Hui-Young Ryu

ϕ photoproduction with coupled-channel effects

We study phi photoproduction with various hadronic rescattering contributions included, in addition to the Pomeron and pseudoscalar meson-exchange diagrams. We find that the hadronic box diagrams can explain the recent experimental data in the vicinity of the threshold. In particular, the bump-like structure at the photon energy E_gamma \approx 2.3

Hadronic description for Omega baryon photoproduction

GeV is well explained by the K Lambda(1520) rescattering amplitude in the intermediate state, which is the dominant contribution among other hadronic contributions. We also find that the hadronic box diagrams are consistent with the observed spin-density matrix elements near the threshold region.

PHOTOPRODUCTION OF PHI MESON NEAR THE THRESHOLD

In the present work, we investigate subsequential production of three kaons and

Magnetic Susceptibility of the QCD Vacuumat a Finite Quark-chemical Potential

\Omega^-

Photoproduction of the scalar mesons

baryon based on an effective Lagrangian approach. We only consider the intermediate states with the light mass baryon to suggest the minimum of the total cross section. Coupling constants for verteces of meson-octet baryons are fixed from the empirical data and/or quark models together with SU(3) symmetry considerations and these for meson-decouplet are predicted not only quark model but also Chiral-quark soliton model calculation. Gauge invariance of the resulting amplitude is maintained by introducing the contact currents by extending the gauge-invariant approach of Haberzettl for one-meson photoproduction to two-meson photoproduction.

f_0(500)

Photoproduction of ϕ, γp → ϕ + p, is revisited in a model with Pomeron and hadron degrees of freedom. The scattering equation is written such that the final state interaction is included at the one-loop level of hadrons. We find a parameter set to reproduce the bump-like structure near the threshold in a way consistent with other meson production data.

and

We investigate the QCD magnetic susceptibility chi at the finite quark-chemical potential (mu>0) and at zero temperature (T=0) to explore the pattern of the magnetic phase transition of the QCD vacuum. For this purpose, we employ the nonlocal chiral quark model derived from the instanton vacuum in the presence of the chemical potential in the chiral limit. Focusing on the Nambu-Goldstone phase, we find that the magnetic susceptibility remains almost stable to mu~200 MeV, and falls down drastically until the the quark-chemical potential reaches the critical point mu_c~320 MeV. Then, the strength of the chi is reduced to be about a half of that at mu=0, and the first-order magnetic phase transition takes place, corresponding to the chiral restoration. From these observations, we conclude that the response of the QCD vacuum becomes weak and unstable to the external electromagnetic field near the critical point, in comparison to that for vacuum. It is also shown that the breakdown of Lorentz invariance for the magnetic susceptibility, caused by the finite chemical potential, turns out to be small.We present in this talk a recent investigation on the magnetic susceptibility (chi) of the QCD vacuum at finite density, utilizing the nonlocal chiral quark model from the instanton vacuum. We take into account the nonzero current-quark mass (m_q) explicitly to consider the effect of explicit flavor SU(3) symmetry breaking. It turns out that, when we turn on the current-quark mass, chi becomes smaller, indicating less response to the externally induced electromagnetic field, in comparison to that for m_q=0.

f_0(980)

We investigate photoproduction of the scalar mesona off the nucleon, using the effective Lagrangians and the Regge approach. We first study

off the nucleon

f_0(980)

Photoproduction of the Scalar Meson

photoproduction, replacing the Feynman propagator with the Regge ones for the