arXiv subjects

M. Oka

Publications and source records attributed to M. Oka.

At least 91 records · Page 5Linked to original sources

arXiv1998

Finite Quark Mass Effects in the Improved Ladder Bethe-Salpeter Amplitudes

We study the finite quark mass effects of the low-energy QCD using the improved ladder Schwinger-Dyson and Bethe-Salpeter equations which are derived in the manner consistent with the vector and axial-vector Ward-Takahashi identities. The non-perturbative mass-independent renormalization allows us to calculate the quark condensate for a non-zero quark mass. We explicitly show that the PCAC relation holds. The key ingredients are the Cornwall-Jackiw-Tomboulis effective action, the generalized Noether current and the introduction of the regularization function to the Lagrangian. The reasonable values of the pion mass, the pion decay constant and the quark condensate are obtained with a rather large $Λ_{QCD}$. The pion mass square and the pion decay constant are almost proportional to the current quark mass up to the strange quark mass region. It suggests that the chiral perturbation is applicable up to the strange quark mass region. We study the validity of the approximation often used in solving the Bethe-Salpeter equations too.

hep-ph↗

arXiv1998

Hyperon Non-leptonic Weak Decays in the Chiral Perturbation Theory II

Hyperon non-leptonic weak decay amplitudes are studied in the chiral perturbation theory. The weak interaction vertices caused by the four quark operators are substituted by the products of the hadronic currents and by the phenomenologically introduced weak Hamiltonian of hadron operators. Our study suggests the improvement of the theoretical prediction for the weak decay amplitudes.

hep-ph↗

arXiv1998

Roles of Quarks in Strong and Weak YN Interactions

The short range parts of the strong and weak hyperon-nucleon interactions are studied with quark substructure of the baryons taken into account. A summary of the quark cluster model calculations of the baryon-baryon interactions is presented. It is pointed out that the spin-flavor symmetry structure determines the qualitative behaviors of the short-range YN interactions. We also show that the spin-orbit YN forces give characteristic behaviors, which are distinct from the meson exchange forces. Weak decays of $Λ$ in hypernuclei are described by the direct quark mechanism. We find that the neutron induced nonmesonic decay is enhanced due to the direct quark transition. Importance of the $ΔI=3/2$ decay amplitudes is emphasized both for the nonmesonic weak decays and the $π^+$ emission in the $Λ$ decay.

nucl-th↗

arXiv1998

Hyperon Non-leptonic Weak Decays in the Chiral Perturbation Theory I

Hyperon non-leptonic weak decay amplitudes are studied in the chiral perturbation theory. We employ the low energy effective weak Hamiltonian which contains the perturbative QCD correction. To include the non-perturbative QCD effect, quark currents of the effective Hamiltonian are substituted with hadronic currents which are color singlet and are derived by the chiral perturbation theory. We find that the amplitudes caused by the product of hadronic currents are small. It reproduce the small amplitudes of $ΔI=3/2$, which are derived by the strong interaction correction.

hep-ph↗

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↗

arXiv1998

Chiral Symmetry for Positive and Negative Parity Nucleons

Chiral properties of positive and negative parity nucleons, N and N*$, are studied from the viewpoint of chiral symmetry. Two possible ways to assign chiral transformations to the negative parity nucleon are considered. Using linear sigma models based on the two chiral realizations, theoretical as well as phenomenological consequences of the two different assignments are investigated. We find that the nucleon mass in the chiral restored phase is the key quantity to determine the meson-nucleon couplings and the axial charges of nucleons. We also discuss the role of chiral symmetry breaking in the mass splitting of N and N* in the two sigma models.

hep-ph↗

arXiv1998

Approximation of the Schwinger--Dyson and the Bethe--Salpeter Equations and Chiral Symmetry of QCD

The Bethe--Salpeter equation for the pion in chiral symmetric models is studied with a special care to consistency with low-energy relations. We propose a reduction of the rainbow Schwinger--Dyson and the ladder Bethe--Salpeter equations with a dressed gluon propagator. We prove that the reduction preserves the Ward--Takahashi identity for the axial-vector current and the PCAC relation.

hep-ph↗

arXiv1998

Suppression of Pi NN* Coupling and Chiral Symmetry

Meson-baryon couplings between positive and negative parity nucleons are investigated using two point correlation functions in the soft meson and the chiral limit. We find that the pi NN* coupling vanishes due to chiral symmetry, while the eta NN* coupling remains finite. We perform an analysis based on the algebraic method for SU(2) chiral symmetry, and find that nucleon axial charges play an essential role for vanishing coupling constants. For an illustration, we construct sigma models with N and N*, where we discuss their properties from the view point of the chiral symmetry. We also discuss N and N* in the viewpoint of chiral partner.

hep-ph↗

arXiv1997

Decays of 1/2^- Baryons in Chiral Effective Theory

We construct an $SU(3)_L \times SU(3)_R$ symmetric chiral effective model which includes parity pair baryon fields. It is assumed that the positive and negative parity baryons in the parity pair have opposite chiral transformation properties each other. Using this model, strong decays of the negative parity baryons are studied up to the order of one-loop corrections. The results agree qualitatively with experiment in the $π$ and $K$ channels. We examine how the model parameters are determined from the decay widths and study their physical meanings. Possibilities to build other models and relations to the correlation function analysis are discussed.

hep-ph↗

arXiv1997

Non-mesonic Weak Decays of Light Hypernuclei in the Direct Quark and the One-Pion Exchange Mechanisms

Contributions of the direct quark mechanism are studied in the nonmesonic weak decays of light hypernuclei. The $ΛN \to NN$ transition is described by the one pion exchange mechanism and the direct quark mechanism, induced by the four-quark vertices in the effective weak Hamiltonian. By employing a realistic wave function of the $Λ$ inside the hypernuclei, nonmesonic decay rates of $^4_Λ$H, $^4_Λ$He, and $^5_Λ$He are calculated. The results show that the direct quark mechanism is significantly large and gives a large $ΔI =3/2$ contribution in the J=0 channel. The relative phase between the one-pion exchange and the direct-quark contributions is determined so that the effective weak Hamiltonian for quarks give both of them consistently. We find that the sum of these two contributions reproduce the current available experimental data fairly well.

nucl-th↗

arXiv1997

Suppression of PiNN* Coupling and Chiral Symmetry

Meson-baryon couplings between positive and negative parity baryons are investigated using two point correlation functions in the soft meson limit. We find that the pi NN^* coupling vanishes due to chiral symmetry, while the eta NN^* coupling remains finite. We perform an analysis based on the algebraic method for SU(2) and SU(3) chiral symmetry, and find that baryon axial charges play an essential role for vanishing coupling constants.

hep-ph↗

arXiv1997

Exchange Currents for Hypernuclear Magnetic Moments

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 the ${}^5_ΛHe$, ${}^6_ΛHe$ and ${}^6_ΛLi$ 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 ${}^5_ΛHe$. 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.

nucl-th↗

arXiv1997

Suppression of PiNN(1535) Coupling in the QCD Sum Rule

The πNN^* and ηNN^* coupling constants are studied based on the two point functions between the vacuum and a one meson state in the soft meson limit. In terms of a suitably constructed interpolating field for the nucleon and resonances, we have found that the πNN^* couplings vanish, while ηNN^* couplings remain finite. This result explains a relatively suppressed coupling of πNN(1535) as compared with others. We compare the present result with predictions of low energy effective models.

hep-ph↗

arXiv1997

QCD Sum Rule for 1/2- Baryons

The masses of the flavor octet and singlet baryons with negative parity and spin 1/2 are calculated using the QCD sum rule. We find that the chiral symmetry breaking vacuum condensates cause the mass splitting of the positive and negative baryons. The present sum rule reproduces the observed masses of these baryons within 10%, and predicts the mass of the excited Xi baryon with $J^P = {1/2}^{-}$ at 1.63 GeV. We confirm that the negative-parity state, $Λ_S-$, is the ground state in the flavor singlet baryon spectrum.

hep-ph↗

arXiv1997

Negative Parity Baryons in the QCD Sum Rule

Masses and couplings of the negative parity excited baryons are studied in the QCD sum rule. Separation of the negative-parity spectrum is proposed and is applied to the flavor octet and singlet baryons. We find that the quark condensate is responsible for the mass splitting of the ground and the negative-parity excited states. This is expected from the chiral symmetry and supports the idea that the negative-parity baryon forms a parity doublet with the ground state. The meson-baryon coupling constants are also computed for the excited states in the QCD sum rule. It is found that the πNN^* coupling vanishes in the chiral limit.

hep-ph↗

arXiv1997

Suppression of PiNN(1535) Coupling in the QCD Sum Rule

The πNN^* and ηNN^* coupling constants are studied in the QCD sum rule ($N^* \equiv N(1535)). We investigate the two point functions between the vacuum and a one meson state in the soft meson limit. The operator product expansion is explicitly performed up to terms of dimension eight. We find that \gpNR vanishes to this order, while \geNR has a finite contribution. The vanishing of \gpNR is a rigorous consequence in the chiral limit. The results of the QCD sum rule are compared with those of low energy effective models.

hep-ph↗

arXiv1996

Strong $U_A(1)$ breaking in radiative $η$ 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 $η$-meson mass, the \egg, \egm and \epg decay widths are in good agreement with the experimental values when the $U_{A}(1)$ breaking is strong and the flavor $SU(3)$ singlet-octet mixing angle $θ$ is about zero. The calculated $ηγγ^\ast$ transition form factor has somewhat weaker dependence on the squared four-momentum of the virtual photon. The effects of the $U_A(1)$ anomaly on the scalar quark contents in the nucleon, the $Σ_{πN}$ and $Σ_{KN}$ terms and the baryon number one and two systems are also studied.

hep-ph↗

arXiv1996

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 $\O16$ 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.

nucl-th↗