Pion physics in the Liege intranuclear cascade model

The implementation of pi production in the Liege intranuclear cascade model (INCL4) for spallation reactions is revisited to alleviate the overestimate of the pi yield. Three modifications are proposed for this purpose: a better pi N cross section at high energy, the introduction of a pi average potential, and the modification of the average mass of the Delta resonance. The pi potential is determined from a global fit of a set of data bearing on pi production in proton-induced reactions, on pi-nucleus and absorption cross sections, and on proton production in pi-induced reactions. The resulting pi potential is poorly determined in the nuclear interior and agrees with the phenomenological optical-model potentials in the surface region. With these modifications, the predictions of the INCL4 model concerning pi production cross sections in proton-induced reactions are considerably improved. Predictions of the improved version for pi-nucleus reaction and absorption cross sections and for proton, residue, and fission cross sections in pi-induced reactions are also presented and shown to give reasonably good agreement. Neutron production and some aspects of fission in pi-induced reactions are also investigated and reasonably well predicted. Effects on the modifications on observables, which are not directly linked with pi's, such as the neutron yield and the residue mass and charge spectra in proton-induced reactions are also investigated and shown to improve the description of these observable quantities. Several results on pi production and the relative insentivity to the pi potential in the nuclear interior are shown to be consistent with the fact that most pi's are not produced in early collisions. Importance of rescattering in pi absorption on nuclei is also pointed out. A comparison is made with the so-called Delta-hole model. Residual discrepancies are identified and are interpreted as due to the lack of pi interaction with two nucleons at low energy, to the neglect of quantum motion effects, and to a possible underestimate of rescattering.