Halo structure of strange particles in nuclei

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 Jπ = 0+ and T ≅ 1/2. Its nucleus-Σ potential derived from a realistic ΣN 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. (author)