No abstract

The exchange of two correlated pions or kaons provides the main part of the intermediate-range attraction between two baryons. Here, a dynamical model for correlated two-pion and two-kaon exchange in the baryon-baryon interaction is presented, both in the scalar-isoscalar (σ) and the vector-isovector (ρ) channel. The contribution of correlated ππ and KK exchange is derived from the amplitudes for the transition of a baryon-antibaryon state (BB ′ ) to a ππ or KK state in the pseudophysical region by applying dispersion theory and unitarity. For the BB ′ → ππ, KK amplitudes a microscopic model is constructed, which is based on the hadron-exchange picture. The Born terms include contributions from baryonexchange as well as ρ-pole diagrams. The correlations between the two pseudoscalar mesons are taken into account by means of ππ-KK amplitudes derived likewise from a meson-exchange model, which is in line with the empirical ππ data. The parameters of the BB ′ → ππ, KK model, which are related to each other by the assumption of SU (3) symmetry, are determined by the adjustment to the quasiempirical N N → ππ amplitudes in the pseudophysical region. It is found that correlated KK exchange plays an important role in the σ-channel for baryon-baryon states with non-vanishing strangeness. The strength of correlated ππ plus KK exchange in the σ-channel decreases with the strangeness of the baryon-baryon system becoming more negative. Due to the admixture of baryon-exchange processes to the SU (3)-symmetric ρ-pole contributions the results for correlated ππ-exchange in the vector-isovector channel deviate from what is expected in the naive SU (3) picture for genuine ρ-exchange. In present models of the hyperon-nucleon interaction contributions of correlated ππ and KK exchange are parametrized for simplicity by single σ and ρ exchange. Shortcomings of this effective description, e.g. the missing long-range contributions, are pointed out by comparison with the dispersiontheoretic results.

We perform a momentum-space calculation of the reaction pp ! pp 0 near threshold. Direct production and pion rescattering are taken into account. For the latter a T-matrix obtained from a microscopic model of the N interaction is employed.The e ects of some approximations commonly used in studies of pp ! pp 0 are discussed. We nd that those approximations overestimate the cross section and also modify its energy dependence. We obtain a signi cant contribution from rescattering, but not nearly enough to describe the data.

No abstract

No abstract

We have performed accurate G-matrix calculations for the hypernucleus A 6 0 using the one-and twoboson-exchange Julich hyperon-nucleon potentials. A finite-nuclei Pauli operator defined in terms of oscillator wave functions is employed, and treated essentially exactly in solving the coupled A7V and XA (/-matrix equation. Numerical stability concerning the choice of the fc-space mesh points for discretization and the accuracy of a finite-boundary approximation are examined. Our calculated spectrum for A 6 0 is in good agreement with experiments.

No abstract

No abstract