Effects of ρ exchange in coherent pion production

Abstract

New theoretical calculations have been done that compare {pi}- and {rho}-exchange effects in exclusive coherent production of pions for {sup 12}C+{sup 12}C{r_arrow}{sup 12}C+{sup 12}C+{pi}{sup 0} below and above the pion threshold at incident energies of 100, 250, and 400 MeV/nucleon. Besides the important enhancement effects to the pion distributions due to the constructive coherence of {Delta}-hole and particle-hole terms describing the coherent, intermediate nuclear states of both projectile and target, additional effects have been observed due to the {rho}-exchange transition interaction. Calculations of the pion angular distributions, which are sensitive to the {rho}-exchange effect, are done that compare the ({pi}+{rho})-exchange effects to the {pi}-exchange effect alone. It is shown that at extreme angles, where the projectile scatters forward ({Theta}{sub P}=0{degree}), and the target scatters backward ({Theta}{sub T}=180{degree}), and the pion scattering forward or backward ({theta}{sub {pi}}=0{degree} or 180{degree}), that the {rho} amplitude is zero under the Born approximation. The insensitivity of the pion-energy distributions to {rho} exchange is also explained. It is shown that the {rho}-exchange effect is a rather mild one which affects only the midregion of the pion-angular distributions for the particular physical channels and incident energies considered here. {copyright} {ital 1998} {ital The American Physical Society}