arXiv subjects

Makoto Oka

Publications and source records attributed to Makoto Oka.

At least 109 records · Page 6Linked to original sources

arXiv2005

Anisotropic lattice QCD study of pentaquark baryons in spin 3/2 channel

We perform the comprehensive analysis of the pentaquark (5Q) in J^P=3/2^{\pm} channel using anisotropic quenched lattice QCD. We employ the standard Wilson gauge action at β=5.75 and the O(a) improved Wilson (clover) quark action on a 12^3*96 lattice with the renormalized anisotropy as a_s/a_t = 4. A large number of gauge configurations as Nconf=1000 is analyzed, which is found to be essential to achieve a reliable measurement. We study three types of the Rarita-Schwinger intepolationg fields with I=0: (a) the NK*-type, (b) the (color-)twisted NK*-type, (c) a diquark-type. As a result, we find only massive states as m(5Q)= 2.1-2.2 GeV in J^P=3/2^- channel, and m(5Q) = 2.4-2.6 GeV in J^P=3/2^+ channel in the chiral limit. The analysis with the hybrid boundary condition (HBC) is performed to distinguish whether these resonances are compact 5Q resonances or two-particle scattering states. No low-lying compact 5Q resonance states are found below 2.1GeV.

hep-lat↗

arXiv2005

Spectroscopy of Pentaquark Baryons

A review is given to pentaquark mass predictions in quark models and QCD. It is pointed out that no successful quark model prediction is available for low-lying pentaquark states. Some new results of direct application of QCD, QCD sum rules and lattice QCD, are also presented.

hep-ph↗

arXiv2004

Why do we need instantons in strangeness hadron physics?

Roles of instantons in strangeness hadron physics are discussed. After introducing general features of instantons, hadron spectroscopy under the influence of instanton-quark effective interactions is discussed. Emphases are on the H dibaryon, spin-orbit splittings of P-wave baryons and baryon-baryon interactions and spectrum of the pentaquarks.

hep-ph↗

arXiv2004

Lattice-QCD based Schwinger-Dyson approach for Chiral Phase Transition

Dynamical chiral-symmetry breaking in QCD is studied with the Schwinger-Dyson (SD) formalism based on lattice QCD data, i.e., LQCD-based SD formalism. We extract the SD kernel function $K(p^2)$ in an Ansatz-independent manner from the lattice data of the quark propagator in the Landau gauge. As remarkable features, we find infrared vanishing and intermediate enhancement of the SD kernel function $K(p^2)$. We apply the LQCD-based SD equation to thermal QCD with the quark chemical potential $μ_q$. We find chiral symmetry restoration at $T_c \simeq 100{\rm MeV}$ for $μ_q=0$. The real part of the quark mass function decreases as $T$ and $μ_q$. At finite density, there appears the imaginary part of the quark mass function, which would lead to the width broadening of hadrons.

hep-ph↗

arXiv2004

Contribution of Instanton Induced Interaction for Penta-quarks in MIT Bag Model

Roles of instanton induced interactions (III) in the masses of pentaquark baryons, Theta^+ (J=1/2 and 3/2) and Xi^{--}, and a dibaryon, H, are discussed using the MIT bag model. It is shown that the two-body terms in III give a strong attraction mainly due to the increase of the number of pairs in multi-quark systems. In contrast, the three-body $u$-$d$-$s$ interaction is repulsive. It is found that III lowers the mass of negative-parity Theta^+ as much as 100 MeV from the mass predicted by the bag model without III.

hep-ph↗

arXiv2004

QCD Sum Rules of Pentaquarks

QCD sum rule is applied to the pentaquark spectroscopy. It is concluded that no positive parity state is seen in low energy region, while there may exist negative parity states at around 1.5 GeV. Choice of interpolating local operators and relation to the lattice calculations are discussed.

hep-ph↗

arXiv2004

The lattice QCD simulation of the quark-gluon mixed condensate g<\bar{q} σG q> at finite temperature and the phase transition of QCD

The thermal effects on the quark-gluon mixed condensate g<\bar{q} σG q>, which is another chiral order parameter, are studied using the SU(3)c lattice QCD with the Kogut-Susskind fermion at the quenched level. We perform the accurate measurement of the mixed condensate as well as the quark condensate for 0MeV<=T<=500MeV. We observe the sharp decrease of both the condensates around T_c \simeq 280MeV, while the thermal effects below T_c are found to be weak. We also find that the ratio m_0^2 = g<\bar{q} σG q>/<\bar{q}q> is almost independent of the temperature even in the very vicinity of T_c, which indicates that the two condensates have nontrivial similarity in the chiral behaviors. We also present the correlation between the condensates and the Polyakov loop to understand the vacuum structure of QCD.

hep-lat↗

arXiv2004

Effects of Instanton Induced Interaction on the Pentaquarks

Roles of instanton induced interactions (III) in the masses of pentaquark baryons, Theta^+ (J=1/2 and 3/2) and Xi^{--}, and a dibaryon, H, are discussed using the MIT bag model in the negative parity case. It is shown that the two-body terms in III give a strong attraction mainly due to the increase of the number of pairs in multi-quark systems. In contrast, the three-body u-d-s interaction is repulsive. It is found that III lowers the mass of Theta^+ as much as 100 MeV from the mass predicted by the bag model without III.

hep-ph↗

arXiv2004

Theoretical Overview of the Pentaquark Baryons

A review is given of the theoretical ideas concerning the mysterious pentaquark baryons proposed during the first year after its discovery. We focus on the difficulties involved with the constituent quark models and the discrepancy between the QCD predictions and most of the phenomenological models.

hep-ph↗

arXiv2004

Thermal effects on quark-gluon mixed condensate g<\bar{q} σG q> from lattice QCD

We present the first study of the thermal effects on the quark-gluon mixed condensate g<\bar{q} σG q>, which is another chiral order parameter, in SU(3)c lattice QCD with the Kogut-Susskind fermion at the quenched level. Using the lattices at β=6.0, 6.1, 6.2 in high statistics, we calculate the mixed condensate as well as the quark condensate in the chiral limit for the temperature 0MeV <= T <= 500MeV. Except for the sharp decrease of both the condensates around T_c \simeq 280MeV, the thermal effects are found to be weak below T_c. We also find that the ratio m_0^2 = g<\bar{q} σG q>/ <\bar{q}q> is almost independent of the temperature even in the very vicinity of T_c. This result indicates nontrivial similarity in the chiral behaviors of the two different condensates.

hep-lat↗

arXiv2004

Models of the Nonmesonic Weak Decay

I review the current status of understanding the mechanism of the nonmesonic weak decays (NMWD) of hypernuclei. Long standing problem on the Gamma_n/Gamma_p ratio has been solved by considering short-range weak interactions properly. This leaves a few remaining problems, such as the asymmetry of the emitted proton with respect to the Lambda polarization and validity of the Delta I=1/2 in the nonmesonic weak decay.

nucl-th↗

arXiv2004

Light Scalar Mesons in the Improved Ladder QCD

The light scalar meson spectrum is studied using the improved ladder QCD with the UA(1) breaking Kobayashi-Maskawa-'t Hooft interaction by solving the Schwinger-Dyson and Bethe-Salpeter equations. The dynamically generated momentum-dependent quark mass is large enough in the low momentum region to give rise to the spontaneous breaking of chiral symmetry. Due to the large dynamical quark mass, the scalar mesons become the q bar{q} bound states. Since the parameters have been all fixed to reproduce the light pseudoscalar meson masses and the decay constant, there is no free parameter in the calculation of the scalar mesons. We obtain $M_{sigma} = 667$ MeV, $M_{a_0} = 942$ MeV and $M_{f_0} = 1336$ MeV. They are in good agreement with the observed masses of $sigma(600)$, $a_0(980)$ and $f_0(1370)$, respectively. We therefore conclude that these states are the members of the light scalar meson nonet. The mass of $K_0^{*}$ is obtained between that of $a_0$ and $f_0$ and the corresponding state is not observed experimentally. We also find that the strangeness content in the $sigma$ meson is about 5%.

hep-ph↗

arXiv2003

Dynamical chiral-symmetry breaking at T=0 and $T \ne 0$ in the Schwinger-Dyson equation with lattice QCD data

For the study of dynamical chiral-symmetry breaking (DCSB) in QCD, we investigate the Schwinger-Dyson (SD) formalism based on lattice QCD data. From the quenched lattice data for the quark propagator in the Landau gauge, we extract the SD kernel function K(p^2), which is the product of the quark-gluon vertex and the polarization factor in the gluon propagator, in an Ansatz-independent manner. We find that the SD kernel function K(p^2) exhibits infrared vanishing and a large enhancement at the intermediate-energy region around p \sim 0.6GeV. We investigate the relation between the SD kernel and the quark mass function, and find that the infrared and intermediate energy region (0.35GeV<p<1.5GeV) would be relevant for DCSB. We apply the lattice-QCD-based SD equation to thermal QCD, and calculate the quark mass function at the finite temperature. Spontaneously broken chiral symmetry is found to be restored at the high temperature above 100MeV.

hep-ph↗

arXiv2003

Penta-quark baryon from the QCD Sum Rule

Exotic penta-quark baryon with strangeness +1, Θ^+, is studied in the QCD sum rule approach. We derive sum rules for the positive and negative parity baryon states with J=1/2 and I=0. It is found that the standard values of the QCD condensates predict a negative parity Θ^+ of mass \simeq 1.5 GeV, while no positive parity state is found. We stress the roles of chiral-odd condensates in determining the parity and mass of Θ^+.

hep-ph↗

arXiv2003

The Study of Chiral Restoration using the Quark-Gluon Mixed Condensate g<\bar{q} σG q> in Lattice QCD at Finite Temperature

The quark-gluon mixed condensate g<\bar{q} σG q> is studied using SU(3)c lattice QCD with the Kogut-Susskind fermion at the quenched level. Using the lattices as β= 6.0 with 16^3 * N_t (N_t=16,12,10,8,6,4), β= 6.1 with 20^3 * N_t (N_t=20,12,10,8,6) and β= 6.2 with 24^3 * N_t (N_t=24,16,12,10,8) in high statistics of 100-1000 gauge configurations, we perform accurate measurement of the thermal effects on g<\bar{q} σG q> as well as <\bar{q}q> in the chiral limit. We find that the thermal effects on both the condensates are very weak except for the vicinity of T_c, while both the condensates suddenly vanish around T_c \simeq 280 MeV, which indicates strong chiral restoration near T_c. We also find that the ratio m_0^2 = g<\bar{q} σG q>/ <\bar{q}q> is almost independent of the temperature even in the very vicinity of T_c, which means that these two different condensates obey the same critical behavior. This nontrivial similarity between them would impose constraints on the chiral structure of the QCD vacuum near T_c.

hep-lat↗

arXiv2003

Meson-Baryon Couplings from QCD Sum Rules

Coupling constants of the pseudoscalar mesons to the octet baryons are calculated in the QCD sum rule approach. Two-point correlation function of the baryons are evaluated in a single meson state and the vacuum, which yields the designated coupling. The emphasis is on the flavor SU(3) structure of the coupling constants and reliability in extracting the coupling constants from the two-point correlation functions. We first calculate the baryon-diagonal couplings and study the reliability of the sum rule. The F/D ratio of the coupling is determined in the SU(3) limit. We further formulate the baryon-off-diagonal couplings using the projected correlation functions and the vertex functions, so that the unwanted excited states do not contaminate the sum rule. As an example, the (π- Λ- Σ) coupling constant is calculated and the flavor SU(3) breaking effect is studied. We find that the effect of SU(3) breaking on the (π- Λ- Σ) coupling constant is small.

hep-ph↗

arXiv2003

Lattice-QCD-based Schwinger-Dyson Approach for Chiral Symmetry Restoration at Finite Temperature

We propose the Schwinger-Dyson (SD) formalism based on lattice QCD, i.e., LQCD-based SD formalism, for the study of dynamical chiral-symmetry breaking in QCD. We extract the kernel function K(p^2) in the SD equation from the lattice data of the quark propagator in the Landau gauge. As remarkable features, we find infrared vanishing and intermediate enhancement of the kernel function K(p^2) in the SD equation. We apply the LQCD-based SD equation to thermal QCD, and calculate the quark mass function at the finite temperature. We find chiral symmetry restoration at the critical temperature T_c \sim 100MeV.

hep-ph↗

arXiv2003

Thermal Effects on the Condensates of Chiral Order Parameters, the Quark Condensate and the Quark Gluon Mixed Condensate, and Chiral Restoration from Lattice QCD

We study the quark-gluon mixed condensate g<\bar{q} σG q> using the SU(3)c lattice QCD with the Kogut-Susskind fermion at the quenched level. We perform the first but accurate measurement of the thermal effects on the mixed condensate in lattice QCD at β= 6.0 with 16^3 * Nt (Nt=16,12,10,8,6,4), β= 6.1 with 20^3 * Nt (Nt=20,12,10,8,6), and β= 6.2 with 24^3 * Nt (Nt=24,16,12,10,8). At each temperature, the lattice calculation is carried out with high statistics using more than 100 gauge configurations. In particular, about 1000 gauge configurations are used near the critical temperature T_c. The chiral extrapolation is done from the lattice result with the current-quark mass of m_q = 21,36,52 MeV. While the thermal effects on both of the mixed condensate and the quark condensate are very weak except for the vicinity of T_c, both the condensates suddenly vanish around T_c \simeq 280 MeV, which indicates strong chiral restoration near T_c. To compare the thermal effects on the two condensates, we analyze the ratio m_0^2 = g<\bar{q} σG q>/ <\bar{q}q>, and find that m_0^2 is almost independent of the temperature, even in the very vicinity of T_c. This means that these two different condensates obey the same critical behavior, and this nontrivial similarity between them would impose constraints on the chiral structure of the QCD vacuum near T_c.

hep-lat↗