Stringy Threshold Corrections in D-brane Systems

We investigate string amplitudes by using the partial modular transformation which we introduced in our previous works. This enables us to extract stringy threshold corrections from the full string amplitudes and interpret them in terms of the Wilsonian effective field theory in a natural way. We calculate mass shifts and wave function renormalizations for massless scalar fields on brane-antibrane systems. We find that the mass shift can be exponentially small and negative. We also propose a strategy for realizing a hierarchical mass spectrum on D-branes.