Energy Dependence of Low-Energy Pion Double-Charge-exchange on the Calcium Isotopes

The forward angle differential cross sections of (π+, π−) reactions on the calcium isotopes to the double-isobaric-analog (DIAS) and ground states (GS) were measured between 25 and 65 MeV. These low-energy cross sections are up to an order of magnitude larger than those at resonance energies. The forward-angle excitation functions of 42Ca, 44Ca DIAS and 44Ca GS transitions peak around 50 MeV. The 48Ca DIAS cross section exhibits a minimum at about 50 MeV and that for the GS has a peak which is shifted to lower energy with a smaller cross section than that of the other nuclei. The observed behaviour is inconsistent with theoretical expectations.