arXiv subjects

Su Houng Lee

Publications and source records attributed to Su Houng Lee.

176 records · Page 10Linked to original sources

arXiv1998

J/psi and eta_c in the nuclear medium: QCD sum rule approach

We investigate the masses of the lowest c\bar{c} states, the J/psi and eta_c, in nuclear matter using QCD sum rules. Up to dimension four, the differences between the operator product expansions in vacuum and in medium arise from the density-dependent change in the gluon condensate and from a new contribution proportional to the nucleon expectation value of the twist-2 gluon operator. Both terms together give an attractive shift of about 5-10 MeV to the J/psi and eta_c masses in nuclear matter.

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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

QCD Properties of Hot/Dense Matter

In the introduction, we will discuss the symmetries of QCD, its breaking in the physical vacuum and its restoration at finite temperature and/or density. Then we will discuss what exact relations can be made between chiral symmetry restoration and the imaginary part of the vector vector correlation function, which is directly related to the dilepton spectrum coming from a hot and/or dense medium. Finally, we will focus on the vector meson pole appearing in the imaginary part and discuss results on how the shape changes using QCD sum rules in medium. In particular, we will discuss three characteristic changes of the vector meson peaks; namely, shift of the peak position, increase of its width and the dispersion effect due to the breaking of Lorentz invariance in medium.

hep-ph↗

arXiv1997

Vector mesons in medium with finite three momentum

We formulate a QCD sum rule to find the three momentum dependence of the peak position of the vector meson spectral density in nuclear medium. We find less than 2 % (0.1 %) shift of the peak position at nuclear matter density and at ($q =0.5$GeV/c) for the $ρ,ω(ϕ$) mesons. However, at higher density and three momentum, its effect becomes non-negligible. The relevance of our result in the dilepton spectrum in A-A and p-A reaction is discussed.

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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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arXiv1996

QCD Sum Rule for $Λ$(1405)

Motivated by the recently constructed interpolation field for S$_{11}$(1535), we propose a new interpolating field for $Λ$(1405). Using this current, we calculate the mass of $Λ$(1405) based on the conventional QCD sum rule analysis. By calculating the Wilson coefficients up to dimension 8 operators and taking into account the mass corrections from s-quark, we find the calculated mass of $Λ$(1405) to be very close to its experimental value.

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arXiv1996

Delta Decay in the Nuclear Medium

The $Δ$ decay in the nuclear medium is calculated in the relativistic meson-nucleon model. The delta spreading width is calculated and compared with the Pauli-blocked $π$N decay width. The influence of relativistic mean fields is also studied. We stress the importance of understanding the delta spreading width in interpreting experiments involving delta resonances.

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arXiv1995

$g_{K N Λ}$ and $g_{K N Σ}$ from QCD sum rules

$g_{K N Λ}$ and $g_{K N Σ}$ are calculated using a QCD sum rule motivated method used by Reinders, Rubinstein and Yazaki to extract Hadron couplings to goldstone bosons. The SU(3) symmetry breaking effects are taken into account by including the contributions from the strange quark mass and assuming different values for the strange and the up down quark condensates. We find $g_{K N Λ}/\sqrt{4 π} = - 1.96 $ and $g_{K N Σ}/\sqrt{4 π} = 0.33 $

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arXiv1995

Thermal effects on dilepton production from $π-π$ annihilation

We study finite temperature effects on dilepton production from pion-pion annihilation in hot hadronic matter. The softening of the pion dispersion relation in a medium is found to enhance the production rate of dileptons with invariant masses in the region of $2m^*_π(T)<M<m_ρ$. On the other hand, the reduction of the pion electromagnetic form factor at finite temperature leads to a suppression of the dilepton production rate. Including both effects, we have found that the dilepton yield is slightly enhanced in the invariant mass region $M= 270\sim 600$ MeV but is suppressed around the vector meson resonance. We further discuss the relevance of our results to recent experimental data from the S+Au collisions at CERN/SPS energies by the CERES collaboration.

hep-ph↗

arXiv1995

QCD Sum Rules, Scattering Length and the Vector Mesons in Nuclear Medium

Critical examination is made on the relation between the mass shift of vector mesons in nuclear medium and the vector-meson $-$ nucleon scattering length. We give detailed comparison between the QCD sum rule approach by two of the present authors (Phys. Rev. {\bf C46} (1992) R34) and the scattering-length approach by Koike (Phys. Rev. {\bf C51} (1995) 1488). It is shown that the latter approach is mortally flawed both technically and conceptually.

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arXiv1994

Suppression of dilepton production in hot hadronic matter

The pion electromagnetic form factor at finite temperature is studied using an effective chiral lagrangian that includes explicitly vector mesons. We find that in the time-like region around the rho meson resonance it decreases with increasing temperature and leads to a suppression of dilepton production from pion-pion annihilation in a hot hadronic matter. Effects on dilepton production in high energy heavy ion collisions and its relevance to the phase transition in a hot hadronic matter are discussed.

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arXiv1994

QCD sum rules and chiral logarithms

Standard QCD sum-rule analyses of the nucleon mass give results that are inconsistent with chiral perturbation theory due to an overly simple continuum ansatz on the phenomenological side of the sum rule. We show that a careful treatment of the continuum, including $π$-$N$ states and other states with virtual pions, resolves the inconsistency associated with chiral logs.

hep-ph↗

arXiv1994

Photon Vector Meson Coupling and Vector Meson Properties at Low Temperature Pion Gas

The vector and axial vector current mixing phenomena at low temperature pion gas by Dey, Eletsky and Ioffe, leads to the low temperature correction of the photon-vector meson coupling ($g_ρ$) at order $ε=T^2/6f_π^2$ and the $ρ$ meson mass at order $ε^2$. We show how this {\it low temperature theorems} involving the photon and vector mesons are satisfied in the chiral models based on hidden gauge symmetry and the massive Yang-Mills approach with an explicit $a_1$ meson. We discuss possible phenomenological consequences of the low temperature corrections in RHIC experiments.

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arXiv1993

Contribution of Quark-Mass-Dependent Operators to Higher Twist Effects in DIS

We look at the contribution of Quark-Mass-dependent twist-4 operators to the second moments of spin averaged structure functions and the Bjorken sum rule. Its contribution is non-negligible in the former case due to large Wilson coefficients. We also discuss the values of the twist- 4 spin-2 nucleon matrix element within present experimental constraints.

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