Systematics of proton-induced pion production at subthreshold energies

A compilation of available data on proton-induced pion production is presented for the subthreshold region E(proton)less than or equal to275 MeV. In this energy range the transition from coherent to quasifree pion production occurs and can be studied by observing the dependence on nuclear structure and proton beam energy. Large variations in the pion cross sections were found. Within a single data set the variations can be accounted for on the basis of energy (phase space) and isospin dependence. A scaling procedure was derived to relate the production probabilities of pi(+), pi(0) and pi(-) as functions of target mass and proton beam energy. A comparison of data sets obtained by different authors shows inconsistencies that appear to a certain extent to be of experimental origin.