The ccbar Pentaquarks by a Quark Model

Recent LHCb experiments have shown us that there are two resonances in the $J/\psi p$ channel in the $\Lambda_b$ decay, whose spin and parity are most probably (3/2$^-$ 5/2$^+$). In this work, we investigate the $I(J^P)=1/2(1/2^-)$, $1/2(3/2^-)$, and $1/2(5/2^-)$ $uudc{\bar c}$ pentaquark states by employing the quark cluster model. It is found that the color-octet isospin-1/2 spin-3/2 $uud$ configuration gives an attraction to such five-quark systems. This configuration together with the color-octet $c{\bar c}$ pair gives structures around the $\Sigma_c^{(*)}{\bar D}{}^{(*)}$ thresholds: one bound state, two resonances, and one large cusp are found in the $uudc{\bar c}$ negative parity channels. We argue that these resonances and cusp may correspond to, or combine to form, the negative parity pentaquark peak observed by LHCb.Comment: 4 pages, 4 figures, Proceedings of the 14th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon (MENU2016

Pentaquark with diquark correlations in a quark model

We have investigated uudd\sbar pentaquarks by employing quark models with the meson exchange and the effective gluon exchange as qq and q\qbar interactions. The system for five quarks is dynamically solved; two quarks are allowed to have a diquark-like qq correlation. It is found that the lowest mass of the pentaquark is about 1947 -- 2144 MeV. There are parameter sets where the mass of the lowest positive-parity state become lower than that of the negative-parity states. Which parity corresponds to the observed peak is still an open question. Relative distance of two quarks with the attractive interaction is found to be by about 1.2 -- 1.3 times closer than that of the repulsive one. The diquark-like quark correlation seems to play an important role in the pentaquark systems.Comment: 8 pages, 1 figure, Talk given at International Workshop PENTAQUARK04 at SPring-8, Japan, July 20-23, 200

Effects of Instantons on the YN Interaction

We investigate the symmetric and anti-symmetric spin-orbit forces (SLS and ALS) of the effective $\Lambda$N interaction derived from a quark cluster model with the instanton-induced interaction (\III), which can reproduce the observed YN cross sections as well as the observed NN scattering data. It is found that coupling to the $\Sigma$N channel enhances $\Lambda$N ALS, and therefore that the cancellation between SLS and ALS in the $\Lambda$N channel becomes more complete. This may be one of the major reasons why the single-particle spin-orbit force of $\Lambda$ in nuclei is weak.Comment: 3 pages, 2 figures, FewBody XV

On the origin of the narrow peak and the isospin symmetry breaking of the XX(3872)

The $X$(3872) formation and decay processes in the $B$-decay are investigated by a $c\bar c$-two-meson hybrid model. The two-meson state consists of the $D^0{\bar D}^*{}^0$, $D^+D^{*-}$, $J/\psi\rho$, and $J/\psi\omega$ channels. The energy-dependent decay widths of the $\rho$ and $\omega$ mesons are introduced. The $D$-${\bar D}^*$ interaction is taken to be consistent with a lack of the $B{\bar B}^*$ bound state. The coupling between the $D{\bar D}^*$ and $J/\psi\rho$ or the $D{\bar D}^*$ and $J/\psi\omega$ channels is obtained from a quark model. The $c{\bar c}$-$D{\bar D}^*$ coupling is taken as a parameter to fit the $X$(3872) mass. The spectrum is calculated up to 4 GeV. It is found that very narrow $J/\psi\rho$ and $J/\psi\omega$ peaks appear around the $D^0{\bar D}^*{}^0$ threshold. The size of the $J/\psi\pi^3$ peak we calculated is 1.29-2.38 times as large as that of the $J/\psi\pi^2$. The isospin symmetry breaking in the present model comes from the mass difference of the charged and neutral $D$ and $D^*$ mesons, which gives a sufficiently large isospin mixing to explain the experiments. It is also found that values of the ratios of the transfer strengths can give the information on the $X$(3872) mass or the size of the $c{\bar c}$-$D{\bar D}^*$ coupling.Comment: 33 pages, 12 figure

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.Comment: 4 pages, 2 figures, Talk given at International Workshop PENTAQUARK04 at SPring-8, Japan, July 20-23, 200

Pauli-blocking Effect in a Quark Model

Pauli-Blocking effect on the kinetic term is investigated by employing the quark cluster model. The effect can be understood by the change of the degrees of the mixing between the incoming wave and the 0$\ell$ state of the inter-cluster wave function, which can be expressed by a potential which is highly nonlocal. We look into the properties of this effect by comparing equivalent local potentials. In the channel where the Pauli-blocking effect is small, the on-shell equivalent local potential simulates the nonlocal potential well even for the off-shell behavior. On the other hand, the off-shell behavior is very different from the original one where the effect is large. This off-shell behavior, however, can well be simulated by considering the nonlocal matrix elements only between the $0s$ state and the other states. The energy dependent potentials are also constructed and found to be helpful to understand the energy dependence of the effect.Comment: 14 pages, 12 figure