We consider the elastic scattering of two open strings living on two D-branes
separated by a distance r. We compute the high-energy behavior of the
amplitude, to leading order in string coupling, as a function of the scattering
angle ϕ and of the dimensionless parameter v=r/(πα′s​) with s​ the center-of-mass energy. The
result exhibits an interesting phase diagram in the (v,Ď•) plane, with a
transition at the production threshold for stretched strings at v=1. We also
discuss some more general features of the open-string semiclassical
world-sheets, and use T-duality to give a quantum tunneling interpretation of
the exponential suppression at high-energy.Comment: JHEP class, 18 pages, 8 figure