Ki-Seok Choi

Axial charges of octet and decuplet baryons

We present a study of axial charges of baryon ground and resonant states with relativistic constituent quark models. In particular, the axial charges of octet and decuplet N, {Sigma}, {Xi}, {Delta}, {Sigma}*, and {Xi}* baryons are considered. The theoretical predictions are compared to existing experimental data and results from other approaches, notably from lattice quantum chromodynamics and chiral perturbation theory. The relevance of axial charges with regard to {pi}-dressing and spontaneous chiral-symmetry breaking is discussed.

Asymmetry in the neutrino and anti-neutrino reactions in a nuclear medium

Abstract We study the effect of the density-dependent axial and vector form factors on the electron–neutrino ( ν e ) and anti-neutrino ( ν ¯ e ) reactions for a nucleon in nuclear matter or in 12C. The nucleon form factors in free space are presumed to be modified for a bound nucleon in a nuclear medium. We adopt the density-dependent form factors calculated by the quark–meson coupling (QMC) model, and apply them to the ν e and ν ¯ e induced reactions with the initial energy E = 8 – 80 MeV . We find that the total ν e cross sections on 12C as well as on a nucleon in nuclear matter are reduced by about 5% at the nuclear saturation density, ρ 0 . This reduction is caused by the modification of the nucleon structure in matter. Although the density effect for both cases is relatively small, it is comparable with the effect of Coulomb distortion on the outgoing lepton in the ν-reaction. In contrast, the density effect on the ν ¯ e reaction reduces the cross section significantly in both nuclear matter and 12C cases, and the amount maximally becomes of about 35% around ρ 0 . Such large asymmetry in the ν e and ν ¯ e cross sections, which seems to be nearly independent of the target, is originated from the differences in the helicities of ν ¯ e and ν e . It is expected that the asymmetry influences the r-process and also the neutrino-process nucleosynthesis in core-collapse supernovae.

eta Photoproduction and N* resonances

We investigate the eta photoproduction using the effective Lagrangian approach at the tree level. We focus on the new nucleon resonance N*(1675), which was reported by the GRAAL, CB-ELSA and Tohoku LNS, testing its possible spin and parity states theoretically (J^P=1/2^+-,3/2^+-). In addition, we include six nucleon resonances, D_13(1520), S_11(1535), S_11(1650), D_15(1675), P_11(1710), P_13(1720) as well as the possible background contributions. We calculate various cross sections including beam asymmetries for the neutron and proton targets. We find noticeable isospin asymmetry in transition amplitudes for photon and neutron targets. This observation may indicate that the new resonance can be identified as a non-strangeness member of the baryon antidecuplet.

Test of the reaction mechanism for γN → K(1520) using the polarized photon

We study the reaction mechanism for the photoproduction of the �(1520), based on an effective Lagrangian approach. We investigate each contribution of the s-, u-, t-channel processes and contact term, separately. One of the most characteristic features of this reaction is the contact-term dominance which governs the photoproduction from the proton, when the K ∗ -exchange contribution is possibly not too large. We suggest several different ways of the polarizations and arrangement of the beam and target to make to understand the role of each contribution separately in future experiments.

A new N∗(1675)N∗(1675) resonance in the γN→ηNγN→ηN reaction

We study the

A New N*(1675) resonance in the gamma N ---> eta N reaction

\eta

Relativistic quark-model spectroscopy of light and heavy baryons

-photoproduction focusing on the new nucleon resonance which was observed at

Effects of the density-dependent weak form factors on the neutrino reaction via neutral current for the nucleon in nuclear medium and

\sqrt{s}=1675

^{12}

MeV with a narrow decay width (

C

\sim 10