Pion Double Charge Exchange onHe4and Meson Exchange Currents

We have measured the differential cross section $\frac{d^2\sigma }{d\Omega }$ $\mathrm{dT}$ and estimated the total cross section ${\sigma }_T$ for the double charge exchange reaction ${}_{}{}^4{}_{}{}^{}\mathrm{He}({\pi }^{+},{\pi }^{-})4p$ at 140, 200, and 295 MeV. The values for ${\sigma }_T$ are at least an order of magnitude larger than predictions based on sequential single charge exchange $\pi -N$ scatterings. It is possible, therefore, that ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(${\pi }^{+}$,${\pi }^{-}$) is dominated by other processes, such as the formation of a $\Delta$ in the intermediate state or scattering from meson exchange currents.