Abstract

Angular distributions for the ^ {12}C(pi^{+/- }, pi^{+/-'} p) and ^{208}Pb( pi^{+/-}, pi^ {+/-'} p or n) reactions near the giant resonance region have been measured at T _{pi} = 180 MeV, and found different between pi^{+ } and pi^{-} data. This observation is interpreted as evidence for different excitation mechanisms dominating the pi^{-}-nucleus and pi^{+}-nucleus interactions in the giant resonance region of these targets. A comparison with the single-nucleon knock-out distorted-wave impulse approximation calculations shows, even though these calculations underestimate (pi^{+/-}, pi^{+/-'} N) data for both targets, the dominance of direct process (due to quasi-free scattering) for (pi^ {+}, pi^{+'} p) or (pi^{-}, pi^{-'} n) in contrast to (pi^{-}, pi^ {-'} p) or (pi ^{+}, pi{+'} n). In the (pi^{+}, pi{+'} p) (or ( pi^{-}, pi^{-' } n)) reaction proton-proton hole states (neutron -neutron hole states) are excited directly and appear to have a large probability for direct decay with escape width, whereas in (pi^{-}, pi ^{-'}p) (or ( pi^{+}, pi{+' } n)) the preferentially excited neutron-neutron hole doorway states (proton-proton hole doorway states) couple to resonance states and decay with spreading width. This interpretation led us to suggest that the ratio of cross -sections for inelastic scattering to the giant resonance region should be written in terms of an incoherent sum of cross-sections to neutron and proton doorway states. In a heavy nucleus such as ^{208} Pb, neutron and proton doorway states contribute incoherently because the different decay processes do not populate the same final states of the residual nucleus. A reanalysis of the previous ^{208} Pb(pi,pi^{' }) data extracted for the giant quadrupole resonance in ^{208}Pb, assuming an incoherent addition of cross-sections to neutron and proton doorway states gives R = M_ n /M_ p = 2.1 compared to 3.8 from the previous analysis using the coherent addition. This new result is in better agreement with the hydrodynamical model prediction R = 1.54.