Pion physics in the Liège intranuclear cascade model

The implementation of {π} production in the Li\`ege intranuclear cascade model (INCL4) for spallation reactions is revisited to alleviate the overestimate of the {π} yield. Three modifications are proposed for this purpose: a better πN cross section at high energy, the introduction of a {π} average potential, and the modification of the average mass of the {Δ} resonance. The {π} potential is determined from a global fit of a set of data bearing on {π} production in proton-induced reactions, on π-nucleus and absorption cross sections, and on proton production in {π}-induced reactions. The resulting {π} 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 {π} production cross sections in proton-induced reactions are considerably improved. Predictions of the improved version for π-nucleus reaction and absorption cross sections and for proton, residue, and fission cross sections in {π}-induced reactions are also presented and shown to give reasonably good agreement. Neutron production and some aspects of fission in {π}-induced reactions are also investigated and reasonably well predicted. Effects on the modifications on observables, which are not directly linked with {π}'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 {π} production and the relative insentivity to the {π} potential in the nuclear interior are shown to be consistent with the fact that most {π}'s are not produced in early collisions. Importance of rescattering in {π} absorption on nuclei is also pointed out. A comparison is made with the so-called {Δ}-hole model. Residual discrepancies are identified and are interpreted as due to the lack of {π} interaction with two nucleons at low energy, to the neglect of quantum motion effects, and to a possible underestimate of rescattering.