energies by solving the Tolman-Oppenheimer-Volkoff equation. We obtain that the critical density at which the phase transition occurs varies over the density (3.6--6.7)$ρ_0$ depending on the symmetry energy and the strength of the vector coupling $G_v$. The maximum mass of the neutron star (NS) is susceptible to $G_v$. When there is no repulsive force, the NS maximum mass is only about $1.5M_\odot$, but it becomes larger than $2.0M_\odot$ when the vector coupling constant is about half of the {attractive} scalar coupling constant. Surprisingly, the presence of the quark matter does not affect the canonical mass of NS ($1.4M_\odot$), so observing the canonical mass of NSs can provide unique constraints to the EoS of hadronic matter at high densities.

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