arXiv subjects

Kiyotaka Shimizu

Publications and source records attributed to Kiyotaka Shimizu.

10 recordsLinked to original sources

arXiv2016

Radiative Decays of the $X(3872)$ in the Charmonium-Molecule Hybrid Picture

The X(3872) radiative decay is discussed by employing a two-meson multichannel model with the charmonium components, which explains many of the other observed features of X(3872). We have found that the ratio of the branching fractions of the X(3872) radiative decays, $R_γ=\text{Br}(X(3872)\rightarrowψ(2S)γ)$ / Br$(X(3872)\rightarrow J/ψγ)$, is 1.1 $\sim$ 3.4 if one assumes that the decay occurs only from the charmonium components. The invariant mass spectra of $cc^{bar}$($2P$)$\rightarrow J/ψγ$ and $cc^{bar}$($2P$)$\rightarrow ψ(2S) γ$ are also investigated. It is found that an enhancement appears at around $E\sim 3960$ MeV in the $ψ(2S) γ$ spectrum but not in the $J/ψγ$ spectrum. This corresponds to the missing $cc^{bar}$($2P$) pole which was predicted by the quark models but has not been observed due to the coupling to the $DD^{bar}$ states. We argue that the fluctuation seen in the experimental $ψ(2S) γ$ spectrum reported by LHCb may correspond to this $cc^{bar}$($2P$) pole.

hep-ph↗

arXiv2015

On the origin of the narrow peak and the isospin symmetry breaking of the $X$(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 $D^0{\bar D}^*{}^0$, $D^+D^{*-}$, $J/ψρ$, and $J/ψω$ channels. The energy-dependent decay widths of the $ρ$ and $ω$ 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 $J/ψρ$ or the $D{\bar D}^*$ and $J/ψω$ 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 $J/ψρ$ and $J/ψω$ peaks appear around the $D^0{\bar D}^*{}^0$ threshold. The size of the $J/ψπ^3$ peak we calculated is 1.29-2.38 times as large as that of the $J/ψπ^2$. 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.

hep-ph↗

arXiv2011

Charmonium and meson-molecule hybrid tetraquarks - Vector meson width and the isospin breaking in the X(3872) decay

In the X(3872) decay, both of the J/ψππ and J/ψπππ branching fractions are observed experimentally, and their sizes are comparable to each other. In order to clarify the mechanism to cause such a large isospin violation, we investigate X(3872) employing a model of coupled-channel two-meson scattering with a ccbar core. The two-meson states consist of D0Dbar*0, D+Dbar*-, J/ψρ, and J/ψω. The effects of the ρ and ω meson width are also taken into account. We calculate the transfer strength from the ccbar core to the final two-meson states. It is found that very narrow J/ψρ and J/ψω peaks appear very close to the D0Dbar*0 threshold for a wide range of variation in the parameter sets. The size of the J/ψρ peak is almost the same as that of J/ψω, which is consistent with the experiments. The large width of the ρ meson makes the originally small isospin violation by about five times larger.

hep-ph↗

arXiv2008

Lambda(1405) in a baryon-meson scattering with a bound state embedded in the continuum

We investigate Lambda(1405) as a resonance in a coupled channel baryon-meson scattering with a `bound state embedded in the continuum'(BSEC). This BSEC is introduced by hand, as a state not originated from simple baryon-meson systems. We assume it comes from the three-quark state. There appears a resonance in the Sigma pi scattering below the N Kbar threshold when the N Kbar channel is taken to be strongly attractive. It occurs without introducing a BSEC, just like the chiral unitary approach. When a BSEC is introduced, a resonance also appears with a weaker baryon-meson interaction. The corresponding peak also has a large width, and the N Kbar scattering length is reproduced well. The interaction whose channel dependence is the same as the one originated from the color-magnetic interaction, where no N Kbar attraction exists, also gives a broad peak like Lambda(1405) with help of a BSEC. It is found that the BSEC coupling which does not vanish at the zero momentum transfer give a larger width. In order to reproduce the observed N Kbar scattering length, introducing a BSEC seems preferable. In our calculation, a model gives an appropriate value when the BSEC contribution to the resonance is roughly half of that of the N Kbar in size.

hep-ph↗

arXiv2007

Lambda(1405) as a Resonance in the Baryon-Meson Scattering Coupled to the q^3 State in a Quark Model

In order to describe Lambda(1405) as a resonance in the baryon-meson scattering, we have investigated q^3-q qbar scattering system with the flavor-singlet q^3 (0s)^2(0p) state (the Lambda^1 pole). The scattering is treated by the quark cluster model (QCM). The Lambda^1 pole is treated as a bound state embedded in the continuum. We found that the peak appears below the N Kbar threshold in the spin one half, isospin 0 channel even if the mass of the Lambda^1 pole is above the threshold. This peak disappears when the coupling to the Lambda^1 pole is switched off. To use the observed hadron mass in the kinetic part of QCM is also found to be important to reproduce a peak just below the N Kbar threshold.

hep-ph↗

arXiv2004

Pentaquark as a NK^* bound state with TJ^P=0{3/2}^-

We have investigated negative-parity uudd\bar{s} pentaquarks by employing a quark model with the meson exchange and the effective gluon exchange as qq and q\bar{q} interactions. The system for the five quarks is dynamically solved; the effects of qq and q\bar{q} correlations on the wave function are taken into account. It is found that the masses of the pentaquarks are reasonably low. The lowest-mass state is found to be TJ^P=0{1/2}^-, and the next lowest one is 0{3/2}^-. The former is reported to have a large width. We argue the observed narrow peak corresponds to the latter state. It is still necessary to introduce an extra attraction to reduce the mass further by 140 -- 280 MeV to reproduce the observed Θ^+ mass. Since their level splitting is found to be less than 80 MeV, the lower level will not become a bound state below the NK threshold even after such an attraction is introduced. It is also found that the 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 two-body correlation seems important in the pentaquark systems.

hep-ph↗

arXiv2004

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.

hep-ph↗

arXiv2002

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.

hep-ph↗

arXiv2002

Quark Model and Equivalent Local Potential

In this paper, we investigate the short-range repulsion given by the quark cluster model employing an inverse scattering problem. We find that the local potential which reproduces the same phase shifts to those given by the quark cluster model has a strong repulsion at short distances in the NN1s0 channel. There, however, appears an attractive pocket at very short distances due to a rather weak repulsive behavior at very high energy region. This repulsion-attractive-pocket structure becomes more manifest in the channel which has an almost forbidden state, SigmaN(T=3/2)3s1. In order to see what kinds of effects are important to reproduce the short-range repulsion in the quark cluster model, we investigate the contribution coming from the one-gluon-exchange potential and the normalization separately. It is clarified that the gluon exchange constructs the short-range repulsion in the NN1s0 while the quark Pauli-blocking effect governs the feature of the repulsive behavior in the SigmaN(T=3/2)3s1 channel.

hep-ph↗

arXiv2000

H-dibaryon

The quark cluster model studies concerning the H-dibaryon are reviewed. The covered topics are the H-dibaryon itself, the interaction between a nucleon and an H-dibaryon, and the one between two H-dibaryons. A related study on the H-dibaryon in nuclear matter is also reviewed and its implication to double hypernuclei is discussed.

nucl-th↗