Low-energy pion double charge exchange on Ca isotopes

Differential cross sections for the double-isobaric-analog-state transition in the ${}_{}{}^{42}{}_{}{}^{}$,44Ca(${\pi }^{+}$,${\pi }^{\mathrm{-}}$) reactions were measured at an incident energy of 35 MeV and 40 degrees. The ${}_{}{}^{42}{}_{}{}^{}\mathrm{Ca}$ cross section (2.0±0.5 μb/sr) is nearly as large as those of the lighter T=1 nuclei and only slightly smaller than that previously measured for ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$ (2.4±0.7 μb/sr). The cross section for ${}_{}{}^{44}{}_{}{}^{}\mathrm{Ca}$ (1.1±0.3 μb/sr) is half of this amount. These results are surprising when compared with simple expectations and point to the importance of the nonanalog route contribution at low energies.