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C. H. Hyun

Publications and source records attributed to C. H. Hyun.

At least 19 recordsLinked to original sources

Polarization of the neutron induced from hadronic weak interactions in the photo-disintegration of the deuteron

New observables with which we can study the two-nucleon weak interactions at low energies are considered. In the breakup of the deuteron by photons, polarization of outgoing neutrons can depend on the parity-violating component of two-nucleon interactions. We express the parity-violating polarization in general forms, and perform numerical calculations with a pionless effective field theory. The theory has unknown parity-violating low energy constants, and the results are expressed in linear combination of them. We discuss the results and their implication to the understanding of the hadronic weak interactions.

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Parity-violating asymmetry in $γd \to \vec{n}p$ with a pionless effective theory

Nuclear parity violation is studied with polarized neutrons in the photodisintegration of the deuteron at low energies. A pionless effective field theory with di-baryon fields is used for the investigation. Hadronic weak interactions are treated by parity-violating di-baryon-nucleon-nucleon vertices, which have undetermined coupling contants. A parity-violating asymmetry in the process is calculated for the incident photon energy up to 30 MeV. If experimental data for the parity-violating asymmetry become available in the future, we will be able to determine the unknown coupling contants in the parity-violating vertices.

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Spin observables in the two-nucleon capture and dissociation processes at low energies

Spin observables in radiative neutron capture on a proton and its inverse process, photodisintegration of the deuteron are calculated using a pionless effective field theory with di-baryon fields. Good agreement with the results of existing standard nuclear physics approach is obtained at very low energies. As energy increases, however, the discrepancy between the effective field theory and the standard nuclear physics approach becomes substantial. We discuss the origin of the difference.

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Spin Polarization in $γd \to \vec{n}p$ at Low Energies with a Pionless Effective Field Theory

With the use of pionless effective field theory including dibaryon fields, we study the $γd \to \vec{n} p$ reaction for the laboratory photon energy $E_γ^{lab}$ ranging from threshold to 30 MeV. Our main goal is to calculate the neutron polarization $P_{y'}$ defined as $P_{y'} = (σ_+ - σ_-)/(σ_+ + σ_-)$, where $σ_+$ and $σ_-$ are the differential cross sections for the spin-up and spin-down neutrons, respectively, along the axis perpendicular to the reaction plane. We also calculate the total cross section as well as the differential cross section $σ(θ)$, where $θ$ is the colatitude angle. Although the results for the total and differential cross sections are found to agree reasonably well with the data, the results for $P_{y'}$ show significant discrepancy with the experiment. We comment on this discrepancy.

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Magnetic moments of octet baryons at finite density and temperature

We investigate the change of magnetic moments of octet baryons in nuclear matter at a finite density and temperature. Quark-meson coupling models are employed in describing properties of octet baryons and their interactions. Magnetic moments of octet baryons are found to increase non-negligibly as density and temperature increase, and we find that temperature dependence can be strongly correlated with the quark-hadron phase transition. Model dependence is also examined by comparing the results from the quark-meson coupling (QMC) model to those by the modified QMC (MQMC) model where the bag constant is assumed to depend on density. Both models predict sizable dependence on density and temperature, but the MQMC model shows a more drastic change of magnetic moments. Feasible changes of the nucleon mass by strong magnetic fields are also reported in the given models.

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Neutron Stars with Bose-Einstein Condensation of Antikaons as MIT Bags

We investigate the properties of an antikaon in medium, regarding itas a MIT bag. We first construct the MIT bag model for a kaon with$σ^*$ and $ϕ$ in order to describe the interaction of$s$-quarks in hyperonic matter in the framework of the modifiedquark-meson coupling model. The coupling constant $g'^{B_K}_σ$in the density-dependent bag constant $B(σ)$ is treated as afree parameter to reproduce the optical potential of a kaon in asymmetric matter and all other couplings are determined by usingSU(6) symmetry and the quark counting rule. With various values ofthe kaon potential, we calculate the effective mass of a kaon inmedium to compare it with that of a point-like kaon. We thencalculate the population of octet baryons, leptons and $K^-$ and theequation of state for neutron star matter. The results show thatkaon condensation in hyperonic matter is sensitive to the $s$-quarkinteraction and also to the way of treating the kaon. The mass andthe radius of a neutron star are obtained by solving theTolmann-Oppenheimer-Volkoff equation.

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Parity-violating polarization in $np \to dγ$ with a pionless effective field theory

We consider the two-nucleon weak interaction with a pionless effective field theory. Dibaryon fields are introduced to facilitate calculations and ensure precision in the initial and final state propagators. Weak interactions are accounted for with the parity-violating dibaryon-nucleon-nucleon vertices, which contain unknown weak dibaryon-nucleon-nucleon coupling constants. We apply the model to the calculation of a parity-violating observable in the neutron-proton capture at threshold. Result is obtained up to the linear order in the unknown dibaryon-nucleon-nucleon coupling constants. We compare our result to the one obtained from a hybrid calculation, and discuss the extension to weak interactions in the few-body systems.

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Parity violation in $d \vecγ -> np$ with pionless effective theory

We consider a pionless effective theory with dibaryon fields for the description of the weak process involving two nucleons. We construct leading order Lagrangians that contain nucleon-dibaryon weak coupling constants. We calculate the physical observable in the photodisintegration of the deuteron at threshold and obtain the result in terms of the nucleon-dibaryon weak coupling constants. Relation to existing calculations is discussed.

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Magnetic moment of hyperons in nuclear matter by using quark-meson coupling models

We calculate the magnetic moments of hyperons in dense nuclear matter by using relativistic quark models. Hyperons are treated as MIT bags, and the interactions are considered to be mediated by the exchange of scalar and vector mesons which are approximated as mean fields. Model dependence is investigated by using the quark-meson coupling model and the modified quark-meson coupling model; in the former the bag constant is independent of density and in the latter it depends on density. Both models give us the magnitudes of the magnetic moments increasing with density for most octet baryons. But there is a considerable model dependence in the values of the magnetic moments in dense medium. The magnetic moments at the nuclear saturation density calculated by the quark meson coupling model are only a few percents larger than those in free space, but the magnetic moments from the modified quark meson coupling model increase more than 10% for most hyperons. The correlations between the bag radius of hyperons and the magnetic moments of hyperons in dense matter are discussed.

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Parity-violating nucleon-nucleon interaction from different approaches

Two-pion exchange parity-violating nucleon-nucleon interactions from recent effective field theories and earlier fully covariant approaches are investigated. The potentials are compared with the idea to obtain better insight on the role of low-energy constants appearing in the effective field theory approach and the convergence of this one in terms of a perturbative series. The results are illustrated by considering the longitudinal asymmetry of polarized protons scattering off protons, $\vec{p}+p -> p+p$, and the asymmetry of the photon emission in radiative capture of polarized neutrons by protons, $\vec{n}+p -> d+γ$.

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The old and the new of parity-violating two-pion-exchange $NN$ potential

We consider the parity-violating two-pion-exchange potential obtained from the covariant formalism in the past and the state-of-the-art effective field theory approach. We discuss the behavior of the potential in coordinate space and its application to the parity-violating asymmetry in $\vec{n} p \to d γ$ at threshold.

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Proton-proton fusion in pionless effective theory

The proton-proton fusion reaction, $pp\to de^+ν$, is studied in pionless effective field theory (EFT) with di-baryon fields up to next-to leading order. With the aid of the di-baryon fields, the effective range corrections are naturally resummed up to the infinite order and thus the calculation is greatly simplified. Furthermore, the low-energy constant which appears in the axial-current-di-baryon-di-baryon contact vertex is fixed through the ratio of two- and one-body matrix elements which reproduces the tritium lifetime very precisely. As a result we can perform a parameter free calculation for the process. We compare our numerical result with those from the accurate potential model and previous pionless EFT calculations, and find a good agreement within the accuracy better than 1%.

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Hadronic parity violation in $\vec{n} p \to d γ$ with effective field theory

The parity-violating nucleon-nucleon (NN) potential is considered up to next-to-next-to leading order in heavy-baryon chiral perturbation theory. We include one-pion exchange at leading order and the two-pion exchange and two-nucleon contact terms at next-to-next-to-leading order. The effects of intermediate (two-pion exchange) and short-range (two-nucleon contact) terms are probed by calculating the photon asymmetry $A_γ$ in $\vec{n} p \to d γ$ employing Siegert's theorem and an accurate phenomenological potential for the parity-conserving NN interaction. We explore in detail the uncertainties due to the parameters that control the contribution of the short-range interaction. We obtain about 20% uncertainty in the value of $A_γ$ up to the next-to-next-to leading order. We discuss the implication of this uncertainty for the determination of the weak pion-nucleon coupling constant and how the uncertainty can be reduced.

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Kaon condensation and composition of neutron star matter in modified quark-meson coupling model

We use the modified quark-meson coupling (MQMC) model to study the composition profile of neutron star matter and compare the results with those calculated by quantum hadrodynamics (QHD). Both MQMC and QHD model parameters are adjusted to produce exactly the same saturation properties so that we can investigate the model dependences of the matter composition at high densities. We consider the possibility of deep kaon optical potential and find that the composition of matter is very sensitive to the interaction strength of kaons with matter. The onset densities of the kaon condensation are studied in detail by varying the kaon optical potentials. We find that the MQMC model produces the kaon condensation at lower densities than QHD. The presence of kaon condensation changes drastically the population of octet baryons and leptons. Once the kaon condensation takes place, the population of kaons builds up very quickly, and kaons become the dominant component of the matter. We find that the $ω$-meson plays an important role in increasing the kaon population and suppressing the hyperon population.

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Kaon condensation in neutron star using relativistic mean field models

We use the modified quark-meson coupling and the quantum hadrodynamics models to study the properties of neutron star. Coupling constants of both models are adjusted to reproduce the same saturation properties. The onset of kaon condensation in neutron star matter is studied in detail over a wide range of kaon optical potential values. Once the kaon condensation takes place, the population of kaons increases very rapidly, and kaons become the dominant component, possibly making the neutron star matter a kaonic matter if the kaon optical potential is large.

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Two-pion-exchange parity-violating potential and $\vec{n} p \to d γ$

We calculate the parity-violating nucleon-nucleon potential in heavy-baryon chiral perturbation theory up to the next-to-next-to-leading order. The one-pion exchange comes in the leading order and the next-to-next-to-leading order consists of two-pion-exchange and the two-nucleon contact terms. In order to investigate the effect of the higher order contributions, we calculate the parity-violating asymmetry in $\vec{n} p \to d γ$ at the threshold. The one-pion dominates the physical observable and the two-pion contribution is about or less than 10% of the one-pion contribution.

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Radiative neutron capture on a proton at BBN energies

The total cross section for radiative neutron capture on a proton, $np \to d γ$, is evaluated at big bang nucleosynthesis (BBN) energies. The electromagnetic transition amplitudes are calculated up to next-to leading order within the framework of pionless effective field theory with dibaryon fields. We also calculate the $dγ\to np$ cross section and the photon analyzing power for the $d\vecγ\to np$ process from the amplitudes. The values of low energy constants that appear in the amplitudes are estimated by a Markov Chain Monte Carlo analysis using the relevant low energy experimental data. Our result agrees well with those of other theoretical calculations except for the $np\to dγ$ cross section at some energies estimated by an R-matrix analysis. We also study the uncertainties in our estimation of the $np\to dγ$ cross section at relevant BBN energies and find that the estimated cross section is reliable to within $\sim$1% error.

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Parity Nonconservation in Elastic $\vec{p} p$ Scattering

By looking at the parity-nonconserving (PNC) asymmetries at different energies in $\vec{p} p$ scattering, it is in principle possible to determine the PNC $ρNN$ and $ωNN$ couplings of a single-meson-exchange model of the PNC $NN$ force. The analysis of the experimental data at 13.6, 45 and 221 MeV simultaneously has been performed by Carlson \textit{et al.}, who concluded to an agreement with the original DDH estimates for the PNC meson-nucleon couplings. In this work, it is shown first that the comparison with updated hadronic predictions of these couplings rather suggests the existence of some discrepancy for the PNC $ωNN$ coupling. The effect of variations on the strong coupling constants and introduction of cutoffs in the one-boson-exchange weak potential is then investigated. As expected, it turns out that the resulting asymmetry is quite sensitive to these parameters regardless of the energy. However the above discrepancy persists. The dependence of this conclusion on various ingredients entering an improved description of the PNC $NN$ force is also examined. These include the two-pion resonance nature of the rho meson and some momentum dependence of the isoscalar PNC $ρNN$ vertex. It is found that none of these corrections is able to remove or even alleviate the above discrepancy. Their impacts on the theoretical determination of the vector meson-nucleon couplings, the description of the PNC force in terms of single-meson exchanges, or the interpretation of measurements, are finally examined.

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