arXiv subjects
Publications and source records attributed to Youngman Kim.
We study the quark number susceptibility, an indicator of QCD phase transition, in the hard wall and soft wall models of hQCD. We find that the susceptibilities in both models are the same, jumping up at the deconfinement phase transition temperature. We also find that the diffusion constant in the soft wall model is enhanced compared to the one in the hard wall model.
We investigate the quark-gluon mixed condensate based on an AdS/QCD model. Introducing a holographic field dual to the operator for the quark-gluon mixed condensate, we obtain the corresponding classical equation of motion. Taking the mixed condensate as an additional free parameter, we show that the present scheme reproduces very well experimental data. A fixed value of the mixed condensate is in good agreement with that of the QCD sum rules.
We consider a holographic QCD model for light mesons beyond the leading order in the context of 5-dim gauged linear sigma model on the interval in the AdS$_5$ space. We include two dimension-6 operators in addition to the canonical bulk kinetic terms, and study chiral dynamics of $π$, $ρ$, $a_1$ and some of their KK modes. As novel features of dim-6 operators, we get non-vanishing Br$(a_1 \to πγ)$, the electromagnetic form factor and the charge radius of a charged pion, which improve the leading order results significantly and agree well with the experimental results.
Chiral symmetry is an essential concept in understanding QCD at low energy. We treat the chiral condensate, which measures the spontaneous breaking of chiral symmetry, as a free parameter to investigate the effect of partially restored chiral symmetry on the physical quantities in the frame work of an AdS/QCD model. We observe an interesting scaling behavior among the nucleon mass, pion decay constant and chiral condensate. We propose a phenomenological way to introduce the temperature dependence of a physical quantity in the AdS/QCD model with the thermal AdS metric.
We consider gluon condensation (GC) at finite temperature using AdS/CFT. We first show that in the presence of regular horizon, the GC is forbidden in high temperature. Then we consider gravity back-reaction to dilaton coupling and show that the back-reaction develops an singularity, and non-vanishing value of gluon condensation is allowed. We also study thermodynamic quantities and the trace anomaly in the presence of the GC. We discuss how to define a temperature in the presence of the singularity which forbids Hawking temperature. Finally we describe the thermodynamics of the gluon condensation including the effect of the Hawking-Page transition.
We study the physics with finite nuclear density in the framework of AdS/QCD with holographic baryon field included. Based on a mean field type approach, we introduce the nucleon density as a bi-fermion condensate of the lowest mode of the baryon field and calculate the density dependence of the chiral condensate and the nucleon mass. We observe that the chiral condensate as well as the mass of nucleon decrease with increasing nuclear density. We also consider the mass splitting of charged vector mesons in iso-spin asymmetric nuclear matter.
Assuming that at sufficiently high densities the constituent quarks become relevant degrees of freedom, we study within the framework of a chiral quark model the influence of s-wave $K^-$ condensation on the quark-antiquark condensates. We find that, in linear density approximation, the presence of a $K^-$ condensate quenches the $\bar{u}u$ condensate, but that the $\bar{d}d$ condensate remains unaffected up to the chiral order under consideration. We discuss the implication of the suppressed $\bar{u}u$ condensate for flavor-dependent chiral symmetry restoration in dense matter
Encouraged by recent developments in AdS/QCD models for light quark system, we study heavy quarkonium in the framework of the AdS/QCD models. We calculate the masses of $c\bar c$ vector meson states using the AdS/QCD models at zero and at finite temperature. Among the models adopted in this work, we find that the soft wall model describes the low-lying heavy quark meson states at zero temperature relatively well. At finite temperature, we observe that once the bound state is above $T_c$, its mass will increase with temperature until it dissociates at a temperature of around $494 {\rm MeV}$. It is shown that the dissociation temperature is fixed by the infrared cutoff of the models. The present model serves as a unified non perturbative model to investigate the properties of bound quarkonium states above $T_c$.
In the framework of a holographic QCD approach we study an influence of matters on the deconfinement temperature, $T_c$. We first consider quark flavor number ($N_f$) dependence of $T_c$. We observe that $T_c$ decreases with $N_f$, which is consistent with a lattice QCD result. We also delve into how the quark number density $ρ_q$ affects the value of $T_c$. We find that $T_c$ drops with increasing $ρ_q$. In both cases, we confirm that the contributions from quarks are suppressed by $1/N_c$, as it should be, compared to the ones from a gravitational action (pure Yang-Mills).
In the framework of heavy-baryon chiral perturbation theory, we investigate the fixed point structure of renormalization group equations (RGE) for the ratios of the renormalized low energy constants (LECs) that feature in the pion-nucleon chiral Lagrangian. The ratios of the LECs deduced from our RGE analysis are found to be in semi-quantitative agreement with those obtained from direct fit to the experimental data. The naturalness of this agreement is discussed using a simple dimensional analysis combined with Wilsonian RGEs.
We study the effects of Lorentz non-invariance on the physical pion velocity at the critical temperature $T_c$ in an effective theory of hidden local symmetry (HLS) with the "vector manifestation" fixed point. We match at a "matching scale" $Λ_M$ the axial-vector current correlator in the HLS with the one in the operator product expansion for QCD, and present the matching condition to determine the bare pion velocity. We find that the physical pion velocity, which is found to be one at $T=T_c$ when starting from the Lorentz invariant bare HLS, remains close to one with the Lorentz non-invariance, $v_π(T_c) = 0.83 - 0.99$. This result is quite similar to the pion velocity in dense matter.
The question as to what the relevant effective degrees of freedom at the chiral phase transition are remains largely unanswered and must be addressed in confronting both terrestrial and space laboratory observations purporting to probe matter under extreme conditions. We address this question in terms of the vector susceptibility χ_V (VSUS in short) and the axial-vector susceptibility χ_A (ASUS in short) at the temperature-induced chiral transition. We consider two possible, albeit simplified, cases that are contrasting, one that is given by the standard chiral theory where only the pions figure in the vicinity of the transition and the other that is described by hidden local symmetry (HLS) theory with the Harada-Yamawaki vector manifestation (VM) where nearly massless vector mesons also enter. We find that while in the standard chiral theory, the pion velocity v_πproportional to the ratio of the space component f_π^s of the pion decay constant over the time component f_π^t tends to zero near chiral restoration with f_π^t\neq 0, in the presence of the vector mesons with vanishing mass, the result is drastically different: HLS with VM {\it predicts} that χ_V automatically equals χ_A in consistency with chiral invariance and that v_π\sim 1$ with $f_π^t\approx f_π^s\to 0 as T\to T_c. These results are obtained in the leading order in power counting but we expect their qualitative features to remain valid more generally in the chiral limit thanks to the VM point.
We describe in-medium properties of hadrons in dense matter near chiral restoration using a Wilsonian matching to QCD of an effective field theory with hidden local symmetry at the chiral cutoff $Λ$. We find that chiral symmetry is restored in vector manifestation à la Harada and Yamawaki at a critical matter density $n_c$. We express the critical density in terms of QCD correlators in dense matter at the matching scale. In a manner completely analogous to what happens at the critical $N_f^c$ and at the critical temperature $T^c$, the vector meson mass is found to vanish (in the chiral limit) at chiral restoration. This result provides a support for Brown-Rho scaling predicted a decade ago.
We investigate the influence of charged kaon condensation in hadronic sector on the formation of the electron-free charge neutral quark matter of equipartition of three light flavors, i.e., a phenomenon called ``enforced electrical neutrality" of the color-flavor locked (CFL) phase. We employ the chiral quark model which is assumed to be applicable, "bottom-up," toward chiral restoration while accessible to kaon condensation in the hadronic sector. Approaching "bottom-up" the high density regime is more appropriate in addressing the properties of compact stars than approaching "top-down" from asymptotic QCD. It turns out that the presence of hadronic kaon condensation makes the electron-free charge-neutral CFL phase energetically more favorable with a relatively small color-superconducting gap $Δ_0\sim 20$ MeV compared to $Δ_0\sim 70$ MeV required in the absence of such kaon condensation.
We investigate the effects of pion and gluon exchanges on the formation of two-flavor color superconductivity at moderate density, $μ<1 GeV$. The chiral quark model proposed by Manohar and Georgi containing pions as well as gluons is employed to show that the pion exchange reduces substantially the value of the superconducting gap gotten with the gluon exchange only. It turns out that the pion exchanges produce a repulsion between quark-quark pair in a spin and isospin singlet state. We suggest that the phase consisiting of pions, gluons and quarks is one of the candidates of in-medium QCD phase at moderate density.
We study the competition between induced symmetry breaking (ISB), Cooper pairing (superconductivity) and chiral condensation at finite non-asymptotic density. Using a quantum mechanical model studied recently by Ilieva and Thirring, we analyze the expectation value of the fermion number density $\la a^*a\ra$ in the presence of chemical potential and discuss relevance of the ISB phenomenon in the model. By Bogoliubov transformation, we obtain the ground state energy and find that a phase with both superconducting and mean field gap is a true ground state. We also study the effective potentials of a two-dimensional field theory model for competitions between $\la\barψψ\ra$, $\laψψ\ra$ and $\laψ^\daggerψ\ra$. We find that in the regime $μ<μ_c$, where $μ_c$ is found to be around $0.4 m_F$ with $m_F$ being the renormalizaton point of chiral condensate, we have a chiral symmetry-breaking phase with almost zero fermion number density and in the case of $μ>μ_c$ the system sits in the mixed phase characterized by $\la ψ^\daggerψ\ra\neq 0$ and $\la ψψ\ra\neq 0$ as long as the renormalization point, $M_0$, corresponding to a superconducting gap is not zero, $M_0\neq 0$.
Based on schematic two-level models extended to $a_1$-meson degrees of freedom, we investigate possible mechanisms of chiral restoration in the vector/axialvector channels in cold nuclear matter. In the first part of this article we employ the massive Yang-Mills framework to construct an effective chiral Lagrangian based on low-energy mesonic modes at finite density. The latter are identified through nuclear collective excitations of `meson'-sobar type such as $π\leftrightarrow [Δ(1232)N^{-1}]\equiv\hatπ$, $ρ\leftrightarrow [N^* (1520)N^{-1}]\equiv\hatρ$, etc.. In a mean-field type treatment the in-medium gauge coupling $\hat g$, the (axial-) vector meson masses and $\hat f_π$ are found to decrease with density indicating the approach towards chiral restoration in the language of in-medium effective fields. In the second part of our analysis we evaluate the (first) in-medium Weinberg sum rule which relates vector and axialvector correlators to the pion decay constant. Using in-medium $ρ$/$a_1$ spectral functions (computed in the two-level model) also leads to a substantial reduction of the pion decay constant with increasing density.
We calculate the density dependence of the $ρ$-meson mass and coupling constant($g_{ρNN}$) for $ρ$-nucleon-nucleon vertex at one loop using the lagrangian where the $ρ$-meson is included as a dynamical gauge boson of a hidden local symmetry. From the condition that thermodynamic potential should not depend on the arbitrary energy scale, renormalization scale, one can construct a renormalization group equation for the thermodynamic potential and argue that the various renormalization group coefficients are functions of the density or temperature. We calculate the $β$-function for $ρ$-nucleon-nucleon coupling constant ($g_{ρNN}$) and $γ$-function for $ρ$-meson mass ($γ_{m_ρ}$). We found that the $ρ$-meson mass and the coupling constant for $g_{ρNN}$ drop as density increases in the low energy limit.