String-brane scattering at high energy

The main purpose of this Thesis is to provide an analysis for tree-level amplitudes involving massive string states in a D-brane background. String-brane interactions provide an ideal framework to study the dynamics of the massive states of the string spectrum in a non-trivial background. The infinite tower of massive modes in the string spectrum is essential for the perturbative finiteness of the string amplitudes. The high-energy limit of string theory is a promising regime to study the relation between string amplitudes and gravitational phenomena. We present here an analysis of tree-level amplitudes for processes in which an NS-NS string state from the leading Regge trajectory scatters from a D-brane into another state from the leading Regge trajectory, in general of a different mass, at high energies and small scattering angles. This is done by using world-sheet OPE methods and effective vertex operators.We find that this class of processes has a universal dependence on the energy of the projectile and that the tree-level amplitudes are in agreement with the eikonal operator form of the string S-matrix.