I. Chepelev

We compute long-distance interaction potentials between certain 1/2 and 1/4 supersymmetric D-brane configurations of type IIB theory, demonstrating detailed agreement between classical supergravity and one-loop instanton matrix model results. This confirms the interpretation of D-branes as described by classical matrix model backgrounds as being `populated' by large number of D-instantons, i.e. as corresponding to non-marginal bound states of branes of lower dimensions. In the process, we establish precise relation between matrix model expressions and non-abelian F^4 terms in the super Yang-Mills effective action.

We discuss long-distance, low-velocity interaction potentials for processes involving longitudinally boosted M5-brane (corresponding in type IIA theory language to the 1/4 supersymmetric bound state of 4-brane and 0-brane). We consider the following scattering configurations: (a) D=11 graviton off longitudinal M5-brane, or, equivalently, 0-branes off marginal 4+0 bound state; (b) M2-brane off longitudinal M5-brane, or a non-marginal 2+0 bound state off marginal 4+0 bound state; (c) two parallel longitudinal M5-branes, or two 4+0 marginal bound states. We demonstrate the equivalence between the classical closed string theory (supergravity) and M(atrix) model (one-loop super Yang-Mills) results for the leading terms in the interaction potentials. The supergravity results are obtained using a generalisation of the classical probe method which allows one to treat bound states of D-branes as probes by introducing non-trivial world-volume gauge field backgrounds.