Search

Pionic decays of light-to-heavy Λ-hypernuclei are summarized including a review and the updated estimates which are based on the pion distorted waves generated by a sophisticated full-form optical potential. For heavy hypernuclei, the pion distortion effects yield a remarkable enhancement as large as two orders of magnitudes in comparison with the estimate without any distortion, where the Coulomb distortion is found to play an important role. The systematic calculations for all the sd-shell hypernuclei (including the updated p-shell results) predict that the totalπ0 and π− decay rates vary strikingly depending on the structures of individual hypernucleus and final nucleus. It is also shown by presenting the excitation functions that the partial pionic decay is selective enough to be utilized as a hypernuclear spectroscopic tool.

The G-matrices are derived on the basis of meson-theoretical hyperon-nucleon and hyperon-hyperon potentials from the Nijmegen group (model-D, model-F, soft-core model) and the Jülich group (version-A, version-B). In order to make easy applications to hypernuclei, they are parametrized to express in terms of three-range Gaussian radial functions with the density dependence being in the quadratic form of the Fermi momentum. Through the structure calculations of Λ-, Σ-, ΛΛ- and Ξ-hypernuclei, properties of these G-matrix interactions have been investigated in the ΛN, ΣN, ΛΛ and ΞN channels, respectively.

A scenario is given for the formation of Ξ− states and the transitions to states with double-Λ. First, the production cross sections of Ξ hypernuclear states by (K−, K+) reactions on typical two targets (12C and 16O) are calculated within the framework of the distorted-wave impulse approximation. Next, the transition rates from Ξ− hypernuclear states to possible double-Λ states are evaluated, which are closely related to single- and double-Λ emission after Ξ−p→ΛΛ conversion in nuclei. The partial conversion widths for the atomic Ξ− absorption by 12C and 16O are also estimated in the similar way, discussing various possible final states. In order to extract an information on the Ξ-N interaction, the cross sections and Ξ state widths are given as a function of the Woods-Saxon well depth for the Ξ particle. The result is discussed in anticipation of observations in future experiments.

On the basis of the phenomenological parametrization for the elementary γ+p→Λ+K+ process amplitudes, the cross sections and polarizations have been estimated for the (γ, K+) reactions on typical p-shell targets. Characteristic aspects of the reaction are discussed in comparison with the π- and K-induced reactions.

The structure of light hypernuclei with strangeness S = −1 is investigated with the three- and four-body cluster model and the Gaussian Expansion method (GEM). Interesting phenomena such as shrinkage effect of the core nucleus and the halo and skin structure due to the glue-like role of Λ particle are demonstrated from the study of structure of Λ6He, Λ7Li and Λ13C. Precise three- and four-body cluster calculation of Λ7He, Λ7Li, Λ7Be, Λ9Be and Λ13C provide us with information about ΛN interaction such as spin-spin, spin-orbit, charge symmetry breaking term, etc., by comparing with the recent experimental data. Precise four-body calculation taking NNNΛ and NNNΣ for Λ4H and Λ4He are performed and the role of Λ-Σ conversion and the size of the virtual Σ component in Λ4H and Λ4He are discussed.

Recently hypernuclear electroproduction experiments have been successfully carried out at the Thomas Jefferson Laboratory, providing high-resolution spectroscopic data. This article discusses several facets inherent to this reaction process: First, numerous theoretical attempts in describing the elementary kaon photoproduction are summarized so as to choose appropriate elementary amplitudes to be used for hypernuclear production estimates. The reaction process is characterized by both the sizable momentum transfer and the spin-flip dominace involved in the transition operator. The DWIA theoretical excitation spectra of producing p-shell hypernucli are discussed together with the experimental ones obtained recently. Encouraged with the successful theory-experiment comparison, the calculational framework has been applied beyond p-shell to typical medium-heavy hypernuclear production. The hypernuclear photo-production spectra predicted with the 19F, 28Si, and 40Ca targets suggest that the unique characters of the electroproduction are more clearly appeared in the medium-mass region, providing interesting opportunities of spectroscopic study beyond p-shell.

The recent theoretical studies of Λ-hypernuclear weak decays of the nonmesonic and π-mesonic ones are developed with the aim to disclose the link between the experimental decay observables and the underlying basic weak decay interactions and the weak decay mechanisms. The expressions of the nonmesonic decay rates Γnm and the decay asymmetry parameter α1 of protons from the polarized hypernuclei are presented in the shell model framework. We then introduce the meson theoretical ΛN → NN interactions which include the one-meson exchanges, the correlated-2π exchanges, and the chiral-pair-meson exchanges. The features of meson exchange potentials and their roles on the nonmesonic decays are discussed. With the adoption of the π + 2π/ρ + 2π/σ + ω + K + ρπ/a1 + σπ/a1 exchange potentials, we have carried out the systematic calculations of the nonmesonic decay observables for light-to-heavy hypernuclei. The present model can account for the available experimental data of the decay rates, Γn/Γp ratios, and the intrinsic asymmetry parameters αΛ (αΛ is related to α1) of emitted protons well and consistently within the error bars. The hypernuclear lifetimes are evaluated by converting the total weak decay rates Γtot = Γπ+Γnm to τ , which exhibit saturation property for the hypernuclear mass A ≥ 30 and agree grossly well with experimental data for the mass range from light to heavy hypernuclei except for the very light ones. Future extensions of the model and the remaining problems are also mentioned. The π-mesonic weak processes are briefly surveyed, and the calculations and predictions are compared and confirmed by the recent high precision FINUDA π-mesonic decay data. This shows that the theoretical basis seems to be firmly grounded.

For the last few decades the (π+, K+) reaction has played an important role in heavy hypernuclear spectroscopy where the Λ spin-orbit splitting should be one of the most interesting subjects. In this article, first, various stages of calculational frameworks for hypernuclear production cross sections are discussed. Then, focusing on the 89Y(π+, K+)89ΛY experimental data, new DWIA analysis has been carried out by taking full account of the core nuclear excitation effects. As a result we found that one of the doublet peaks comes from the nuclear core excitation and that the Λ spin-orbit splitting is small in consistent with the finding in the p-shell hypernuclei.

The structure of light hypernuclei with strangeness S = −2 is investigated with the three-, four-, and five-body cluster model and the Gaussian Expansion method (GEM). Energy levels of the double-Λ hypernuclei ΛΛΛ7He, ΛΛΛ7Li, ΛΛΛ8Li, ΛΛΛ9Li, ΛΛΛ9Be, and ΛΛΛ10Be are predicted with the framework of αxΛΛ four-body model, where x = n, p, d, t, 3He, and α, respectively. Also, energy levels of the double Λ hypernucleus, ΛΛΛ11Be are calculated within the framework of a ααnΛΛ five-body model. The Demachi-Yanagi event, identified as ΛΛΛ10Be, is interpreted as an observation of its 2+ state excited state. The Hida event, recently observed at KEK-E373 experiment, is interpreted as an observation of the ground state of the ΛΛΛ11Be. Detailed structure calculations in Ξ12_Be, Ξ5_H, Ξ9_Li, Ξ7_H and Ξ10_Li are performed within the framework of the microscopic two-, three- and four-body cluster models. Ξ7_H and Ξ10_Li are predicted to have bound states.

The properties of the new {itYN} · {itYY} interaction model ESC08 are studied on the basis of the {itG}-matrix theory. G-matrix calculations in nuclear matter are performed in the cases of Λ{itN}-, Σ{itN}-, Ξ{itN}- and ΛΛ-starting channels. In the Λ{itN} case, the spin-spin and spin-orbit terms are shown to be of reasonable strengths. The folding-model analyses are done extensively for observed spectra of Λ hypernuclei. The experimental energy spectra are nicely reproduced. The shell-model analyses for typical Λ hypernuclei are performed with the {itG}-matrix interactions. In the Σ{itN} case, the repulsive values of {itU}{inΣ} are obtained owing to the effect of the quark Pauli-forbidden states which are taken into account phenomenologically in ESC08. In the Ξ{itN} case, the moderately attractive values of {itU}{inΞ} are obtained mainly due to the strongly attractive contributions in {su3}{itS}{in1}{itT} = 1 Ξ{itN}-ΛΣ-ΣΣ coupled states, which leads to extensive existence of Ξ hypernuclei. In the ΛΛ case, the {itB}{inΛΛ} values derived from ESC08 are found to be consistent with the experimental values.

The present status of hypernuclear physics is outlined and the future prospect of this field is described on the basis of the works presented in this volume.

Detailed investigations have been made on the applications of the (fp)n shell model in describing the light fp-shell nuclei with A ≦44. Many levels of these nuclei are shown to be well accounted for by the shell model. The roles of core-polarization renormalization and the quadrupole-quadrupole dominant character in the realistic effective interaction are discussed. Characteristics of the wavefunctions of 44Ti have been studied in detail from several viewpoints. The shell and α-cluster aspects observed in 44Ti can be well described by the extended model proposed here in which the (fp)4 shell and 40Ca + α-cluster configurations are incorporated. It is clarified that the α-like four-body correlation has important effects on reproducing the low-spin yrast rotational feature and the α-transfer spectroscopic factors of the ground and high-lying molecule-like bands in 44Ti. Another extension of the model space has been tried for 42Sc and 44Ti by including the core-excited configurations.