arXiv subjects

Makoto Oka

Publications and source records attributed to Makoto Oka.

141 records · Page 8Linked to original sources

arXiv1999

Weak Decay of $Λ$ in Nuclei: Direct Quark Mechanism vs Meson Exchange

Nonmesonic decays of $Λ$ in nuclear medium and light hypernuclei are studied by using the $ΛN \to NN$ weak transition potential derived from the meson exchange mechanism and the direct quark mechanism. The long range part of the transition potential is described by exchanges of the pseudoscalar mesons ($π$, $K$, $η$), while the vector mesons ($ρ$, $ω$, $K^\ast$) may be considered as the medium- and short-range part in the meson exchange picture. We propose the direct quark transition potential as the short range part, which is derived from the matrix elements of the $ΔS=1$ effective weak Hamiltonian in the two baryon states. The results indicate that the direct quark contribution is significantly large and its behavior is qualitatively different from the vector meson exchanges. We also find that the decay rate is sensitive to the choice of form factor and that a soft cutoff must be used for the pion-baryon verteces so that the strong tensor transition is suppressed. We find that the $π+ K + DQ$ results are compatible with experiment although the $n/p$ ratio is still too large. The $π^+$ decays of light hypernuclei are related to the $\DI=3/2$ amplitudes of the nonmesonic decay. The role of chiral symmetry for the pionic decays are discussed.

nucl-th↗

arXiv1999

Two-point correlation function with pion in QCD sum rules

Within the framework of the conventional QCD sum rules, we study the pion two-point correlation function, $i\int d^4x e^{iq\cdot x} < 0| T J_N(x) {\bar J}_N(0)|π(p)>$, beyond the soft-pion limit. We construct sum rules from the three distinct Dirac structures, $i γ_5 \notp, i γ_5, γ_5 σ_{μν} {q^μp^ν}$ and study the reliability of each sum rule. The sum rule from the third structure is found to be insensitive to the continuum threshold, $S_π$, and contains relatively small contribution from the undetermined single pole which we denote as $b$. The sum rule from the $i γ_5$ structure is very different even though it contains similar contributions from $S_π$ and $b$ as the ones coming from the $γ_5 σ_{μν} {q^μp^ν}$ structure. On the other hand, the sum rule from the $i γ_5 \notp$ structure has strong dependence on both $S_π$ and $b$, which is clearly in constrast with the sum rule for $γ_5 σ_{μν} {q^μp^ν}$. We identify the source of the sensitivity for each of the sum rules by making specific models for higher resonance contributions and discuss the implication.

nucl-th↗

arXiv1999

QCD sum rule calculation of the $πNN$ coupling-revisited

In the conventional QCD sum rule calculation for the $πNN$ coupling beyond the soft-pion limit, there are three distinct Dirac structures, each of which can be used to construct separate sum rule. We point out subtleties in the previous sum rule results, based on one of the Dirac structure, associated with using either the PV or the PS coupling schemes in modeling the phenomenological side. We propose a sum rule coming from a different Dirac structure, which is independent of the coupling schemes used and has less uncertainty in the OPE. The obtained value is $g_{πN}=9.76 \pm 2.04$ where the uncertainty mainly comes from the quark-gluon mixed condensate.

nucl-th↗

arXiv1999

QCD sum rules with two-point correlation function

We construct three different sum rules from the two-point correlation function with pion, $i\int d^4x e^{iq\cdot x} <0| T J_N(x) {\bar J}_N(0)|π(p)>$, beyond the soft-pion limit. The PS and PV coupling schemes in the construction of the phenomenological side are carefully considered in each sum rule. We discuss the dependence of the result on the specific Dirac structure and identify the source of the dependence by making specific models for higher resonances.

nucl-th↗

arXiv1999

$π^+$ Emission from Hypernuclei and the Weak $ΔI=3/2$ Transitions

Low energy $π^+$ emission in hypernuclear weak decays is studied in the soft pion limit. It is found that the $π^+$ decay amplitude is dominated by the $ΔI=3/2$ part of nonmesonic weak decays according to the soft pion theorem. The ratios, $R(π^+/soft π^-) $, for light hypernuclei are estimated in the direct quark mechanism, which predicts violation of the $ΔI=1/2$ rule, and found to be about 1/3 for $^4_ΛHe$.

nucl-th↗

arXiv1998

Direct Quark Mechanism for Weak $\Lam N \leftrightarrow NN$ Processes

Two body weak processes $ΛN \leftrightarrow NN$ are studied from the viewpoint of quark substructure of baryons.They can be studied in nonmesonic weak decays of hypernuclei and also hyperon production in the $NN$ scattering. The direct quark mechanism in contrast to the one-pion exchange mechanism is shown to give sizable $\di=3/2$ contribution in the J=0 channel. Nonmesonic decay rates of $^4_Λ$H, $^4_Λ$He, and $^5_Λ$He agree with the current experimental data fairly well. Especially the $n/p$ ratio is enhanced by the direct quark contribution. The cross sections for $pn\to \Lam p$ scattering are studied in the quark cluster model approach.

nucl-th↗

arXiv1997

Roles of Quark Degrees of Freedom in Hypernuclei

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.

hep-ph↗

arXiv1995

The pion at finite temperature

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} = \sqrt{2} f_π^{T=0} $. 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 $\sim 15\%$ and $\sim 10\%$, respectively, and that the pion mass slightly ($\sim 3\%$) increases at $T=160MeV$, where the pion gas approximation seems to break down.

hep-ph↗

arXiv1995

QCD Analysis of the H dibaryon

The status of theoretical studies of the $H$ dibaryon is reviewed. Some recent developments including the effect of the instanton induced interaction and the QCD sum rule results are discussed in detail.

hep-ph↗

arXiv1995

Direct quark transition potential for $ΛN \to NN$ decay

The weak $ΛN\to NN$ transition is studied in the valence quark model approach. The quark component of the two baryon system is described in the quark cluster model and the weak transition potential is calculated by evaluating the matrix elements of the $ΔS=1$ effective weak Hamiltonian. The transition potential is applied to the decay of hypernuclei and the results are compared with available experimental data. The results indicate that direct quark process is significant and qualitatively different when compared with those in conventional meson-exchange calculations. The direct quark mechanism predicts the violation of the $ΔI = 1/2$ rule for this transition.

hep-ph↗

arXiv1995

Strong and Weak Interactions of Strange Hadrons

I review hadronic processes involving strange hadrons, especially hyperons from the quark structure point of view. The strong interaction of quarks expects several important new features when the strangeness is introduced upon the non-strange degrees of freedom. I concentrate on the instanton induced interaction, effects of the Pauli principle for strangeness and the nonleptonic weak interaction of strangeness. The single hadron as well as the two baryon systems including the $H$ dibaryon and other exotic hadronic systems with strangeness are studied in this context.

hep-ph↗

arXiv1993

Short-range YN interactions in the Quark Cluster Model

A phenomenological model for the hyperon-nucleon interactions is constructed by using the quark cluster model approach to the short-distance baryon-baryon interactions. The model contains the SU(3) symmetric meson exchange interaction at large distances and the quark-exchange short-distance interaction. The main feature of the model is that strong channel dependences of the short range repulsions due to the quark model symmetry. It is pointed out that two channels, ($I$, $S$)= (1/2, 0) and (3/2, 1), of the S-wave sigma-nucleon interactions have extremely strong repulsions at short-distances.

hep-ph↗

arXiv1993

Weak $\Lam N\to NN$ Transition in the Direct Quark Mechanism

The weak $\Lam N\to NN$ transition is studied in the valence quark model approach. The momentum transfer for this transition is so large that the short-distance two baryon dynamics must be taken into account. The two baryon system is described in the quark cluster model and the weak transition amplitude is calculated by evaluating the matrix elements of the effective weak $ΔS= 1$ hamiltonian. The results indicate some qualitative differences when compared with those in conventional meson-exchange calculations. Especially, we conclude that contributions of the $ΔI={3\over2}$ transition are significant and that the discrepancy in the $n-p$ ratio between theory and experiment could be resolved by including the direct-quark processes.

hep-ph↗

arXiv1993

Hyperon-Nucleon Interaction in a Quark Model

A lecture given at the International School Seminar on {\sl Hadrons and Nuclei from QCD}, Tsuruga-Vladivostok-Sapporo, August-September, 1993. A realistic hyperon ($Y$)-nucleon ($N$) interaction based on the quark model and the one-boson-exchange potential is constructed. The Nijmegen potential model D with the SU(3) flavor symmetry is modified with a quark exchange interaction at the short-distance, which replaces the short-range repulsive core in the original model. The flavor-spin dependences of the short-range repulsion are qualitatively different from the original hard-core potential. We also study a two-body weak decay, $ΛN \to NN$, in the quark model. An effective weak interaction, where one-loop QCD corrections are explicitly taken into account, is employed. Differences from the conventional meson-exchange processes are discussed.

hep-ph↗

arXiv1993

Nuclear Force in the Skyrme Model: Quasistatic Approach

Dynamics of two-Skyrmion systems is studied in the quasistatic approach. The quasistatic approach enables us to formulate the collective coordinate quantization of two-Skyrmion systems consistently with the 1/Nc expansion. By constructing quasistatic field configurations for largely separated two Skyrmions, their energies are computed for three independent relative orientations of the Skyrmions in the isospin space. The obtained energies are much lower than those for the product approximation. In particular, the repulsion in the two-pion range is considerably reduced.

hep-ph↗