Studio del Sistema D(P)-D(P+n) in Teoria di Campo di Stringa Aperta

In this thesis, we study a bosonic D-branes setup made up of a D(P)-brane parallel to a toroidally compactified D(P +n)-brane along a given Dirichlet direction y. For a range of values of n, this system is found to be unstable, ending up in a bound state of lower energy where the D(P )-brane dissolves into the D(P +n)-brane. We provide an in-depth analysis from the world-sheet point of view, in the framework of string field theory, using recently developed effective action techniques, and we explicitly write down the effective potential up to quartic order in the massless fields. We then study the obtained potential and the structure of the vacua as a function of the Neumann-Dirichlet directions n and the compactification radii. We also discuss some subtle issues with quasi-massless modes.