arXiv subjects

Yoshihiko Kondo

Publications and source records attributed to Yoshihiko Kondo.

13 recordsLinked to original sources

arXiv2024

QCD sum rules for positive and negative parity heavy baryons at next-to-leading order in $α_s$-expansion

QCD sum rules for positive and negative parity heavy baryons in the heavy quark limit are formulated. We apply the method to $Λ$ and $Σ$ channels. We include the next-to-leading order corrections in $α_s$-expansion to dimension 0 and 3 terms in the operator product expansion. The corrections lead to the considerable reduction of the predicted masses and significantly improves the stability with respect to the Borel parameter, especially for negative parity states. It is also found that in the heavy quark limit chiral odd condensates do not contribute to the negative parity states.

hep-ph↗

arXiv2007

ppK- bound states from Skyrmions

The bound kaon approach to the strangeness in the Skyrme model is applied to investigating the possibility of deeply bound $ppK^-$ states. We describe the $ppK^-$ system as two-Skyrmion around which a kaon field fluctuates. Each Skyrmion is rotated in the space of SU(2) collective coordinate. The rotational motions are quantized to be projected onto the spin-singlet proton-proton state. We derive the equation of motion for the kaon in the background field of two Skyrmions at fixed positions. From the numerical solution of the equation of motion, it is found that the energy of $K^-$ can be considerably small, and that the distribution of $K^-$ shows molecular nature of the $ppK^-$ system. For this deep binding, the Wess-Zumino-Witten term plays an important role. The total energy of the $ppK^-$ system is estimated in the Born-Oppenheimer approximation. The binding energy of the $ppK^-$ state is $B.E.\simeq 126$ MeV. The mean square radius of the $pp$ subsystem is $\sqrt{< r_{pp}^2>}\simeq 1.6$ fm.

hep-ph↗

arXiv2007

Bound kaon approach for the ppK- system in the Skyrme model

The bound kaon approach to the strangeness in the Skyrme model is applied to exploring the possibility of deeply bound $ppK^-$ states. We derive the equation of motion for the kaon in the background of baryon number two Skyrmion expressed by the product ansatz. Collective coordinate quantization is performed to extract the spin-singlet proton-proton state. The numerical solution of the equation of motion shows that the kaon can acquire large binding energy for reasonable proton-proton relative distances. For this deep binding, the Wess-Zumino-Witten term plays an important role. The kaon tends to be centered between the protons.

hep-ph↗

arXiv2006

Positive and negative-parity flavor-octet baryons in coupled QCD sum rules

We apply the method of the QCD sum rule, in which positive- and negative-parity baryons couple with each other, to the flavor-octet hyperons and investigate the parity splittings. We also reexamine the nucleon in the method, which was studied in our previous paper, by carefully choosing the Borel weight. Both in the nucleon and hyperon channels the obtained sum rules turn out to have a very good Borel stability and also have a Borel window, an energy region in which the OPE converges and the pole contribution dominates over the continuum contribution. The predicted masses of the positive- and negative-parity baryons reproduce the experimental ones fairly well in the $Λ$ and $Σ$ channels, if we assign the $Λ(1670)$ and the $Σ(1620)$ to the parity partners of the $Λ$ and the $Σ$, respectively. This implies that the $Λ(1405)$ is not the party partner of the $Λ$ and may be a flavor-singlet or exotic state. In the $Ξ$ channel, the sum rule predicts the mass of the negative-parity state to be about 1.8 GeV, which leads to two possibilities; one is that the observed state with the closest mass, $Ξ(1690)$, is the parity partner and the other is that the parity partner is not yet found but exists around 1.8 GeV.

hep-ph↗

arXiv2005

Coupled QCD sum rules for positive and negative-parity nucleons

A new approach of the QCD sum rule is proposed in which positive and negative-parity baryons couple with each other. With positive and negative-parity states explicitly taken into account, sum rules are derived by means of the dispersion relation in energy. The method is applied to the nucleon channel and the parity splitting of the nucleon resonance states is studied. It is found that the obtained sum rules have a very good Borel stability. This suggests that the ansatz for the spectral function in the present sum rule approximates the physical spectrum better than the usual lowest pole plus continuum ansatz. The predicted masses of the positive and negative nucleons reproduce the experimental ones fairly well. Especially, the mass difference is extremely close to the experimental value.

hep-ph↗

arXiv2005

Spin-3/2 pentaquark in the QCD sum rule

We study $IJ^P=0{3/2}^\pm$ and $1{3/2}^\pm$ pentaquark states with $S=+1$ in the QCD sum rule approach. The QCD sum rule for positive parity states and that for negative parity are independently derived. The sum rule suggests that there exist the $0{3/2}^-$ and the $1{3/2}^-$ states. These states may be observed as extremely narrow peaks since they can be much below the $S$-wave threshold and since the only allowed decay channels are $NK$ in $D$-wave, whose centrifugal barriers are so large that the widths are strongly suppressed. The $0{3/2}^-$ state may be assigned to the observed $Θ^+(1540)$ and the $1{3/2}^-$ state can be a candidate for $Θ^{++}$.

hep-ph↗

arXiv2004

Theta++ from QCD sum rule

We study the pentaquark $uudd\bar s$ with $J=3/2$ and I=1 ($Θ^{++}$) in the QCD sum rule approach. We derive the QCD sum rules for positive and negative parity states of the pentaquark. The QCD sum rule predicts that there exists $Θ^{++}$ with negative parity and its mass is $1.5\sim1.6$ GeV. The negative parity $Θ^{++}$ can be extremely narrow, since it lies much below the $ΔK$ threshold and the decay into $KN$ state is strongly suppressed due to the $D$-wave centrifugal barrier. Also, the possibility of the existence of the $Θ^{++}$ with positive parity is not excluded. Although it nearly degenerates with the negative parity state, it may be broader than the negative parity state.

hep-ph↗

arXiv2004

Two-Hadron-Irreducible QCD Sum Rule for Pentaquark Baryon

We point out that naive pentaquark correlations function include two-hadron-reducible contributions, which are given by convolution of baryon and meson correlation functions and have nothing to do with pentaquark. We show that the two-hadron-reducible contributions are large in the operator product expansion of the correlation functions of three existing works on the pentaquark. Therefore, it is dangerous to draw a conclusion from the sum rules using naive pentaquark correlation functions with naive ansatz for the spectral function under the dispersion integral. Instead, we propose to use the two-hadron-irreducible correlation function, which is obtained by subtracting the two-hadron-reducible contribution from the naive correlation function. Taking one of the works as an example we demonstrate how drastically the results can change if we remove the two-hadron-reducible part from the naive correlation function. We obtain the result opposite to the original work for the parity of the pentaquark.

hep-ph↗

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

QCD sum rules for hyperon-nucleon interactions

We investigate the hyperon-nucleon interactions in the QCD sum rule starting from the nucleon matrix element of the hyperon correlation function. Through the dispersion relation, the correlation function in the operator product expansion (OPE) is related with its integral over the physical energy region. The dispersion integral around the hyperon-nucleon ($YN$) threshold is identified as a measure of the interaction strength in the $YN$ channel. The Wilson coefficients of the OPE for the hyperon correlation function are calculated. The obtained sum rules relate $YN$ interaction strengths to the nucleon matrix elements of the quark-gluon composite operators, which include strange quark operators as well as up and down quark operators. It is found that the $YN$ interaction strengths are smaller than the $NN$ interaction strength since the nucleon matrix elements of strange quark operators are smaller than those of up and down quark operators. Among $YN$ channels $ΛN$ channel has stronger interaction than $ΣN$ and $ΞN$ channels. Also found is that the interaction strength is greater in the $Σ^+N$ ($Ξ^0 N$) channel than in the $Σ^-N$ ($Ξ^-N$) channel since the nucleon matrix elements of up quark operators are greater than those of down quark operators. The spin-dependent part is much smaller than the spin-independent part in the $YN$ and $NN$ channels. The results of the sum rules are compared with those of the phenomenological meson-exchange models.

nucl-th↗

arXiv2002

The F/D Ratio and Meson-Baryon Couplings from QCD Sum Rules

Coupling constants of the pseudoscalar mesons to the octet baryons are computed in the QCD sum rule approach. The pi-NN, eta-NN, pi-XiXi, eta-XiXi, pi-SigmaSigma, eta-SigmaSigma as well as pi-Lambda-Sigma couplings are studied. Determining the pertinent Dirac structure in the correlation function, we analyze the couplings in the SU(3) limit. We find the F/D ratio to be \sim 0.6-0.8 that is consistent with the SU(6) value. We also estimate the SU(3) breaking effect using a projected correlation function method.

hep-ph↗

arXiv2002

A projected correlation function approach to the pi NN coupling constant in QCD sum rules

We propose a new approach to construct QCD sum rules for the pi NN coupling constant, g, starting from the vacuum-to-pion correlation function of the interpolating fields of two nucleons and taking its matrix element with respect to nucleon spinors. The new approach with the projected correlation function is advantageous because even in the chiral limit the dispersion integral can be parametrized with well-defined physical parameters. Another advantage of the new approach is that unwanted pole contribution is projected out. Calculating the Wilson coefficients of the operator product expansion of the correlation function up to O(M_B^{-4}) and O(m_pi) where M_B and m_pi are the Borel mass and the pion mass, respectively, we construct new QCD sum rules for the pi NN coupling constant from the projected correlation function with consistently including O(m_pi) corrections. By numerically analyzing the obtained four sum rules we identified the most prominent one. After roughly estimating errors we obtaind, g=10 +/- 3, as a result of this sum rule, which is in reasonable agreement with the empirical value. It is also found that the O(m_pi) correction is about 5%.

nucl-th↗

arXiv2002

Model-independent study of the QCD sum rule for the pi NN coupling constant

We reinvestigate the QCD sum rule for the pi NN coupling constant, g, starting from the vacuum-to-pion matrix element of the correlation function of the interpolating fields of two nucleons. We study in detail the physical content of the correlation function without referring to the effective theory. We consider the invariant correlation functions by splitting the correlation function into different Dirac structures. We show that the coefficients of the double-pole terms are proportional to g but that the coefficients of the single-pole terms are not determined by g. In the chiral limit the single-pole terms as well as the continuum terms are ill defined in the dispersion integral. Therefore, the use of naive QCD sum rules obtained from the invariant correlation functions is not justified. A possible procedure to avoid this difficulty is discussed.

nucl-th↗