Two-nucleon processes in near-threshold (p vector,π-) negative pion) reactions in the carbon and calcium regions

With the increasing theoretical interest in two-nuclear models (TNM) of pion production, a set of experimental measurements were proposed to show the dominance of two-nucleon processes over other processes in pion production and to show that, in spite of detailed theoretical complexities of TNM's, simple two-nucleon signatures exist. By choosing nuclear systems in which the nuclear structure restricts the target neutrons which might contribute to selected transitions, we are able to make simple predictions regarding cross section and analyzing power behavior for (p vector,π-) reactions induced on carbon isotopes. The measurements confirm the predictions of a j-dependent signature of the sign of the vector analyzing power for transitions involving target neutrons from j = l + 1/2 and J = l - 1/2 shell model orbitals, and of a simple scaling of the ground-state cross section with neutron excess in the target carbon isotope. All of the measurements presented provide evidence for the dominance of two-nucleon processes in pion production, and also confirm our original hypothesis that the simple aspects of pion production in nuclei may be more clearly manifested in (p,π-) than in (p,π+) reactions