A chiral D = 4, N = 1 string vacuum with a finite low energy effective field theory

Supersymmetric N=1, D=4 string vacua are known to be finite in perturbationtheory. However, the effective low energy D=4, N=1 field theory lagrangian doesnot yield in general finite theories. In this note we present the first (to ourknowledge) such an example. It may be constructed in three dual ways: i) as a$Z_3$, SO(32) heterotic orbifold; ii) as a Type -IIB, $Z_3$ orientifold withonly ninebranes and a Wilson line or iii) as a Type-IIB, $Z_6$ orientifold withonly fivebranes. The gauge group is $SU(4)^3$ with three chiral generations.Although chiral, a subsector of the model is continuosly connected to a modelwith global N=4 supersymmetry. From the $Z_6$, Type IIB orientifold point ofview the above connection may be understood as a transition of four dynamicalfivebranes from a fixed point to the bulk. The N=1 model is thus also expectedto be S-dual. We also remark that, using the untwisted dilaton and modulifields of these constructions as spurion fields, yields soft SUSY-breakingterms which preserve finiteness even for N=0.Comment: 8 pages, LaTe