Excitation Functions for Simple Nuclear Reactions ofπ−Mesons withAr40

Abstract

Cross sections have been measured for the reactions ${\mathrm{Ar}}^{40}({\pi }^{-}, {\pi }^{-}p){\mathrm{Cl}}^{39}$ and ${\mathrm{Ar}}^{40}({\pi }^{-}, {\pi }^{-}pn){\mathrm{Cl}}^{38}$ in the energy range 500-1100 MeV. The excitation function of the former reaction has peaks at about 600 and 900 MeV pion kinetic energy, corresponding to known resonances in the ${\pi }^{-}-p$ scattering cross section. This implies that a quasifree pion-proton collision occurs as the initial step of this simple reaction. The (${\pi }^{-}, {\pi }^{-}\mathrm{pn}$) excitation function, in contrast, is essentially independent of energy in this energy range, indicating a mechanism or combination of mechanisms sufficiently complex to average out the structure in the ${\pi }^{-}-p$ and ${\pi }^{-}-n$ cross sections. The effect of the proton momentum distribution on the ${\pi }^{-}-p$ resonances is calculated, and by comparison with the (${\pi }^{-}, {\pi }^{-}p$) cross section a momentum distribution is deduced which is in agreement with the results of ($p, 2p$) quasifree scattering experiments. Cross sections for the reactions ${\mathrm{Ar}}^{40}({\pi }^{-}, {\pi }^{-}2p){\mathrm{S}}^{38}$, ${\mathrm{Ar}}^{40}(p, 2p){\mathrm{Cl}}^{39}$, and ${\mathrm{Ar}}^{40}(p, 2pn){\mathrm{Cl}}^{38}$ at a single energy were also measured.