Makoto Oka

Negative parity nucleon resonance in the QCD sum rule

The negative-parity baryons are studied by a novel approach in the QCD sum rule. It is found that the parity of the ground-state nucleon is determined by the sign of the quark condensate. We predict the mass of the negative-parity nucleon. {copyright} {ital 1996 The American Physical Society.}

Negative parity baryons in the QCD sum rule

Abstract Masses and couplings of negative parity baryons are studied in the QCD sum rule. A technique to single out the negative-parity spectrum is proposed. The quark condensates that break the chiral symmetry cause mass splittings between the ground state and negative parity baryons, suggesting that they form parity doublet. (In the flavor singlet spectra the negative parity baryon is the lowest state.) We also compute the πNN∗ coupling and found that it vanishes in the soft pion limit due to the chiral structure of the interpolating field.

Roles of quark degrees of freedom in hypernuclei

Abstract The quark model description of the hyperon nucleon forces, especially the antisymmetric spin-orbit forces, is studied from the spin-flavor SU(6) and the flavor SU(3) symmetry point of view. It is pointed out that the quark exchange interaction predicts strong antisymmetric spin-orbit force between the hyperon and nucleon.

Exchange currents for hypernuclear magnetic moments

Abstract The meson (K and π) exchange currents for the hypernuclear magnetic moments are calculated using the effective Lagrangian method. The seagull diagram, the mesonic diagram and the Σ 0 -excitation diagram are considered. The Λ-N exchange magnetic moments for Λ 5 He and A=6 hypernuclei are calculated employing the harmonic oscillator shell model. It is found that the two-body correction is about −9% of the single particle value for Λ 5 He. The π exchange current, induced only in the Σ 0 -excitation diagram, is found to give dominant contribution for the isovector magnetic moments of hypernuclei with A = 6.

The pion at finite temperature

Abstract Properties of the pion are studied at finite temperature with the help of the PCAC and the QCD sum rule. The pionic mode is treated consistently with the thermal pions which consist of the ground state at finite temperature. The temperature dependence of the pion decay constant is estimated for m q = 0. No consistent solution is found above the critical temperature T c = 2 f T=0 φ . The QCD sum rule shows that the finite quark mass makes the critical temperature move up to a temperature where the dilute pion gas approximation is no more valid. It also suggests that the pion decay constant and the continuum threshold decrease by ∼ 15% and ∼ 10%, respectively, and that the pion mass slightly (∼ 3%) increases at T = 160 MeV, where the pion gas approximation seems to break down.

Nuclear G-Matrix Elements from Nonlocal Potentials

We study effects of nonlocality in the nuclear force on the G-matrix elements for finite nuclei. Nuclear G-matrix elements for 16O are calculated in the harmonic oscillator basis from a nonlocal potential which models quark exchange effects between two nucleons. We employ a simple form of potential that gives the same phase shifts as a realistic local nucleon potential. The G-matrix elements calculated from the nonlocal potential show moderate increase in repulsion from those derived from the local potential.

Electromagnetic Energy Shift of Nucleus between Conducting Plates

Electromagnetic energy shifts of a single particle quantum state sandwiched between parallel conducting plates are studied. We find an energy shift of order 1/b 2 , with b the interval of the boundary plates. Applying the calculation to a polarized nucleus in a uniform magnetic field, we find significant energy shifts which depend on the magnetic quantum number. The relation of this effect and the enhancement factor in E1 photo nuclear reactions is discussed.

Strong U-A(1) breaking in radiative eta decays

We study the egg, egm and epg decays using an extended three-flavor Nambu-Jona-Lasinio model that includes the t~Hooft instanton induced interaction. We find that the

Direct quark transition potential for

eta

\Lambda N \to NN

-meson mass, the egg, egm and epg decay widths are in good agreement with the experimental values when the