Double Charge Exchange of PositiveπMesons on Complex Nuclei

The differential cross section for the reaction ${\pi }^{+}+N(z, n)\to {\pi }^{-}+X$ was measured at lab angles 8.5° and 16°, with incident 200-MeV ${\pi }^{+}$ mesons. Several target nuclei were employed: $V^{51}$, ${Z\mathrm{r}}^{90}$, and ${\mathrm{Li}}^7$. The momentum analysis of the final-state ${\pi }^{-}$ was accomplished with a magnetic spectrometer, using digitized wire spark chambers for track location. Recent theoretical papers indicate the possibility of exciting isobaric analog states [for which $X=N(z+2, n-2)$: a two-body final state]. Our data show no evidence of this state, and provide an upper limit of 1-2 μb/sr on its excitation at both 8.5° and 16°. The contribution to the cross section from all final states is 40-60 μb/sr for the various targets and angles and for $110\ge T_{{\pi }^{-}}\ge 200$ MeV. The data are discussed in terms of a pion-complex-nucleus interaction model, including effects of higher-order inelastic scattering.