Heavy-Ion Collision Study at Intermediate Energies with DJBUU

2026.08.24 8th International Workshop on Nuclear Dynamics in Heavy-ion Reactions (IWND2026)

Kyungil Kim (IRIS, IBS)

Campus Area: 952,066 m²

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Initial condition

Detection in experiments

How can we understand the process of heavy-ion collisions?

Ex.) How can we know the impact parameter of an event?

¹²⁹Xe+¹²⁰Sn, E_beam = 65 MeV/n, b=1 fm

• Transport models (BUU, QMD, …)
• Hydrodynamics
• Time-dependent Hatree-Fock models
• Multifragmentation models
• Statistical models

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DaeJeon Boltzmann-Uehling-Uhlenbeck (DJBUU)

• Lagrangian (QHD)

• Phase-space density

• Equation of motion

Ref.) M.Kim et al. PRC 101 (2020) 064614

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Relativistic Thomas-Fermi model calculation with same Lagrangian

Check the stability of an initialized nuclei

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• ¹⁹⁷Au
• for 300 fm/c

Comparison of the density evolution in a heavy-ion collision

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• ¹⁹⁷Au + ¹⁹⁷Au
• Beam energy 100 MeV/n
• for 140 fm/c

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Ref.) M.Kim et al. Universe 8 (2022) 564

NN->N𝚫 𝚫 -> N𝛑

Production of pion in DJBUU

Isospin-dependent in-medium cross-sections

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• Bubble nucleus ?

Nucleus which has a much smaller central density than the maximum density

In many nuclei, these bubble structures are observed in the proton density.

Since these nuclei have different density distributions, total and isospin densities at the collision center would be changed. We expect that these changes make the differences in the momentum distribution and isospin-dependent observables.

• Chiral symmetry restoration at saturation density

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Density as a function of radius for ²⁰⁶Hg from RCHB and RTF calculations

• We use two density configurations for ²⁰⁶Hg for comparison.
• The density distribution from the RCHB calculation has a bubble configuration.
• The DF values for total and proton densities are 13% and 18%, respectively.

Baryon and proton density distributions that are used in DJBUU calculations

• Because of the width of the test particle (here, 4.2fm), with the same configuration obtained from the RCHB calculation, it is hard to get the bubble configuration. So, we modify the configuration.
• For similar reasons, it is difficult to obtain small bubble nuclei with the test particle method which uses finite width.

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• v₁ of free protons

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• We choose test particles that are in effectively low density as free nucleons.

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• The differences are observed at high p_T, but still within the error bar.

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• It is hard to reduce the error at high p_T since the number of free nucleons is small in this region.

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• DJBUU with the extended Parity Doublet Model (J. Zhou et al. Symmetry 2025, 17, 2155)

• DJBUU with the Quark-Meson Coupling Model (D. I. Kim et al. PRC 113, 024615 (2026))

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• Density evolution in the collision center

<Effective mass>

<QMC Lagrangian>

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• Directed flow (v1) comparison with experiment

<Directed flow>

<Transverse flow>

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• Delta baryon and pion production

• Pion production ratios

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• Density evolution in the collision center

<Effective mass>

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• Effective masses with PDM model

Sn108+Sn112

Sn132+Sn124

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• DJBUU with the extended Parity Doublet Model (J. Zhou et al. Symmetry 2025, 17, 2155)

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N.M.Fortier et al. PRC111 (2025) L011901

The deformation of nuclei causes elliptic flows at ultra-central collisions.

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P. Danielewics et al. Science 298, 1592

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• Heavy-ion collisions at intermediate beam energies, from tens to hundreds of MeV per nucleon, offer a wide range of opportunities to explore interesting physics.
• The DaeJeon Boltzmann-Uehling-Uhlenbeck (DJBUU) model has been developed to simulate and analyze these heavy-ion collisions at RAON.
• Since the DJBUU model is based on relativistic mean-field calculations, it provides an opportunity to investigate fundamental physics, such as chiral symmetry, by adopting various mean-field models.
• Bubble configurations in nuclei lead to differences in the directed flow, but the effect is marginal.
• We are interested in the impact of nuclear deformation on heavy-ion collisions, and flow observables provide a useful tool for investigating these effects.

Thank you for your attention!