Interaction between two nonthreshold bound states

A general nonthreshold Bogomol'nyi-Prasad-Sommerfield (F,Dp)[or (Dp-2,Dp)] bound state can be described by a boundary state with a quantized world-volume electric (or magnetic) flux and is characterized by a pair of integers (m,n). With this, we calculate explicitly the interaction amplitude between two such nonthreshold bound states with a separation Y when each of the states is characterized by a pair of integers (mi,ni) with i=1,2. With this result, one can show that the nondegenerate (i.e., mini≠0) interaction is, in general, attractive for the case of (Dp-2,Dp), but this is true and for certain only at large separation for the case of (F,Dp). In either case, this interaction vanishes only if m1/n1=m2/n2 and n1n2>0. We also study the analytic structure of the corresponding amplitude and calculate, in particular, the rate of pair production of open strings in the case of (F,Dp).