String Loop Threshold Corrections for N=1 Generalized Coxeter Orbifolds

We discuss the calculation of threshold corrections to gauge coupling constants for the, only, non-decomposable class of abelian (2, 2) symmetric N=1 four dimensional heterotic orbifold models, where the internal twist is realized as a generalized Coxeter automorphism. The latter orbifold was singled out in earlier work as the only N=1 heterotic $Z_N$ orbifold that satisfy the phenomenological criteria of correct minimal gauge coupling unification and cancellation of target space modular anomalies.Comment: 10 pages, LaTe

Standard Model Compactifications of IIA Z3 x Z3 Orientifolds from Intersecting D6-branes

We discuss the construction of chiral four dimensional ${\bf T^6/(Z_3 \times Z_3)}$ orientifold compactifications of IIA theory, using D6-branes intersecting at angles and not aligned with the orientifold O6 planes. Cancellation of mixed U(1) anomalies requires the presence of a generalized Green-Schwarz mechanism mediated by RR partners of closed string untwisted moduli. In this respect we describe the appearance of three quark and lepton family $SU(3)_C \times SU(2)_L \times U(1)_Y$ non-supersymmetric orientifold models with only the massless spectrum of the SM at low energy that can have either no exotics present and three families of $\nu_R$'s (A$^{\prime}$-model class) or the massless fermion spectrum of the N=1 SM with a small number of massive non-chiral colour exotics and in one case with extra families of $\nu_R$'s (B$^{\prime}$-model class). Moreover we discuss the construction of SU(5), flipped SU(5) and Pati-Salam $SU(4)_c \times SU(2)_L \times SU(2)_R$ GUTS - the latter also derived from adjoint breaking - with only the SM at low energy. Some phenomenological features of these models are also briefly discussed. All models are constructed with the Weinberg angle to be 3/8 at the string scale.Comment: 31 pages, 1 figure; v2: typos corrected, comments in the introduction added v3:35 pages, Pati-Salam G422 models & comments on split susy scenario and refs added; to appear in NP

4D GUT (and SM) Model Building from Intersecting D-Branes

We provide a general overview of the current state of the art in three generation model building proposals - using intersecting D-brane toroidal compactifications of IIA string theories - which have, only, the SM at low energy. In this context, we focus on these model building directions, where natural non-supersymmetric constructions based on $SU(4)_C \times SU(2)_L \times SU(2)_R$, SU(5) and flipped SU(5) GUT groups, have at low energy only the Standard Model. In the flipped SU(5) GUTS, the special build up structure of the models accommodates naturally a see-saw mechanism and a new solution to the doublet-triplet splitting problem.Comment: 8 pages, LaTex, Contribution to the Proceedings of the "String Phenomenology 2003" conference, Durham, England, 29th July - 4th August 200