# How to handle D-brane particles in the S-matrix using string worldsheets?

In string theory, it's possible to collide a number of strings only in the asymptotic past, and get a final asymptotic state which contains a D0-brane, an anti D0-brane, and a number of strings. If space is compactified, the D0-brane can be replaced by some wrapped D-brane particle, or a stable bound state of a number of them.

How is this handled using string worldsheets only? It's true if the string coupling is weak, many strings have to collide to produce a pair of D-branes, but the string coupling is always nonzero, even if tiny. Does this mean a worldsheet formalism only is incomplete? It's true this is a nonperturbative effect, but does this mean perturbation theory is incomplete?

Perturbation theory is usually "effective and accurate enough" for all questions in which the coupling is weak. However, the actual description of perturbation theory is that it is a systematic expansion around $g=0$. For $g=0$, the number of strings and/or their energy needed to produce a Dirichlet brane-antibrane pair – whose mass goes like $1/g$ – is infinite simply because $1/g=\infty$. So the problem is ill-posed for $g=0$ and we can't expand around $g=0$ in the naive way.
However, in principle, it's fair to say that it's a nonperturbative process given by a D-instanton (an unstable one, like in Coleman's papers), more precisely D$(p-1)$-instanton if you have $D$p-branes in your state with D-branes.