On the migdal-watson approach to FSI effects in meson production in NN collisions
The influence of the nucleon-nucleon final-state interaction (FSI) on properties of the meson-production amplitude near threshold is discussed. For nucleon-nucleon interaction, a simple Yamaguchi potential and realistic potential models are considered. It is shown that FSI effects cannot be factorized from the production amplitude. The absolute magnitude of FSI effects depends on the momentum transfer (or on the mass of the produced meson) and hence is not universal. Only in the case of the production of rather heavy mesons like η′ or φ FSI do effects become universal. The Jost function approach to FSI effects is critically examined.
Many-body Systems Interacting via a Two-body Random Ensemble (I): Angular Momentum distribution in the ground states
In this paper, we discuss the angular momentum distribution in the ground states of many-body systems interacting via a two-body random ensemble. Beginning with a few simple examples, a simple approach to predict P(I)'s, angular momenta I ground state (g.s.) probabilities, of a few solvable cases, such as fermions in a small single-j shell and d boson systems, is given. This method is generalized to predict P(I)'s of more complicated cases, such as even or odd number of fermions in a large single-j shell or a many-j shell, d-boson, sd-boson or sdg-boson systems, etc. By this method we are able to tell which interactions are essential to produce a sizable P(I) in a many-body system. The g.s. probability of maximum angular momentum $I_{max}$ is discussed. An argument on the microscopic foundation of our approach, and certain matrix elements which are useful to understand the observed regularities, are also given or addressed in detail. The low seniority chain of 0 g.s. by using the same set of two-body interactions is confirmed but it is noted that contribution to the total 0 g.s. probability beyond this chain may be more important for even fermions in a single-j shell. Preliminary results by taking a displaced two-body random ensemble are presented for the I g.s. probabilities.
eta N S-wave scattering length in a three-coupled-channel, multiresonance, unitary model.
The {ital S}-wave scattering length for the {eta}{ital N} elastic scattering is extracted from the {ital S}-wave {ital T} matrix in a three-coupled-channel, multiresonance unitary model. Results are compared with values already reported in the literature which are obtained on the basis of simpler models, usually single-resonance--no background. The dispersion among the previously published values of the real part of the {ital S}-wave scattering length is observed. We show that the {eta}{ital N} scattering length value obtained in single-resonance--no background models significantly increases if a background term is added in a unitary way. We question the reliability of previously reported values based only on the single-resonance--no background models, and demonstrate that the value of the {eta}{ital N} {ital S}-wave scattering length obtained in this publication is much more realistic because of the multiresonance and unitary approach.