A breakdown of the Impulse Approximation is studied in pion photoproduction on $^3$He at high momentum transfers. The usual DWIA formalism with Faddeev wave functions which works well for small momentum transfers deviates from experimental measurements by up to two orders of magnitude for $Q^2>6 fm^{ -2}$. It is found that the explicit inclusion of two-body mechanisms, where the photon is absorbed on one nucleon and the pion is emitted from another nucleon can restore agreement with the data.
Categories: nucl-th