arXiv subjects

Chang-Hwan Lee

Publications and source records attributed to Chang-Hwan Lee.

69 records · Page 4Linked to original sources

arXiv2005

Pentaquark $Θ^+$ mass and width in dense matter

We investigate medium modifications of the pentaquark $Θ^+$ in dense medium, taking into account different parities of the exotic $Θ^+$ baryon. We find that the chemical potential of the $Θ^+$ is shifted in a density-dependent way to one-loop order. We also investigated the effect of the scaled nucleon mass in dense medium on the $Θ^+$ propagator. The results turn out to depend sensitively on the scaled nucleon mass and on the parity of the $Θ^+$.

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arXiv2004

The \bar q q Bound States and Instanton Molecule at T >~ T_c

The main objective of this work is to explore the evolution in the structure of the quark-anti-quark bound states in going down in the chirally restored phase from the so-called "zero binding points" $T_{zb}$ to the full (unquenched) QCD critical temperature $T_c$ at which the Nambu-Goldstone and Wigner-Weyl modes meet. In doing this, we adopt the idea recently introduced by Shuryak and Zahed for charmed $\bar c c$, light-quark $\bar q q$ mesons $π, σ, ρ, A_1$ and gluons that at $T_{zb}$, the quark-anti-quark scattering length goes through $\infty$ at which conformal invariance is restored, thereby transforming the matter into a near perfect fluid behaving hydrodynamically, as found at RHIC. We show that the binding of these states is accomplished by the combination of (i) the color Coulomb interaction, (ii) the relativistic effects, and (iii) the interaction induced by the instanton-anti-instanton molecules. The spin-spin forces turned out to be small. While near $T_{zb}$ all mesons are large-size nonrelativistic objects bound by Coulomb attraction, near $T_c$ they get much more tightly bound, with many-body collective interactions becoming important and making the $σ$ and $π$ masses approach zero (in the chiral limit). The wave function at the origin grows strongly with binding, and the near-local four-Fermi interactions induced by the instanton molecules play an increasingly more important role as the temperature moves downward toward $T_c$.

hep-ph↗

arXiv2004

A new state of matter at high temperature as "sticky molasses"

The main objective of this work is to explore the evolution in the structure of the quark-antiquark bound states in going down in the chirally restored phase from the so-called "zero binding points" $T_{zb}$ to the QCD critical temperature $T_c$ at which the Nambu-Goldstone and Wigner-Weyl modes meet. In doing this, we adopt the idea recently introduced by Shuryak and Zahed for charmed $\bar c c$, light-quark $\bar q q$ mesons $π, σ, ρ, A_1$ and gluons that at $T_{zb}$, the quark-antiquark scattering length goes through $\infty$ at which conformal invariance is restored, thereby transforming the matter into a near perfect fluid behaving hydrodynamically, as found at RHIC. We name this new state of matter as "sticky molasses". We show that the binding of these states is accomplished by the combination of (i) the color Coulomb interaction, (ii) the relativistic effects, and (iii) the interaction induced by the instanton-anti-instanton molecules. The spin-spin forces turned out to be small. While near $T_{zb}$ all mesons are large-size nonrelativistic objects bound by Coulomb attraction, near $T_c$ they get much more tightly bound, with many-body collective interactions becoming important and making the $σ$ and $π$ masses approach zero (in the chiral limit). The wave function at the origin grows strongly with binding, and the near-local four-Fermi interactions induced by the instanton molecules play an increasingly more important role as the temperature moves downward toward $T_c$.

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arXiv2003

The Case for Case C Mass Transfer in the Galactic Evolution of Black Hole Binaries

Earlier works, which we review, have shown that if the Fe core in a presupernova star is to be sufficiently massive to collapse into a black hole, earlier in the evolution of the star the He core must be covered (clothed) by a hydrogen envelope during He core burning and removed only following this, in, e.g. common envelope evolution. This is classified as Case C mass transfer. These previous arguments were based chiefly on stellar evolution, especially depending on the way in which 12C burned. In this work we argue for Case C mass transfer on the basis of binary evolution. The giant progenitor of the black hole will have a large radius 1000 Rsun at the end of its supergiant stage. Its lifetime at that point will be short, about 1000 yrs, so it will not expand much further. Thus, the initial giant radius for Case C mass transfer will be constrained to a narrow band about 1000 Rsun. This has the consequence that the final separation a_f following common envelope evolution will depend nearly linearly on the mass of the companion m_d which becomes the donor after the He core of the giant has collapsed into the black hole. The separation at which this collapse takes place is essentially a_f, because of the rapid evolution of the giant. (In at least two binaries the black hole donor separation has been substantially increased because of mass loss in the black hole formation. These can be reconstructed from the amount of mass deposited on the donor in this mass loss.) We show that the reconstructed preexplosion separations of the black hole binaries fit well the linear relationship.

astro-ph↗

arXiv2002

On the Theory of Gamma Ray Bursts and Hypernovae: The Black Hole Soft X-ray Transient Sources

We show that a common evolutionary history can produce the black hole binaries in the Galaxy in which the black holes have masses of ~ 5-10 M_sun. In with low-mass, <~ 2.5 M_sun, ZAMS (zero age main sequence) companions, the latter remain in main sequence during the active stage of soft X-ray transients (SXTs), most of them being of K or M classification. In two intermediate cases, IL Lupi and Nova Scorpii with ZAMS ~ 2.5 M_sun companions the orbits are greatly widened because of large mass loss in the explosion forming the black hole, and whereas these companions are in late main sequence evolution, they are close to evolving. Binaries with companion ZAMS masses >~ 3 M_sun are initially "silent" until the companion begins evolving across the Herzsprung gap. We provide evidence that the narrower, shorter period binaries, with companions now in main sequence, are fossil remnants of gamma ray bursters (GRBs). We also show that the GRB is generally accompanied by a hypernova explosion (a very energetic supernova explosion). We further show that the binaries with evolved companions are good models for some of the ultraluminous X-ray sources (ULXs) recently seen by Chandra in other galaxies. The great regularity in our evolutionary history, especially the fact that most of the companions of ZAMS mass <~ 2.5 M_sun remain in main sequences as K or M stars can be explained by the mass loss in common envelope evolution to be Case C; i.g., to occur only after core He burning has finished. Since our argument for Case C mass transfer is not generally understood in the community, we add an appendix, showing that with certain assumptions which we outline we can reproduce the regularities in the evolution of black hole binaries by Case C mass transfer.

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arXiv2002

Discovery of a Black-hole Mass - Period Correlation in Soft X-ray Transients and its Implication for Gamma-Ray Burst and Hypernova Mechanisms

We investigate the soft X-ray transients with black hole primaries which may have been the sources of gamma-ray bursts and hypernovae earlier in their evolution. We find that the black-hole mass increases with the orbital period of the binary, both for systems with main-sequence donors and for those with evolved donors. This correlation can be understood in terms of angular-momentum support in the helium star progenitor of the black hole, if the systems with shorter periods had more rapidly rotating primaries prior to their explosion: centrifugal support will then prevent more of its mass from collapsing into the black hole. This trend of more rapidly rotating stars in closer binaries is very generally seen, and in the present case can be understood in terms of spin-up during spiral in and subsequent tidal coupling. We investigate the relation quantitatively and obtain reasonable agreement with the observed mass-period correlation. A natural consequence of this observation is that black holes formed in soft X-ray transients acquire significant Kerr parameters. This makes them good sources of power for gamma-ray bursts and hypernovae, via the Blandford-Znajek mechanism, and thus supports our model for the origin of gamma-ray bursts in soft X-ray transients.

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arXiv2001

Afterglow Light Curve Modulated by a Highly Magnetized Millisecond Pulsar

We investigate consequences of a continuously energy-injecting central engine of gamma-ray burst (GRB) afterglow emission, assuming that a highly magnetized pulsar is left beaming in the core of a GRB progenitor. Beaming and continuous energy-injection are natural consequences of the pulsar origin of GRB afterglows. Whereas previous studies have considered continuous energy-injection from a new-born pulsar to interpret the deviation of afterglow light curves of GRBs from those with the simple power law behavior, a beaming effect, which is one of the most important aspects of pulsar emissions, is ignored in earlier investigations. We explicitly include the beaming effect and consider a change of the beaming with time due to a dynamical evolution of a new-born pulsar. We show that the magnitude of the afterglow from this fireball indeed first decreases with time, subsequently rises, and declines again. One of the most peculiar optical afterglows light curve of GRB 970508 can be accounted for by continuous energy injection with beaming due to a highly magnetized new-born pulsar. We discuss implications on such observational evidence for a pulsar.

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arXiv2000

On Companion-Induced Off-Center Supernova-Like Explosions

We suggest that a neutron star with a strong magnetic field, spiraling into the envelope of a companion star, can generate a ``companion induced SN-like off-center explosion". The strongly magnetized neutron star ("magnetar") is born in a supernova explosion before entering into an expanding envelope of a supergiant companion. If the neutron star collapses into a black hole via the hypercritical accretion during the spiral-in phase, a rapidly rotating black hole with a strong magnetic field at the horizon results. The Blandford-Znajek power is sufficient to power a supernova-like event with the center of explosion displaced from the companion core. The companion core, after explosion, evolves into a C/O-white dwarf or a neutron star with a second explosion. The detection of highly eccentric black-hole, C/O-white dwarf binaries or the double explosion structures in the supernova remnants could be an evidence of the proposed scenario.

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arXiv1999

Can precessing jets explain the light curves of Gamma-ray Bursts?

We present a phenomenological model to explain the light curves of gamma-ray bursts. In the model a black hole is orbited by a precessing accretion disc which is fed by a neutron star. Gamma-rays are produced in a highly collimated beam via the Blandford-Znajek mechanism. The gamma-ray beam sweeps through space due to the precession of the slaved accretion disc. The light curve expected from such a precessing luminosity cone can explain the complex temporal behavior of observed bright gamma-ray bursts.

astro-ph↗

arXiv1995

The Role of Lambda(1405) in Kaon-Proton Interactions

S-wave $K^-p$ scattering into various channels near threshold are analyzed in heavy-baryon chiral perturbation theory with $Λ(1405)$ introduced as an independent field. This is the approach that predicted the critical density $2<=ρ_c/ρ_0 <=3$ for negatively charged kaon condensation. We show that chiral perturbation expansion treating the $Λ(1405)$ as elementary is consistent with {\it all} threshold data including a double-charge-exchange process suppressed at leading order of chiral expansion in the absence of the $Λ(1405)$. We also discuss S-wave $K^+ p$ scattering phase shifts at low energy.

hep-ph↗

arXiv1995

Kaon Condensation in Dense Stellar Matter

This article combines two talks given by the authors and is based on works done in collaboration with G.E. Brown and D.-P. Min on kaon condensation in dense baryonic medium treated in chiral perturbation theory using heavy-baryon formalism. It contains, in addition to what was recently published, astrophysical backgrounds for kaon condensation discussed by Brown and Bethe, a discussion on a renormalization-group analysis to meson condensation worked out together with H.K. Lee and S.-J. Sin, and the recent results of K. M. Westerberg in the bound-state approach to the Skyrme model. Negatively charged kaons are predicted to condense at a critical density $2\lsim ρ/ρ_0\lsim 4$, in the range to allow the intriguing new phenomena predicted by Brown and Bethe to take place in compact star matter.

hep-ph↗

arXiv1994

Kaon Condensation in Dense Stellar Matter

This talk is based on work done in collaboration with G.E. Brown and D.-P. Min on kaon condensation in dense baryonic medium treated in chiral perturbation theory using heavy-baryon formalism. It contains, in addition to what was recently published, some new results based on the analysis on kaonic atoms by Friedman, Gal and Batty and a discussion on a renormalization-group analysis to meson condensation made together with H.K. Lee and Sin. Negatively charged kaons are predicted to condense at the critical density $2\lsim ρ/ρ_0\lsim 4$, in the range to allow all the intriguing new phenomena predicted by Brown and Bethe to take place in compact star matter.

hep-ph↗

arXiv1994

An Effective Chiral Lagrangian Approach to Kaon-Nuclear Interactions: Kaonic atom and kaon condensation

On-shell kaon-nucleon scattering, kaonic atom and kaon condensation are treated on the same footing by means of a chiral perturbation expansion to the next-to-next-to-leading order (``N$^2$LO"). Constraining the low-energy constants in the chiral Lagrangian by on-shell $KN$ scattering lengths and kaonic atom data, the off-shell s-wave scattering amplitude up to one-loop order corresponding to N$^2$LO and the critical density of kaon condensation up to in-medium two-loop order are computed. The effects on kaon-proton scattering of the quasi-bound $Λ(1405)$ and on kaonic atoms and kaon condensation of $Λ(1405)$-proton-hole excitations through four-Fermi interactions are studied to all orders in density within the in-medium two-loop approximation. It is found that the four-Fermi interaction terms in the chiral Lagrangian play an essential role in providing attraction for kaonic atoms, thereby inducing condensation but the critical density is remarkably insensitive to the strength of the four-Fermi interaction that figures in kaonic atoms. The prediction for the critical density is extremely robust and gives -- for ``natural" values of the four-Fermi interactions -- a rather low critical density, $ρ_c \lsim 4 ρ_0$. When the BR scaling is suitably implemented, the condensation sets in at $ρ_c\simeq 2\, ρ_0$ with loop corrections and four-Fermi interactions playing a minor role.

hep-ph↗

arXiv1994

Kaon Condensation in ``Nuclear Star" Matter

The critical density for kaon condensation in ``nuclear star" matter is computed up to two-loop order {\it in medium} (corresponding to next-to-next-to-leading order in chiral perturbation theory in free space) with a heavy-baryon effective chiral Lagrangian whose parameters are determined from $KN$ scattering and kaonic atom data. To the order considered, the kaon self-energy has highly non-linear density dependence in dense matter. We find that the four-Fermi interaction terms in the chiral Lagrangian play an important role in triggering condensation, predicting for ``natural" values of the four-Fermi interactions a rather low critical density, $ρ_c < 4 ρ_0$.

hep-ph↗

arXiv1994

Kaon-Nucleon Scattering from Chiral Lagrangians

The s-wave $K^{\pm}N$ scattering amplitude is computed up to one-loop order corresponding to next-to-next-to-leading order (or N$^2$LO in short) with a heavy-baryon effective chiral Lagrangian. Constraining the low-energy constants by on-shell scattering lengths, we obtain contributions of each chiral order up to N$^2$LO and find that the chiral corrections are ``natural" in the sense of viable effective field theories. We have also calculated off-shell $s$-wave $K^-N$ scattering amplitudes relevant to kaonic atoms and $K^-$ condensation in ``nuclear star" matter including the effect of $Λ(1405)$. The $K^-p$ amplitude is found to be quite sensitive to the intermediate $Λ(1405)$ contribution, while the $K^-n$ amplitude varies smoothly with the C.M. energy. The crossing-even one-loop corrections are found to play an important role in determining the higher-order chiral corrections.

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