arXiv subjects

Taesoo Song

Publications and source records attributed to Taesoo Song.

At least 19 recordsLinked to original sources

arXiv2026

Charmonium production in p+A collisions at SPS and FAIR energies

We employ the Parton-Hadron-String Dynamics (PHSD) transport approach to investigate the influence of baryon-rich matter on charmonium production and dissociation. The Remler formalism is implemented to dynamically model charmonium formation from charm-anticharm pairs. As a validation step, the formalism is first benchmarked against experimental data from elementary pp collisions and then extended to pA systems to extract the effective nuclear absorption cross section of charmonium. This extracted cross section can subsequently be applied in heavy-ion collisions to quantify medium-induced effects. Our results demonstrate that the Remler formalism provides a quantitatively consistent description of charmonium production in pp and pA collisions at SPS energies. The approach is then extrapolated to GSI/FAIR energies, where predictions for charmonium yields and survival probabilities are presented. These findings highlight the relevance of the Remler formalism as a dynamical framework for studying heavy-quark bound-state formation in baryon-rich matter and offer theoretical guidance for future experimental programs at SPS, FAIR and NICA aimed at mapping the QCD phase structure.

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arXiv2026

Resolving the $\phi$-meson directed-flow puzzle by multi-step meson--baryon dynamics

Recent STAR measurements at fixed-target Beam Energy Scan energies have revealed an unexpectedly large directed flow of $\phi$ mesons in Au+Au collisions, comparable to that of protons and $\Lambda$ baryons and much stronger than that of light strange mesons. Since the $\phi$ is a hidden-strangeness meson with relatively weak interactions with non-strange hadrons, this observation has been interpreted as a possible signal of unconventional baryonic dynamics or exotic baryonic resonances coupled to the $\phi$ channel. Within the framework of the Parton-Hadron-Quantum-Molecular-Dynamics(PHQMD) model, we demonstrate that in the high baryon density region, $\phi$ mesons are produced predominantly through multi-step meson--baryon and meson--hyperon reactions, whose transition amplitudes are constrained by a coupled-channel $T$-matrix calculation based on an extended SU(6) chiral effective Lagrangian. Together with the in-medium broadening of the $\phi$ spectral function, these baryon-driven production channels enhance near-threshold $\phi$ production and imprint the collective motion of the baryon-rich source on the produced $\phi$ mesons.

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arXiv2026

Heavy quark coalescence probability in the presence of a potential

In this study, we explore the role of the heavy quark potential in heavy quark coalescence, whose probability is expected to be unity at low momentum. To this end, we develop a phenomenological heavy quark potential based on the constituent quark model that reproduces the vacuum masses of pseudoscalar and vector heavy mesons. Using this potential, we demonstrate its enhancement effect on the coalescence probability. We also investigate how medium-induced modifications of the heavy quark potential in the quark gluon plasma affect the coalescence process. Our results indicate that the coalescence probability remains close to unity as long as the modification of the potential is sufficiently moderate.

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arXiv2026

Probing the QCD Phase Structure with Dileptons from SIS to LHC Energies

We study the properties of strongly interacting matter at finite temperature and baryon chemical potential in relativistic heavy-ion collisions, with emphasis on dilepton probes of the QCD phase structure. The equilibrium QGP is described within the Dynamical QuasiParticle Model (DQPM), which reproduces the lattice-QCD equation of state and provides $\mu_B$-dependent quasiparticle properties, transport coefficients, and thermal dilepton rates, including elastic and inelastic partonic processes. The dynamical evolution is modeled with the off-shell Parton--Hadron--String Dynamics (PHSD) transport approach, which consistently propagates partonic and hadronic degrees of freedom and incorporates chiral-symmetry restoration effects. We discuss the space--time evolution of heavy-ion collisions over a broad energy range and show that a small deconfined QGP core can already emerge at $\sqrt{s_{NN}}\simeq 3.5$ GeV. We present, for the first time, the baryon-chemical-potential dependence of QGP thermal dilepton radiation in PHSD and demonstrate that its influence increases toward lower collision energies. The excitation function indicates that thermal QGP radiation can exceed dileptons from correlated charm decays in central Au+Au collisions at $\sqrt{s_{NN}}\lesssim 25$--$30$ GeV, making RHIC--BES and FAIR energies particularly promising for the direct observation of QGP electromagnetic radiation after subtraction of heavy-flavor and Drell--Yan contributions.

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arXiv2026

Transport coefficients of strongly interacting quark-gluon plasma including elastic and inelastic scattering within the dynamical quasiparticle model

We study the impact of inelastic gluon-radiation and absorption processes on the transport coefficients of the quark-gluon plasma within the dynamical quasiparticle model (DQPM) in the temperature--baryon-chemical-potential plane $(T,\mu_B)$. Extending the $2\rightarrow2$ baseline established in previous DQPM calculations, we include gluon-radiation $2\rightarrow3$ and the inverse gluon-absorption $3\rightarrow2$ scattering channels with massive partons and effective DQPM propagators and vertices. The corresponding momentum-dependent interaction rates and relaxation times are used to calculate the shear viscosity, bulk viscosity, electric conductivity, and baryon diffusion coefficient as functions of temperature $T$ and baryon chemical potential $\mu_B$. Within the relaxation time approximation, we find that $2\leftrightarrow3$ contributions systematically reduce all considered transport coefficients relative to the $2\rightarrow2$ only results, in accordance with the decrease of the relaxation times. In the thermal regime explored here, however, this reduction remains moderate, since the investigated inelastic rates stay below the $2\rightarrow2$ ones over the considered $(T,\mu_B)$ range. The inelastic channels become more relevant mainly for partonic scatterings at large momenta, which are thermally suppressed in the strongly interacting QGP. At $\mu_B=0$, the resulting $\eta/s$, $\zeta/s$, and $\sigma_Q/T$ are compatible with available lattice-QCD estimates within uncertainties. At finite $\mu_B$, our results provide predictions for the transport properties of QCD matter relevant for beam-energy-scan programs.

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arXiv2026

Accessing Exotic Hadronic States via Charmed-Meson Femtoscopy in Relativistic Heavy-Ion Collisions

The two-particle correlation function measured in femtoscopic analyses provides access to the interaction potentials between emitted particles. This offers a unique opportunity to investigate interactions among charmed mesons and to explore the nature of possible exotic hadronic states. In this Letter, we study femtoscopic correlations of various charmed-meson pairs in relativistic heavy-ion collisions. The dynamical evolution of the system and charm hadron production are described within the Parton-Hadron-String Dynamics (PHSD) transport approach, while the correlation functions are computed using the Correlation Analysis Tool using the Schr\"odinger equation (CATS). We demonstrate that heavy-ion collisions provide a significantly more favorable environment than $pp$ collisions for accessing charmed meson femtoscopic correlations. This arises from enhanced charm-quark production, reduced relative momenta due to in-medium energy loss, and a strong suppression of initial-state correlations. Our results indicate that femtoscopic measurements in heavy-ion collisions offer a sensitive probe of charmed meson interactions and possible hadronic molecular states.

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arXiv2026

Charmonium production at SPS and FAIR energies

In this study we apply the Remler formalism to charmonium production at SPS and GSI/FAIR energies in order to investigate the effects of baryon-rich matter on charmonium production and dissociation in heavy-ion collisions within the Parton-Hadron-String Dynamics (PHSD). As a first step the Remler formalism is tested in p+p collisions and then applied to p+A collisions in order to extract the nuclear absorption cross section of charmonium, which is then utilized in heavy-ion collisions. We find that the Remler formalism successfully describes charmonium production in heavy-ion collisions at SPS energies when an in-medium heavy quark potential is implemented, in which $J/\psi$ dissociates near $T_c$. Finally the same formalism is applied to the low CERN/SPS energy and GSI/FAIR energies, where we estimate charmonium production.

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arXiv2026

Probe charmonium-nucleon interactions in high energy proton-proton collisions

We investigate charmonium production and the charmonium-nucleon correlation function in pp collisions using the EPOS4+CATS framework. For the first time, the emission source of charmonium-proton pairs is dynamically generated and found to be non-Gaussian. This enables a femtoscopic extraction of the charmonium-proton interaction directly from experimental correlation functions. Both ground and excited charmonium states are included. We find that feed-down from excited charmonium states can induce sizable modifications to the observed prompt $J/\psi$-proton correlation function, even when the correlation deviates from unity by less than one, reflecting the stronger interactions of the excited states.

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arXiv2025

The influence of heavy quark potential on quarkonium production in quark-gluon plasma

Remler formalism is the Wigner projection of a two particle state to bound states, which is carried out when the bound state begins to exist or when one of the two particles scatters in medium. This method has been successfully applied to quarkonium production in box simulations and in heavy-ion collisions. In this study this method is extended to strongly bound states with considerable binding energies by taking into account the potential energy between heavy quark pairs in the quark-gluon plasma (QGP). We find that an attractive heavy-quark potential enhances quarkonium production and the results are consistent with those from the statistical model in box simulations, if a proper spatial cutoff is introduced to the potential to mimic quantum effects.

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arXiv2025

Heavy flavor correlations and Quarkonia production in high energy pp collisions in the EPOS4 framework

In QCD, the production of heavy quark-antiquark pairs can proceed through different mechanisms, each imprinting characteristic correlations between the heavy quarks. In this work, we use the EPOS4 event generator to study how these correlations affect quarkonium production in pp collisions. Our results demonstrate that the observed correlations between heavy mesons directly reflect the underlying production mechanisms and simultaneously shape the transverse-momentum distributions of quarkonia.

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arXiv2025

The influence of electromagnetic fields on the generation of the directed and elliptic flows of heavy quark in relativistic heavy-ion collisions

We study the impact of self-generated electromagnetic fields (EMF) on the charm quarks momentum evolution in the partonic and hadronic medium created in heavy-ion collisions at RHIC energy within the Parton-Hadron-String Dynamics (PHSD) off-shell transport approach. In the quark-gluon plasma (QGP) phase, the charm quark interacts with the off-shell partons whose mass and widths are given by the Dynamical Quasi-Particle Model (DQPM), which can reproduce the lattice QCD thermodynamics. The background electromagnetic fields are computed dynamically within the PHSD considering both the spectators and participants protons as well as newly produced charged hadrons, quarks, and antiquarks, which reflects naturally the electric conductivity $\sigma_{el}$ of the medium. We study the directed and elliptic flow of the $D$ mesons in the presence of the electromagnetic fields. We find that electromagnetically induced splitting in the $D$ meson $v_1$ through $D^0$ and $\overline{D}^0$ mesons is consistent with the experimental data. Furthermore, we notice that the splitting in the heavy quark $v_1$ as a function of $p_T$ is more prominent as a probe of the produced electromagnetic fields. However, we find only a small impact of electromagnetic fields on the heavy quark elliptic flow $v_2$.

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arXiv2025

Transport coefficients of heavy quarks by elastic and radiative scatterings in the strongly interacting quark-gluon plasma

We extend our investigation of heavy quark transport coefficients in the effective dynamical quasiparticle model (DQPM) -- which reproduces nonperturbative QCD phenomena in the strongly interacting quark-gluon plasma (sQGP) according to lattice QCD data -- by including inelastic $2 \to 3$ processes with massive gluon radiation, in addition to elastic $2 \to 2$ parton scattering. Both elastic and inelastic reactions are evaluated at leading order using DQPM-based effective propagators and vertices, accounting for all channels and their interferences. Based on the obtained matrix elements, we calculate various observables connected to a charm quark. First, we calculate the total cross section of a charm quark with the medium partons as functions of temperature and collision energy. Second, we obtain the drag $\mathcal{A}$ coefficient and $\hat{q}$ coefficient of a charm quark as functions of temperature and momentum and also compare our results with those obtained using the Zakharov model for the momentum-dependent strong coupling for the elastic and radiative vertices with a heavy quark, highlighting the importance of the choice of the strong coupling in the determination of transport coefficients. Third, we calculate the spatial diffusion coefficient of a charm quark and compare our results with those obtained using other approaches. Finally, we explore the mass dependence of the diffusion coefficient by comparing the results for charm quark, bottom quark, and infinitely-heavy quark. We found that inelastic processes can play a significant role in the determination of the transport coefficients at large transverse momenta, but are strongly suppressed at low transverse momenta.

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arXiv2025

Electromagnetic emission from strongly interacting hadronic and partonic matter created in heavy-ion collisions

We investigate dilepton production in heavy-ion, proton-proton, and proton-nucleus collisions from low energies of 1 AGeV (SIS) to ultra-relativistic energies (LHC) using the Parton-Hadron-String Dynamics (PHSD) transport approach. PHSD is a microscopic, non-equilibrium approach that integrates hadronic and partonic degrees of freedom, providing a comprehensive description of relativistic heavy-ion collisions from initial nucleon-nucleon interactions to quark-gluon plasma (QGP) formation, hadronization, and final-state interactions. Key dilepton sources in PHSD include hadronic decays, bremsstrahlung, QGP radiation ($q+\bar q \to e^+e^-$, \ $q+\bar q \to g+ e^+e^-$, \ $q+g \to q+ e^+e^-$), primary Drell-Yan production, and semileptonic decays of correlated charm and bottom pairs. PHSD well describes dilepton data from HADES, STAR, and ALICE experiments. We examine in-medium effects, such as the vector meson spectral function broadening, and present the excitation function for the dilepton "excess" in the invariant mass range $0.4<M_{ee}<0.75$ GeV/c$^2$. For the first time we report on the baryon chemical potential $\mu_B$-dependence of the QGP radiation calculated on a basis of the Dynamical-Quasi-Particle Model (DQPM) in PHSD. The influence of $\mu_B$ on the QGP yield grows at lower collision energies, where $\mu_B$ becomes large, however, its impact on the total dilepton spectra is small due to the lowering of the QGP volume with decreasing energy. The excitation function of QGP dileptons, is presented versus correlated charm, confirming that the QGP radiation overshines charm contributions at $\sqrt{s} \simeq 25-30$ GeV in central Au+Au collisions, providing access to thermal QGP dileptons at BES RHIC and FAIR.

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arXiv2024

Exploring Dark Photon Production and Kinetic Mixing Constraints in Heavy-Ion Collisions

Vector $U$-bosons, often referred to as 'dark photons', are potential candidates for mediating dark matter interactions. In this study, we outline a procedure to derive theoretical constraints on the upper bound of the kinetic mixing parameter $\epsilon^2(M_U)$ using dilepton data from heavy-ion from SIS to RHIC energies. The analysis is based on the microscopic Parton-Hadron-String Dynamics (PHSD) transport model, which successfully reproduces the measured dilepton spectra in $p+p$, $p+A$, and $A+A$ collisions. Besides the dilepton channels resulting from interactions and decays of Standard Model particles (such as mesons and baryons), we extend the PHSD approach to include the decay of hypothetical $U$-bosons into dileptons, $U \to e^+ e^-$. The production of these $U$-bosons occurs via Dalitz decays of pions, $\eta$-mesons, $\omega$-mesons, Delta resonances, as well as from the decays of vector mesons and $K^+$ mesons. This analysis provides an upper limit on $\epsilon^2(M_U)$ and offers insights into the accuracy required for future experimental searches for dark photons through dilepton experiments.

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arXiv2024

Heavy quark potential and thermal charm production in heavy-ion collisions

Heavy quark mass in QGP is related to the heavy quark potential at a large distance. In this study we test three different heavy quark potentials, namely, the free energy, the internal energy of the heavy quark pair in QGP, and the unscreened potential, which was recently proposed by the HotQCD Collaboration, through the thermal production of charm quarks in heavy-ion collisions at the LHC. We find that the free energy potential overestimates charm production in heavy-ion collisions at the LHC, while the unscreened potential produces results closest to the experimental data from the ALICE Collaboration among the three potentials.

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arXiv2024

Quarkonium production in pp and heavy-ion collisions

We describe bottomonium production not only in pp collisions but also in heavy-ion collisions by using the Remler's formalism where quarkonium density operator is applied to all possible combination of heavy quark and heavy antiquark pairs. In pp collisions heavy (anti)quark momentum is provided by the PYTHIA event generator after rescaling $p_T$ and rapidity to imitate the FONLL calculations. Then spatial separation between heavy quark and heavy antiquark is introduced based on the uncertainty principle. In heavy-ion collisions quarkonium wavefunction changes with temperature assuming heavy quark potential equals the free energy of heavy quark and heavy antiquark system in heat bath. The density operator is updated whenever heavy quark or heavy antiquark scatters in QGP produced in heavy-ion collisions. Our results are consistent with the experimental data from ALICE and CMS Collaborations assuming that the interaction rate of heavy (anti)quark in quarkonium is suppressed to 10 \% that of unbound heavy (anti)quark. We also find that off-diagonal recombination of bottomonium barely happens even in Pb+Pb collisions at $\sqrt{s}=5.02$ TeV.

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arXiv2024

Transport properties of the strongly interacting quark-gluon plasma

We investigate the transport properties of the strongly interacting quark-gluon plasma (sQGP) by comparing the role of elastic and inelastic (radiative) processes in the sQGP medium within the effective dynamical quasi-particle model (DQPM), constructed for the description of non-perturbative quantum chromodynamic (QCD) phenomena of the sQGP in line with the lattice QCD (lQCD) equation of state. First, we present the results for the energy and temperature dependencies of the total radiative cross sections and compare them to the corresponding elastic cross sections. Second, we perform a calculation of the interaction rate and relaxation time of radiative versus elastic scatterings. Finally, we obtain the jet transport coefficient $\hat{q}$ and investigate its dependence on the choice of the strong coupling in thermal, jet parton and radiative vertices.

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arXiv2024

Production and in-medium modification of $\phi$ mesons in proton-nucleus reactions from a transport approach

Production and in-medium modification of hidden strange $\phi$ mesons are studied in proton-nucleus reactions - p+C, p+Cu and p+Pb - at 12 GeV/c, partially measured by the KEK E325 collaboration via the dilepton decay mode. This work is based on the off-shell microscopic Parton-Hadron-String Dynamics (PHSD) transport approach, which provides the dynamical description of strongly interacting hadronic and partonic degrees of freedom in dense matter, and especially makes it possible to simulate in-medium off-shell dynamics of the $\phi$ meson in the reactions studied. Different in-medium scenarios for the modification of the $\phi$ meson spectral function in nuclear matter are investigated: i) dropping pole mass, ii) collisional broadening and iii) simultaneous dropping pole mass and collisional broadening. Even though the $\phi$ meson production in p+A reactions occurs only at densities around or below normal nuclear matter density $\rho_0$, we find visible modifications of the generated dilepton spectra for the in-medium scenarios compared to the vacuum distribution.

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