Stringhe aperte su orbifolds con campi magnetici di fondo

This project is fitted into the context of String Phenomenology: a brach of theoret- ical physics which aim is to construct string models that, at low energy limit, are compatible with Minimal Supersymmetric Standard Model. The aim of the thesis is to combine background magnetic fields with orbifold compactification in closed and open non oriented strings. This construction is well known in the case of the orbifold T4/Z2, but it is completely unexplored for generic groups Z_N with N!=2. In fact, in the past years researchers have been working on the orbifold construction with intersecting branes, whose geometric interpretation is well known. The two descriptions (magnetic field and intersecting branes) are equivalent on a torus, because a T-duality transformation maps one description into the other one. This is no longer true in the case of the orbifold because the T-duality transformation maps a geometrical action on the string coordinates into a non geometrical one.