The reactions $\gamma A\to {\pi }^{\pm }{A}^{*}$ have been studied at four-momentum transfers $-t<~0.5$ ${\mathrm{GeV}}^2$ for seven elements ranging from hydrogen to lead. Exclusion-principle suppression is clearly visible at small-momentum transfer. Neither the $A$ dependence nor the energy dependence of the cross sections agrees with the predictions of the vector-dominance model. The ratio of $\frac{{\pi }^{-}}{{\pi }^{+}}$ production requires equal spatial distributions for the protons and neutrons in nuclei. Some $K^{+}$ data are also presented.

Close

PrevNext

View figure in article