Scattering of Noncommutative Strings: A Note on Signature Change at   Planck Scale
  We compute the S-matrix, for the scattering of two string states, on a noncommutative D3-brane in a path integral formalism. Our analysis attempts to resolve the issue of ``imaginary string'', originally raised by `t Hooft in a point-particle scattering at Planck energy, by incorporating a notion of signature change on an emerging semi-classical D-string in the theory. 
Supriya Kar and Sumit Majumdar