INTERACTIONS INVOLVING D-BRANES

We investigate some aspects of the spectrum of D-branes and their inte ractions with closed strings. As argued earlier, a collection of many D-strings behaves at large dilaton values as a single multiply wound s tring. We use this result and T-duality transformations to show that a similar phenomenon occurs for effective strings produced by wrapping p-branes on a small (p - 1)-dimensional torus, for suitable coupling. To understand the decay of an excited D-string at large dilaton values , we study the decay of an elementary string at small dilaton values. A long string, multiply wound on a circle, with a small excitation ene rgy is found to predominantly decay into another string with the same winding number, and an unwound closed string (rather than two wound st rings), This decay amplitude agrees, under duality, with the decay amp litude computed using the Born-Infeld action for the D-string. We comp ute the absorption cross section for the D-brane model studied by Call an and Maldacena. The absorption cross section for the dilaton equals that for the scalars obtained by reduction of the graviton, and both a gree with the cross section expected from a classical hole with the sa me charges.