Spin - Isospin Nuclear Responses With Hadronic Probes.

We develop a random-phase approximation theory of the spin-isospin nuclear surface response, suitable for the interpretation of recent experiments of polarized (p,p’) and charge exchange (3He,t) reactions. The present approach extends our previous treatment of the volume responses, which was quite successful in accounting for inelastic electron scattering data. Its crucial ingredient is a vertex function which localizes the probe-target interaction on the surface region of the nucleus. Our results compare rather successfully with both the (p,p’) and (3He,t) experiments, although some difficulties remain to be overcome. Indeed, we predict a ratio between the spin-longitudinal and the spin-transverse nuclear responses still somewhat larger than the measured value. Concerning the charge-exchange (3He,t) reaction, we qualitatively account for the progressive softening of the peak of the cross section at large momentum transfer, thus confirming the role of the pion in nuclear structure. We tentatively ascribe the remaining discrepancies with the (3He,t) data to relativistic kinematics effects.