Using reparametrization invariance to define vacuum infinities in string path integrals

Path integrals for interacting world sheet sigma models play a key role in string theory. For open strings, the relevant path integral is one-dimensional and has direct physical interpretation as a source term for closed string fields. This means that the vacuum divergences (Möbius infinities) of the path integral must be renormalized correctly. In this paper we show that reparametrization invariance Ward identities, apart from specifying the equations of motion of space-time background gauge fields, also serve to fix the renormalization scheme of the vacuum divergences.