Production mechanisms and polarization observables for p + d -> He(3) + pi(o) near threshold

Abstract

Pion production at threshold from nucleon-deuteron collisions is considered, with reference to the outgoing channel where the three-nucleon system is bound. The available experimental data are compared with calculations using accurate nuclear wavefunctions coming from rigorous solutions of the three-nucleon quantum mechanical equations. The dominant contributions for pion production are obtained through matrix elements involving pion-nucleon rescattering mechanisms in S- and P-waves. S-wave rescattering includes also an isoscalar contribution which is enhanced because of the off-shell effects. P-wave rescattering includes also diagrams involving explicitly the Delta degrees of freedom. It is found that the pion-nucleon S-wave off-shell effects in the isospin-even channel are of considerable importance. Initial-state interactions between the proton and the deuteron have in general minor effects on the spin-averaged and spin-dependent observables, except when the polarization observable involves interference terms amongst the various helicity amplitudes. Keywords: Polarization phenomena. Pion production. Few-nucleon systems.