On the width of Σ - hyperons in nuclei and atoms

doi:10.1016/0375-9474(86)90570-1

Nuclear Physics A

Volume 450, 10 March 1986, Pages 353-370

https://doi.org/10.1016/0375-9474(86)90570-1 Get rights and content

Abstract

The width of a Σ-hyperon in symmetric nuclear matter is investigated by using a boson exchange model for the ΣN→ΛN amplitude. This amplitude is largely affected in nuclear matter yielding a strong density dependence for the width of Σ-hyperons in matter. At low densities this width is linear to the density, while, at higher density, it is strongly non-linear leading to saturation properties around ϱ≈0.7ϱ₀. This feature allows for a simultaneous description of the width of Σ atoms and the known Σ hypernuclear states. In addition, our model predicts a small width of about 5 MeV for hypernuclear ls-states.

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Some characteristic behaviors of hyperons in nuclei which have recently been revealed experimentally and theoretically are discussed with the emphasis on the repulsive part of the hyperon-nucleus interaction. The observed _Σ⁴He nucleus is a bound state with J^π= 0⁺ and $T ≈ 12$ . Its nucleus-Σ potential derived from a realistic ∑N interaction is characterized by inner repulsion and a strong Lane term, which play important roles in forming the Σ-hypernuclear bound state. In ²⁰⁸Pb a typical Coulomb-assisted bound state is expected, where Σ is trapped in the surface region by the nucleus-Σ potential with the aid of Coulomb and centrifugal interactions. In the double-strangeness (S=−2) sector, there is a possibility that the lightest double-Λ hypernucleus _ΛΛ⁴H is abundantly populated by stopping Ξ⁻ on ⁴He. Its formation branching amounts to about 15%. A stopped Ξ⁻ on ⁹Be will also produce efficiently a variety of double-Λ hyperfragments. Discrete spectra of weak-decay pions from the fragments will provide a means of mass spectroscopy of double-Λ hypernuclei. In the S=−2 five-body system an excited state _Ξ⁵H is predicted to appear with ‘strangeness halo’ and the ground state _ΛΛ⁵H with almost pure ΛΛ component.
A survey of hypernuclear physics
1995, Physics Reports
The present status of experimental and theoretical investigations of hypernuclear physics is reviewed. We emphasize the unique aspects of hadronic many-body systems containing nonzero strangeness and the opportunities that exist to exploit this physics. Our understanding of the elementary hyperon-nucleon interaction and its role in few-body systems is summarized. In particular, we examine the constraints that our knowledge of few-body systems place upon parameterizations of the realistic hyperon-nucleon force models. Also, issues involving the structure of p-shell hypernuclei as well as the weak decays of hypernuclei are summarized in some detail. Finally, the future prospects for strange-particle nuclear physics involving Λs, ∑s, and Ξs at existing facilities are discussed.
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Decay modes of sigma and lambda hypernuclei
1990, Physics Reports
A review is presented of the problem of Σ and Λ decay in nuclei. By means of handy, yet reliable approximations, an approach to Σ decay is developed which allows a clear insight into the different aspects of the problem. The role of nuclear polarization, due to the spin-isospin ΣN → ΛN interaction, in the reduction of the Σ widths is discussed. Many experimentally observable Σ hypernuclei are predicted by showing that their widths are narrower than the separation energies between levels. The mesonic and non-mesonic decays of Λ hypernuclei are analyzed. The non-mesonic width is fairly well reproduced by simplified models for the ΛN → NN weak transition, but quantities like the ratio of proton to neutron induced Λ decay are highly sensitive to the models used. On the other hand, the mesonic width is very sensitive to the pion-nucleus optical potential and can be used to discriminate between different potentials which are equally suited to describe low energy pion-nucleon scattering.
On the production and spectroscopy of Σ hypernuclei
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The production mechanisms and the spectroscopy of Σ hypernuclei are critically examined, with a view towards understanding the data on the interactions of the Σ with nucleons (N) and nuclei in a consistent framework. Data from Σ⁻ atoms and quasi-free Σ production in the (K⁻, π^±) reactions are used to derive information on the Σ-nucleus well depth. Starting from several parametrization of the ΣN effective interaction V _ΣN in the nucleus, we perform a series of shell model calculations of the energy spectrum of the p-shell Σ hypernuclei. The models for V_ΣN are motivated by successful one-boson exchange (OBE) descriptions of the free space ΣN scattering data. Certain levels of the Σ hypernucleaus reflect the coherent energy shifts due to the strong spin and isopin dependence of V_ΣN. We isolate optimum experimental cases for the study of such coherent effects as well as other components of V_ΣN, for instance the spin-orbit part. If the single-particle Σ-nucleus potential is indeed deep enough to support a spectrum of quasi-stable states, which is not clear experimentally, their width in some cases depends sensitively on the isospin purity and the degree of configuration mixing induced by V_ΣN. The production cross sections of selected Σ hypernuclear states in the (K⁻, π^±) reactions are estimated in distorted wave Born approximation (DWBA); these are also sensitive to deviations of the wave functions from pure isospin and/or j-j coupling limits. On the basis of our results, we suggest several experiments which may resolve some of the open questions concerning the ΣN and Σ-nucleus interactions.
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1988, Nuclear Physics, Section A
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^☆: Supported partly by Deutsche Forschungsgemeinschaft (DFG), Comision Aserora and the Joint Spanish-American Committee for Scientific and Technological Cooperation.

^∗: On leave of absence from University of Regensburg, West Germany.

^∗∗: On leave of absence from University of Valladolid, Spain.

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On the width of Σ - hyperons in nuclei and atoms☆

Abstract

Phys. Lett.

Phys. Lett.

Phys. Lett.

Phys. Lett.

Nucl. Phys.

Prog. Theor. Phys.

Phys. Rev. Lett.

Nucl. Phys.

Rev. Mod. Phys.

Nucl. Phys.

Nucl. Phys.

Nucl. Phys.

Phys. Rep.

Nucl. Phys.

Phys. Lett.

Phys. Lett.

Phys. Lett.

Z. Phys.