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Hungchong Kim

Publications and source records attributed to Hungchong Kim.

At least 55 records · Page 3Linked to original sources
arXiv2002

Re-analysis of the $D^* Dπ$ coupling in the light-cone QCD sum rules

The recent measurement from the CLEO experiment presents the $DD^*π$ coupling, $17.9\pm 0.3 \pm 1.9$. This value is much larger than any of QCD sum rule predictions available in literature. We report that, with a relevant treatment of the continuum subtraction as well as with the asymptotic form of the twist-2 pion wave function, the light-cone QCD sum rule can provide the coupling comparable to the experimental value. The stability of the resulting sum rule becomes much better with these corrections.

hep-ph↗
arXiv2001

Couplings between pion and charmed mesons

We compute the couplings $DD^*π$,$D_1D^*π$,$D^*D^*π$, $D_1 D_1π$ using QCD sum rules. These couplings are important inputs in the meson exchange model calculations used to estimate the amount of $J/ψ$ absorption due to pions and rho mesons in heavy ion collisions. Our sum rules are constructed at the first order in the pion momentum $p_μ$, which give the couplings that are not trivially related to the soft-pion theorem. Our calculated couplings, which somewhat depend upon the values of the heavy meson decay constants, are $g_{DD^*π}=8.2 \pm 0.1$, $g_{D_1 D^* π} = 15.8 \pm 2 {\rm GeV}$, $g_{D^* D^* π}= 0.3 \pm 0.03$ and $g_{D_1 D_1 π}= 0.17 \pm 0.04$.

hep-ph↗
arXiv2001

In-medium pion weak decay constants

In nuclear matter, the pion weak decay constant is separated into the two components $f_t, f_s$ corresponding to the time and space components of the axial-vector current. Using QCD sum rules, we compute the two decay constants from the pseudoscalar-axial vector correlation function in the matter $i \int d^4x~ e^{ip\cdot x} < ρ| T[{\bar d}(x) i γ_5 u (x)~ {\bar u}(0) γ_μγ_5 d (0)] | ρ>$. It is found that the sum rule for $f_t$ satisfies the in-medium Gell-Mann--Oakes--Renner (GOR) relation precisely while the $f_s$ sum rule does not. The $f_s$ sum rule contains the non-negligible contribution from the dimension 5 condensate $<{\bar q} i D_0 iD_0 q >_N + {1\over 8} < {\bar q} g_s σ\cdot {\cal G} q >_N$ in addition to the in-medium quark condensate. Using standard set of QCD parameters and ignoring the in-medium change of the pion mass, we obtain $f_t =105$ MeV at the nuclear saturation density. The prediction for $f_s$ depends on values of the dimension 5 condensate and on the Borel mass. However, the OPE constrains that $f_s/f_t \ge 1 $, which does not agree with the prediction from the in-medium chiral perturbation theory. Depending on the value of the dimension 5 condensate, $f_s$ at the saturation density is found to be in the range $ 112 \sim 134$ MeV at the Borel mass $M^2 \sim 1$ GeV$^2$.

hep-ph↗
arXiv2001

QCD sum rules for the pseudoscalar decay constants - To constrain the strange quark mass

We study the higher order corrections of quark masses to the Gell-Mann$-$Oakes$-$Renner (GOR) relation by constructing QCD sum rules exclusively for pseudoscalar mesons from the axial-vector correlation function, $i \int d^4x~ e^{ip\cdot x} < 0| T[A_μ(x) A_ν(0)] | 0>$. To project out the pseudoscalar meson contributions, we apply $-p^μp^ν/p^2$ to this correlation function and construct sum rules for the decay constants of pseudoscalar mesons, $f_π, f_k$ and $f_{η_8}$. The OPE is proportional to quark masses due to PCAC. To leading order in quark mass, each sum rule reproduces the corresponding GOR relation. For kaon and $η_8$, the deviation from the GOR relation due to higher orders in quark mass is found to be substantial. But the deviation gives better agreements with the phenomenology. Our sum rule provides a sensitive relation between $f_K$ and $m_s$, which stringently constrain the value for $m_s$. To reproduce the experimental value for $f_K$, $m_s$ is found to be 186 MeV at 1 GeV scale. The $f_{η_8}$ sum rule also supports this finding.

hep-ph↗
arXiv2000

Pertinent Dirac structure for QCD sum rules of meson-baryon coupling constants

Using general baryon interpolating fields $J_B$ for $B= N, Ξ, Σ, $ without derivative, we study QCD sum rules for meson-baryon couplings and their dependence on Dirac structures for the two-point correlation function with a meson $i\int d^4x e^{iqx} \bra 0|{\rm T}[J_B(x)\bar{J}_B(0)] |{\cal M}(p)\ket$. Three distinct Dirac structures are compared: $iγ_5$, $iγ_5\fslash{p}$, and $γ_5σ_{μν}q^μp^ν$ structures. From the dependence of the OPE on general baryon interpolating fields, we propose criteria for choosing an appropriate Dirac structure for the coupling sum rules. The $γ_5σ_{μν}q^μp^ν$ sum rules satisfy the criteria while the $iγ_5$ sum rules beyond the chiral limit do not. For the $iγ_5\fslash{p}$ sum rules, the large continuum contributions prohibit reliable prediction for the couplings. Thus, the $γ_5σ_{μν}q^μp^ν$ structure seems pertinent for realistic predictions. In the SU(3) limit, we identify the OPE terms responsible for the $F/D$ ratio. We then study the dependence of the ratio on the baryon interpolating fields. We conclude the ratio $F/D \sim 0.6-0.8$ for appropriate choice of the interpolating fields.

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arXiv2000

The $F/D$ ratio and meson-baryon couplings from QCD sum rules-II

Using QCD sum rules, we compute the diagonal meson-baryon couplings, $πNN$, $ηNN$, $πΞΞ$, $ηΞΞ$, $πΣΣ$ and $ηΣΣ$, from the baryon-baryon correlation function with a meson, $i\int d^4x e^{iq\cdot x} < 0| T J_B(x) {\bar J}_B(0)|M(p)>$. The calculations are performed to leading order in $p_μ$ by considering the two separate Dirac structures, $i γ_5 γ_μp^μ$ and $γ_5 σ_{μν} {q^μp^ν}$ separately. We first improve the previous sum rule calculations on these Dirac structures for the $πNN$ coupling by including three-particle pion wave functions of twist 4 and then extend the formalism to calculate the other couplings, $ηNN$, $πΞΞ$, $ηΞΞ$, $πΣΣ$ and $ηΣΣ$. In the SU(3) symmetric limit, we identify the terms responsible for the $F/D$ ratio in the OPE by matching the obtained couplings with their SU(3) relations. Depending on the Dirac structure considered, we find different identifications for the $F/D$ ratio. The couplings including the SU(3) breaking effects are also discussed within our approach.

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arXiv2000

Use of the double dispersion relation in QCD sum rules with external fields

In QCD sum rules with external fields, the double dispersion relation is often used to represent the correlation function. In this work, we point out that the double spectral density, when it is determined by successive applications of the Borel transformation, contains the spurious terms which should be kept in the subtraction terms in the double dispersion relation. They are zero under the Borel transformation but, if the dispersion integral is restricted with QCD duality, they contribute to the continuum. For the simple case with zero external momentum, it is shown that subtracting out the spurious terms is equivalent to the QCD sum rules represented by the single dispersion relation.

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arXiv1999

Meson-baryon couplings and the $F/D$ ratio from QCD sum rules

Motivated by the recent work on the calculation of the $πNN$ coupling constant using QCD sum rule beyond the chiral limit, we construct the corresponding sum rules for the couplings, $ηNN$, $πΞΞ$, $ηΞΞ$, $πΣΣ$ and $ηΣΣ$. In constructing the $η$-baryon sum rules, we use the second moment of the $η$ wave function, which we obtain from the pion wave function after SU(3) rotation. In the SU(3) symmetric limit, we can identify the term responsible for the $F/D$ ratio in the OPE, which after the sum rule analysis gives $F/D\sim 0.2$, a factor of 3 smaller than from other studies. We also present a qualitative analysis including the SU(3) breaking terms.

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arXiv1999

$πNN$ coupling determined beyond the chiral limit

Within the conventional QCD sum rules, we calculate the $πNN$ coupling constant, $g_{πN}$, beyond the chiral limit using two-point correlation function with a pion. We consider the Dirac structure, $iγ_5$, at $m_π^2$ order, which has clear dependence on the PS and PV coupling schemes for the pion-nucleon interactions. For a consistent treatment of the sum rule, we include the linear terms in quark mass as they constitute the same chiral order as $m_π^2$. Using the PS coupling scheme for the pion-nucleon interaction, we obtain $g_{πN}=13.3\pm 1.2$, which is very close to the empirical $πNN$ coupling. This demonstrates that going beyond the chiral limit is crucial in determining the coupling and the pseudoscalar coupling scheme is preferable from the QCD point of view.

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arXiv1999

Two-point correlation function with pion in QCD sum rules

Within the framework of the conventional QCD sum rules, we study the pion two-point correlation function, $i\int d^4x e^{iq\cdot x} < 0| T J_N(x) {\bar J}_N(0)|π(p)>$, beyond the soft-pion limit. We construct sum rules from the three distinct Dirac structures, $i γ_5 \notp, i γ_5, γ_5 σ_{μν} {q^μp^ν}$ and study the reliability of each sum rule. The sum rule from the third structure is found to be insensitive to the continuum threshold, $S_π$, and contains relatively small contribution from the undetermined single pole which we denote as $b$. The sum rule from the $i γ_5$ structure is very different even though it contains similar contributions from $S_π$ and $b$ as the ones coming from the $γ_5 σ_{μν} {q^μp^ν}$ structure. On the other hand, the sum rule from the $i γ_5 \notp$ structure has strong dependence on both $S_π$ and $b$, which is clearly in constrast with the sum rule for $γ_5 σ_{μν} {q^μp^ν}$. We identify the source of the sensitivity for each of the sum rules by making specific models for higher resonance contributions and discuss the implication.

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arXiv1999

Constraints on vector mesons with finite momentum in nuclear matter

Using the QCD operator product expansion, we derive the real part of the transverse and longitudinal vector-vector correlation function with the quantum numbers of the rho and omega mesons to leading order in density and three momentum (q^2) for energy $ω^2 --> -\infty $. The operator product expansion provides, through the Borel transformed energy dispersion relation, a model independent constraint for the momentum dependence of the vector meson spectral density in nuclear matter. Existing model calculations for the dispersion effect of the rho meson, where the vector-meson nucleon scattering amplitude is obtained by resonance saturation in the s-channel, in general violate this constraint. We trace this to an inconsistent choice for the form factor of the $ΔNρ$ vertex. With a consistent choice, where both the form factor and the coupling constant are obtained from the Bonn potential, the contribution of the $Δ$ is substantially reduced and we find good agreement with the constraint equation. We briefly comment on the implications of our result for attempts to interpret the enhancement of low-mass dileptons in heavy-ion collisions.

nucl-th↗
arXiv1999

QCD sum rule calculation of the $πNN$ coupling-revisited

In the conventional QCD sum rule calculation for the $πNN$ coupling beyond the soft-pion limit, there are three distinct Dirac structures, each of which can be used to construct separate sum rule. We point out subtleties in the previous sum rule results, based on one of the Dirac structure, associated with using either the PV or the PS coupling schemes in modeling the phenomenological side. We propose a sum rule coming from a different Dirac structure, which is independent of the coupling schemes used and has less uncertainty in the OPE. The obtained value is $g_{πN}=9.76 \pm 2.04$ where the uncertainty mainly comes from the quark-gluon mixed condensate.

nucl-th↗
arXiv1999

QCD sum rules with two-point correlation function

We construct three different sum rules from the two-point correlation function with pion, $i\int d^4x e^{iq\cdot x} <0| T J_N(x) {\bar J}_N(0)|π(p)>$, beyond the soft-pion limit. The PS and PV coupling schemes in the construction of the phenomenological side are carefully considered in each sum rule. We discuss the dependence of the result on the specific Dirac structure and identify the source of the dependence by making specific models for higher resonances.

nucl-th↗
arXiv1999

About QCD sum rules for pion-nucleon coupling

We point out subtleties in the previous sum rule calculations of the $πNN$ coupling associated with using either the PV or the PS coupling schemes in modeling the phenomenological side. We propose a sum rule which is independent of the coupling schemes used and has less uncertainty in the OPE. The obtained value is $g_{πN}=9.76 \pm 2.04$ where the uncertainty mainly comes from the quark-gluon mixed condensate.

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arXiv1998

Vector mesons in nuclear medium with small three momentum, a QCD sum rule approach

Using the QCD Operator Product Expansion, we derive the real part of the transverse and longitudinal vector vector correlation function with the $ρ,ω$ quantum numbers to leading order in density and in ${\bf q}^2$ at $-ω^2\to \infty $. To dimension 6, only twist-2 and 4 operators contribute. These OPE, through the energy dispersion relation, provide model independent constraints for the ${\bf q}$ dependence of the vector meson spectral density in nuclear medium. We further make a QCD sum rule type of analysis to extract the momentum dependence of the vector meson dispersion relation in medium. The contributions from twist-2 operators are added up to infinite order to check the validity of the OPE at the relevant Borel window.

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arXiv1998

Properties of $N^*$(1535) at Finite Density in the Extended Parity-Doublet Models

We improve so called ``naive'' and ``mirror'' models for the positive and negative parity nucleons, $N$ and $N^{*}$, by introducing nonlinear terms allowed by chiral symmetry. Both models in this improvement reproduce the observed nucleon axial charge in free space and reveal interesting density dependence of the axial charges for $N$ and $N^*$, and the doublet masses. A remarkable difference between the two models is found in the off-diagonal axial charge, $g_{ANN^*}$, which could appear either as suppression or as enhancement of $N^* \to πN$ decay in the medium.

hep-ph↗
arXiv1997

Calculation of $g_{πNS_{11}}$ and $g_{ηNS_{11}}$ Couplings in QCD Sum Rules

We calculate the coupling constants, $g_{πNS_{11}}$ and $g_{ηNS_{11}}, using QCD sum rules in the presence of an external meson field. A covariant derivative is introduced within the S_{11} interpolating field so that in the nonrelativistic limit the field dominantly reduces to two quarks in the s-wave state and one quark in the p-wave state. Our result for the couplings obtained by further making use of the soft-meson theorem qualitatively agrees with its phenomenological value extracted from the S_{11}(1535) decay width. The prediction for the couplings however depend on the value of quark-gluon condensate, $< {\bar q} g_s σ\cdot {\cal G} q >$, which is also important in the calculation of the S_{11}(1535) mass itself within the sum rule approach.

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