Dynamics of Dirichlet-Neumann open strings on D-branes Original Research Article

A method for computing scattering amplitudes of open strings with a Dirichlet boundary on one end and a Neumann boundary condition on the other is described. The vertex operators for these states are constructed using twist fields which have been studied previously in the context of the Ashkin-Teller model and strings on orbifolds. Using these vertex operators, we compute the three- and four-point scattering amplitudes for (5,9) strings on 5-branes and 9-branes. In the low-energy limit, these amplitudes are found to be in exact agreement with the field theory amplitudes for supersymmetric Yang-Mills coupled to hypermultiplets in six dimensions. We also consider the 1-brane 5-brane system and compute the amplitude for a pair of (1,5) strings to collide and to escape the brane as a closed string. (1,5) strings are found to be remarkably selective in their coupling to massless closed strings in the NS-NS sector; they only couple to the dilaton.