Abstract :
Extending the recent work by M. Bershadsky, Z. Kakushadze and C. Vafa (hep-th/9803076), we consider string perturbative expansion in the presence of D-branes and orientifold planes imbedded in orbifolded space-time. In the α′ → 0 limit the weak coupling string perturbative expansion maps to `t Hooftʹs large-N expansion. We focus on four-dimensional N = 1, 2, 4 supersymmetric theories, and also discuss possible extensions to N = 0 cases. Utilizing the string theory perturbation techniques we show that computation of any M-point correlation function in these theories reduces to the corresponding computation in the parent N = 4 theory. In particular, we discuss theories (which are rather constrained) with vanishing β-functions to all orders in perturbation theory in the large-N limit. We also point out that in theories with non-vanishing β-functions the gauge coupling running is suppressed in the large-N limit. Introduction of orientifold planes allows to construct certain gauge theories with SO, Sp and SU gauge groups and various matter (only unitary gauge groups with bi-fundamental/adjoint matter arise in theories without orientifold planes).
