Scale-independent spin effects in D-brane dynamics

We study spin interactions between two moving D-branes using the Green-Schwarz formalism of boundary states. We focus our attention on the leading terms for small velocities v, of the form v4-n/r7-p+n (v2-n/r3-p+n) for p-p (p-p+4) systems, with 16 (8) supercharges. In analogy with standard GS computations of massless four-point one-loop amplitudes in type I theory, the above terms are governed purely by zero-modes, massive state contributions cancel as expected by the residual supersymmetry. This implies the scale invariance of these leading spin effects, supporting the relevant matrix model descriptions of supergravity interactions; in this context, we also discuss similar results for more general brane configurations. We then give a field theory interpretation of our results that allows us in particular to deduce the gyromagnetic ratio g=1 and the presence of a quadrupole moment for D0-branes. (orig.)