Spin observables for pion production from pd collisions

We have calculated the proton analyzing power A_{y0} of the pion-production reaction from pd collisions for one energy close to threshold and for another in the region of the $\Delta$-resonance. A fair reproduction of the experimental data could be obtained in both cases with a model which includes isoscalar and isovector $\pi N$ rescatterings in s waves, as well as the p-wave rescattering mechanisms mediated by the $\pi NN$ and $\pi N \Delta$ vertices. For the analyzing power at threshold we found that the initial-state interaction (ISI) is also quite important.Comment: Talk at XVIth International Conference on Few-Body Problems in Physics, Taipei, Taiwan, March 6-10 2000. 3 pages, latex, 2 figures -- To appear in Nuclear Physics A -- fleqn.sty and espcrc1.sty require

Three-nucleon portrait with pion

We report on recent results obtained by the above collaboration on the collision processes involving three nucleons, where we pay particular attention on the dynamical role of the pion. After discussing the case at intermediate energies, where real pions can be produced and detected, we have considered the case at lower energies, where the pions being exchanged are virtual. The study has revealed the presence of some new pion-exchange mechanisms, which leads to a new three-nucleon force of tensor structure. Recently, the effect of this tensor three-nucleon force to the spin observables for neutron-deuteron scattering at low energy has been analyzed, and will be briefly reviewed.Comment: To appear in the Proceedings of 8th Convegno su Problemi di Fisica Nucleare Teorica (Cortona 2000), Cortona, Italy, 18-21 Oct 2000. 8 pages, uses ws-p8-50x6-00.cl

Particle-unstable light nuclei with a Sturmian approach that preserves the Pauli principle

Sturmian theory for nucleon-nucleus scattering is discussed in the presence of all the phenomenological ingredients necessary for the description of weakly-bound (or particle-unstable) light nuclear systems. Currently, we use a macroscopic potential model of collective nature. The analysis shows that the couplings to low-energy collective-core excitations are fundamental but they are physically meaningful only if the constraints introduced by the Pauli principle are taken into account. The formalism leads one to discuss a new concept, Pauli hindrance, which appears to be important to understand the structure of weakly-bound and unbound systems.Comment: 5 pages, 2 figures, 1 table, contribution to proceedings of "18th International IUPAP Conference on Few-Body Problems in Physics," Santos, Brazil, August 21-26, 200

Complex Conjugate Pairs in Stationary Sturmians

Sturmian eigenstates specified by stationary scattering boundary conditions are particularly useful in contexts such as forming simple separable two nucleon t matrices, and are determined via solution of generalised eigenvalue equation using real and symmetric matrices. In general, the spectrum of such an equation may contain complex eigenvalues. But to each complex eigenvalue there is a corresponding conjugate partner. In studies using realistic nucleon--nucleon potentials, and in certain positive energy intervals, these complex conjugated pairs indeed appear in the Sturmian spectrum. However, as we demonstrate herein, it is possible to recombine the complex conjugate pairs and corresponding states into a new, sign--definite pair of real quantities with which to effect separable expansions of the (real) nucleon--nucleon reactance matrices.Comment: (REVTEX) 8 Pages, Padova DFPD 93/TH/78 and University of Melbourn