Meson-exchange currents in pion-nucleus double-charge-exchange reactions

The scattering of pions by virtual mesons in the nuclear medium is analysed and its contribution to the process of double-charge exchange in pion-nucleus scattering compared to the conventional mechanism which involves charge exchange in the scattering of the pions with two different nucleons. It is shown that at forward angles the cross section is decreased by about 50% around pion energies T ≈ 130 MeV and increased by a similar amount at energies above the resonance, T ≈ 250 MeV. At large angles the meson-exchange contribution changes the cross section appreciably and can become the dominant mechanism.