Narrow Pentaquark States in a Quark Model with Antisymmetrized Molecular Dynamics

The exotic baryon $\Theta^+(uudd\bar{s})$ is studied with microscopic calculations in a quark model by using a method of antisymmetrized molecular dynamics(AMD). We predict narrow states, $J^\pi=1/2^+(I=0)$, $J^\pi=3/2^+(I=0)$, and $J^\pi=3/2^-(I=1)$, which nearly degenerate in a low-energy region of the $uudd\bar{s}$ system. We discuss $NK$ decay widths and estimate them to be $\Gamma< 7$ for the $J^\pi=\{1/2^+,3/2^+\}$, and $\Gamma<1$ MeV for the $J^\pi=3/2^-$ state.Comment: Talk given at International Workshop on PENTAQUARK04, Spring-8, Hyogo, Japan, 20-23 Jul 200

Antisymmetrized molecular dynamics studies for exotic clustering phenomena in neutron-rich nuclei

We present a review of recent works on clustering phenomena in unstable nuclei studied by antisymmetrized molecular dynamics (AMD). The AMD studies in these decades have uncovered novel types of clustering phenomena brought about by the excess neutrons. Among them, this review focuses on the molecule-like structure of unstable nuclei. One of the earliest discussions on the clustering in unstable nuclei was made for neutron-rich Be and B isotopes. AMD calculations predicted that the ground state clustering is enhanced or reduced depending on the number of excess neutrons. Today, the experiments are confirming this prediction as the change of the proton radii. Behind this enhancement and reduction of the clustering, there are underlying shell effects called molecular- and atomic-orbits. These orbits form covalent and ionic bonding of the clusters analogous to the atomic molecules. It was found that this "molecular-orbit picture" reasonably explains the low-lying spectra of Be isotopes. The molecular-orbit picture is extended to other systems having parity asymmetric cluster cores and to the three cluster systems. O and Ne isotopes are the candidates of the former, while the $3\alpha$ linear chains in C isotopes are the latter. For both subjects, many intensive studies are now in progress. We also pay a special attention to the observables which are the fingerprint of the clustering. In particular, we focus on the monopole and dipole transitions which are recently regarded as good probe for the clustering. We discuss how they have and will reveal the exotic clustering.Comment: 96 pages, 44 figure

Structure of Excited States of 10Be studied with Antisymmetrized Molecular Dynamics

We study structure of excited states of 10Be with the method of variation after spin parity projection in the framework of antisymmetrized molecular dynamics. Present calculations describe many excited states and reproduce the experimental data of E2 and E1 transitions and the new data of the $\beta$ transition strength successfully. We make systematic discussions on the molecule-like structures of light unstable nuclei and the important role of the valence neutrons based on the results obtained with the framework which is free from such model assumptions as the existence of inert cores and clusters.Comment: 15 pages, RevTex, seven postscript figures (using epsf.sty

Superdeformation and clustering in 40^{40}Ca studied with Antisymmetrized Molecular Dynamics

Deformed states in $^{40}$Ca are investigated with a method of antisymmetrized molecular dynamics. Above the spherical ground state, rotational bands arise from a normal deformation and a superdeformation as well as an oblate deformation. The calculated energy spectra and $E2$ transition strengths in the superdeformed band reasonably agree to the experimental data of the superdeformed band starting from the $0^+_3$ state at 5.213 MeV. By the analysis of single-particle orbits, it is found that the superdeformed state has particle-hole nature of an $8p$-$8h$ configuration. One of new findings is parity asymmetric structure with $^{12}$C+$^{28}$Si-like clustering in the superdeformed band. We predict that $^{12}$C+$^{28}$Si molecular bands may be built above the superdeformed band due to the excitation of inter-cluster motion. They are considered to be higher nodal states of the superdeformed state. We also suggest negative-parity bands caused by the parity asymmetric deformation.Comment: 13 figures, submitted to Phys. Rev.

Axial vector tetraquark with S=+2

Possibility of an axial vector isoscalar tetraquark with $ud\bar{s}\bar{s}$ is discussed. If the pentaquark $\Theta^+(1540)$ has the $(qq)_{\bar{3}}(qq)_{\bar{3}}\bar{q}$ configuration, the isoscalar $ud\bar{s}\bar{s}$($\vartheta^+$-meson) state with $J^P=1^+$ is expected to exist in the mass region lower than or close to the mass of $\Theta^+(1540)$. Within a flux-tube quark model, a possible resonant state of $ud\bar{s}\bar{s}(J^{P}=1^{+})$ is suggested to appear around 1.4 GeV with the width ${\cal{O}}(20\sim 50)$ MeV. We propose that the $\vartheta^+$-meson is a good candidate for the tetraquark search, which would be observed in the $K^+K^+\pi^-$ decay channel.Comment: 20 pages, 5 figures, submitted to Phys.Rev.

Dipole resonances in light neutron-rich nuclei studied with time-dependent calculations of antisymmetrized molecular dynamics

In order to study isovector dipole response of neutron-rich nuclei, we have applied a time-dependent method of antisymmetrized molecular dynamics. The dipole resonances in Be, B and C isotopes have been investigated. In $^{10}$Be, $^{15}$B, $^{16}$C, collective modes of the vibration between a core and valence neutrons cause soft resonances at the excitation energy $E_x=10-15$ MeV below the giant dipole resonance(GDR). In $^{16}$C, we found that a remarkable peak at $E_x=14$ MeV corresponds to coherent motion of four valence neutrons against a $^{12}$C core, while the GDR arises from the core vibration in the $E_x >20$ MeV region. In $^{17}$B and $^{18}$C, the dipole strengths in the low energy region decline compared with those in $^{15}$B and $^{16}$C. We also discuss the energy weighted sum rule for the $E1$ transitions.Comment: 12 figures, submitted to Phys. Rev.

New effective nuclear forces with a finite-range three-body term and their application to AMD+GCM calculations

We propose new effective inter-nucleon forces with a finite-range three-body operator. The proposed forces are suitable for describing the nuclear structure properties over a wide mass number region, including the saturation point of nuclear matter. The forces are applied to microscopic calculations of $Z=N$ ($A\le 40$) nuclei and O isotopes with a method of antisymmetrized molecular dynamics. We present the characteristics of the forces and discuss the importance of the finite-range three-body term.Comment: 15 pages, 11 figures, submitted to Phys.Rev.

Structure of excited states of Be-11 studied with Antisymmetrized Molecular Dynamics

The structures of the ground and excited states of Be-11 were studied with a microscopic method of antisymmetrized molecular dynamics. The theoretical results reproduce the abnormal parity of the ground state and predict various kinds of excited states. We suggest a new negative-parity band with a well-developed clustering structure which reaches high-spin states. Focusing on a $2\alpha$ clustering structure, we investigated structure of the ground and excited states. We point out that molecular orbits play important roles for the intruder ground state and the low-lying $2\hbar \omega$ states. The features of the breaking of $\alpha$ clusters were also studied with the help of data for Gamow-Teller transitions.Comment: 24 pages, 7 figures, to be submitted to Phys.Rev.