arXiv subjects

Myungkuk Kim

Publications and source records attributed to Myungkuk Kim.

8 recordsLinked to original sources

arXiv2026

Universal Properties of Near-Threshold Single-Neutron Resonances

We establish universal width predictions for near-threshold single-neutron resonances in $L > 0$ partial waves. Our results go beyond Wigner's well-known scaling behavior of cross sections near threshold. We show that the finite square-well potential exhibits discrete scale invariance at zero energy. From this fact, we derive an analytic baseline for the resonance width that depends only on geometry, angular momentum, and resonance energy, and not on internal short-distance nuclear details or radial excitation. This is a nontrivial property that is unique to the finite square-well potential and does not occur for other potentials. Application to observed p-wave and d-wave resonances demonstrates that the square-well result provides a robust baseline. We show that discrete scale invariance erases radial-node information in the sharp-boundary limit, but realistic Woods-Saxon diffuseness breaks this invariance, suppressing the reduced width by a factor sensitive to the internal radial excitation. These results provide a simple geometric benchmark for identifying when observed neutron resonances are controlled by universal threshold physics and when they exhibit systematic deviations driven by structure-dependent effects.

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arXiv2026

The compact neutron star in 4U 1746-37 revisited: Reassessing the mass and radius

A recent analysis of photospheric radius expansion X-ray bursts from the low-mass X-ray binary 4U 1746-37 reported unusually small mass and radius estimates for the neutron star, suggesting it could be a quark star or quark-cluster star. Here, we propose an alternative interpretation: the star's mass and radius could be underestimated from significant blocking of the X-ray flux. Significant blocking factors ($\mathcal{B} \gtrsim 0.8$, reducing the observed flux to $\sim17\%$ of the intrinsic emission) permit neutron star parameters consistent with the canonical values: $M = 1.59 \pm 0.69 M_{\odot}$, $R = 13.0 \pm 5.45\,\mathrm{km}$, or $M = 2.12 \pm 1.08 M_{\odot}$, $R = 9.80 \pm 4.13\,\mathrm{km}$. The blocking factor, which varies with the photospheric radius, provides a natural explanation for the anomalously large peak-to-touchdown flux ratio ($\sim2.0$) and highlights the importance of accounting for geometric system configuration in neutron star mass--radius estimates.

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arXiv2025

Ab Initio Calculations of the Carbon and Oxygen Isotopes: Energies, Correlations, and Superfluid Pairing

We perform \textit{ab initio} nuclear lattice calculations of the neutron-rich carbon and oxygen isotopes using high-fidelity chiral interactions. We find good agreement with the observed binding energies and compute correlations associated with each two-nucleon interaction channel. For the isospin $T=1$ channels, we show that the dependence on $T_z$ provides a measure of the correlations among the extra neutrons in the neutron-rich nuclei. For the spin-singlet S-wave channel, we observe that any paired neutron interacts with the nuclear core as well as its neutron pair partner, while any unpaired neutron interacts primarily with only the nuclear core. For the other partial waves, the correlations among the extra neutrons grow more slowly and smoothly with the number of neutrons. These general patterns are observed in both the carbon and oxygen isotopes and may be universal features that appear in many neutron-rich nuclei.

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arXiv2024

Wavefunction matching for solving quantum many-body problems

Ab initio calculations play an essential role in our fundamental understanding of quantum many-body systems across many subfields, from strongly correlated fermions to quantum chemistry and from atomic and molecular systems to nuclear physics. One of the primary challenges is to perform accurate calculations for systems where the interactions may be complicated and difficult for the chosen computational method to handle. Here we address the problem by introducing a new approach called wavefunction matching. Wavefunction matching transforms the interaction between particles so that the wavefunctions up to some finite range match that of an easily computable interaction. This allows for calculations of systems that would otherwise be impossible due to problems such as Monte Carlo sign cancellations. We apply the method to lattice Monte Carlo simulations of light nuclei, medium-mass nuclei, neutron matter, and nuclear matter. We use high-fidelity chiral effective field theory interactions and find good agreement with empirical data. These results are accompanied by new insights on the nuclear interactions that may help to resolve long-standing challenges in accurately reproducing nuclear binding energies, charge radii, and nuclear matter saturation in ab initio calculations.

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arXiv2022

Transport Model Comparison Studies of Intermediate-Energy Heavy-Ion Collisions

Transport models are the main method to obtain physics information from low to relativistic-energy heavy-ion collisions. The Transport Model Evaluation Project (TMEP) has been pursued to test the robustness of transport model predictions in reaching consistent conclusions from the same type of physical model. Calculations under controlled conditions of physical input and set-up were performed with various participating codes. These included both calculations of nuclear matter in a box with periodic boundary conditions, and more realistic calculations of heavy-ion collisions. In this intermediate review, we summarize and discuss the present status of the project. We also provide condensed descriptions of the 26 participating codes, which contributed to some part of the project. These include the major codes in use today. We review the main results of the studies completed so far. They show, that in box calculations the differences between the codes can be well understood and a convergence of the results can be reached. These studies also highlight the systematic differences between the two families of transport codes, known as BUU and QMD type codes. However, when the codes were compared in full heavy-ion collisions using different physical models, as recently for pion production, they still yielded substantially different results. This calls for further comparisons of heavy-ion collisions with controlled models and of box comparisons of important ingredients, like momentum-dependent fields, which are currently underway. We often indicate improved strategies in performing transport simulations and thus provide guidance to code developers. Results of transport simulations of heavy-ion collisions from a given code will have more significance if the code can be validated against benchmark calculations such as the ones summarized in this review.

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arXiv2021

Comparison of Heavy-Ion Transport Simulations: Mean-field Dynamics in a Box

Within the transport model evaluation project (TMEP) of simulations for heavy-ion collisions, the mean-field response is examined here. Specifically, zero-sound propagation is considered for neutron-proton symmetric matter enclosed in a periodic box, at zero temperature and around normal density. The results of several transport codes belonging to two families (BUU-like and QMD-like) are compared among each other and to exact calculations. For BUU-like codes, employing the test particle method, the results depend on the combination of the number of test particles and the spread of the profile functions that weight integration over space. These parameters can be properly adapted to give a good reproduction of the analytical zero-sound features. QMD-like codes, using molecular dynamics methods, are characterized by large damping effects, attributable to the fluctuations inherent in their phase-space representation. Moreover, for a given nuclear effective interaction, they generally lead to slower density oscillations, as compared to BUU-like codes. The latter problem is mitigated in the more recent lattice formulation of some of the QMD codes. The significance of these results for the description of real heavy-ion collisions is discussed.

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arXiv2021

Measuring Masses and Radii of Neutron Stars in Low-Mass X-ray Binaries: Effects of Atmospheric Composition and Touchdown Radius

Methods. We use both a Monte Carlo (MC) sampling and a Bayesian analysis to find the effects of the photospheric composition and the touchdown radius. We apply these two methods to six LMXBs that show PRE XRBs. In both methods, we solve the Eddington flux equation and the apparent angular area equation both of which include the correction terms. For the MC sampling, we have developed an iterative method in order to solve these two equations more efficiently. Results. We confirm that the effects of the photospheric composition and the touchdown radius are similar in the statistical and analytical estimation of mass and radius even when the correction terms are considered. Furthermore, in all of the six sources, we find that a H-poor photosphere and a large touchdown radius are favored statistically regardless of the statistical method. Our Bayesian analysis also hints that touchdown can occur farther from the neutron star surface when the photosphere is more H-poor. This correlation could be qualitatively understood with the Eddington flux equation. We propose a physical explanation for this correlation between the photospheric composition and the touchdown radius. Our results show that when accounting for the uncertainties of the photospheric composition and the touchdown radius, most likely radii of the neutron stars in these six LMXBs are less than 12.5 km, which is similar to the bounds for the neutron star radius placed with the tidal deformability measured from the gravitational wave signal.

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arXiv2020

Extended parity doublet model with a new transport code

A new transport code "DaeJeon Boltzmann-Uehling-Uhlenbeck (DJBUU)" had been developed and enables to describe the dynamics of heavy-ion collisions in low-energy region. To confirm the validity of the new code, we first calculate Au + Au collisions at Ebeam = 100 and 400A MeV and also perform the box calculation to check the detail of collisions and Pauli blocking without mean-field potential as suggested by the Transport Code Comparison Project. After confirming the validity of new transport code, we study low-energy heavy-ion collisions with an extended parity doublet model. Since the distinctive feature of the parity doublet model is the existence of the chiral invariant mass that contributes to the nucleon mass, we investigate how physical quantities depend on the chiral invariant mass in heavy ion collisions at low energies. For this, we calculate physical quantities such as the effective nucleon mass in central collisions and transverse flow in semi-central collisions of Au + Au at Ebeam = 400A MeV with different values of the chiral invariant masses.

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