The Normalization of Perturbative String Amplitudes: Weyl Covariance and   Zeta Function Regularization
  This is a self-contained pedagogical review of Polchinski's 1986 analysis from first principles of the Polyakov path integral based on Hawking's zeta function regularization technique for scale-invariant computations in two-dimensional quantum gravity, an approach that can be adapted to any of the perturbative string theories. In particular, we point out the physical significance of preserving both Weyl and global diffeomorphism invariance while taking the low energy field theory limit of scattering amplitudes in an open and closed string theory, giving a brief discussion of some physics applications. We review the path integral computation of the pointlike off-shell closed bosonic string propagator due to Cohen, Moore, Nelson, and Polchinski. The extension of their methodology to the case of the macroscopic loop propagator in an embedding flat spacetime geometry has been given by Chaudhuri, Chen, and Novak. We examine the macroscopic loop amplitude from the perspective of both the target spacetime massive type II supergravity theory, and the boundary state formalism of the worldsheet conformal field theory, clarifying the precise evidence it provides for a Dirichlet (-2)brane, an identification made by Chaudhuri. The appendices contain extensive detail. 
Shyamoli Chaudhuri