High energy scattering on distant branes

We consider the elastic scattering of two open strings living on two D-bran es separated by a distance r. We compute the high-energy behavior of the am plitude, to leading order in string coupling, as a function of the scatteri ng angle phi and of the dimensionless parameter upsilon = r/(pi alpha'root s) with root s the center-of-mass energy. The result exhibits an interestin g phase diagram in the (upsilon, phi) plane, with a transition at the produ ction threshold for stretched strings at upsilon = 1. We also discuss some more general features of the open-string semiclassical world-sheets, and us e T-duality to give a quantum tunneling interpretation of the exponential s uppression at high-energy.