Exponential suppression of string interactions on orbifolds

Nonperturbative world-sheet instanton effects can be seen in the classical bosonic contribution to scattering amplitudes on orbifolds. These instanton contributions have the form exp(-cR2/α') and are exponentially suppressed when the orbifold radius is large. Amplitudes whose leading contribution comes from instantons are identified with processes in which the corresponding string states are pinned to the orbifold fixed points, and unpinned amplitudes are shown to lack this suppression at large radius. Blowing up the fixed points of an orbifold eliminates any pinning and with it the corresponding exponential suppression. (orig.)