Soft supersymmetry breaking in D-brane models

Spontaneous breaking of supersymmetry in a hidden sector, communicated gravitationally to the visible world of the Minimal Supersymmetric Standard Model (MSSM) gives rise to soft supersymmetry breaking. In string theoretic models with D-branes, the MSSM gauge and matter fields originate from open strings localized on these branes, while the hidden sector is given by closed string excitations or other distant branes. To obtain chiral non-abelian gauge symmetries with semi-realistic properties, we consider intersecting or magnetized D-branes, for which a computable low energy Lagrangian is known, giving rise to a specific pattern of soft breaking parameters. Since in D-brane models the universality of gravity is partly violated, gauge coupling unification is usually negated, and one can put bounds from experiments on the parameters that characterize the model, which are however not very restrictive and model dependent. A possible source of supersymmetry breaking is the presence of background fluxes in the compactification, which allows a combined approach to moduli stabilization and supersymmetry breaking.