Fortschr. Phys. 59, No. 11 – 12, 1144 – 1148 (2011) / DOI 10.1002/prop.201100026 Avoiding disastrous couplings in SU(5) orientifolds P. Anastasopoulos 1, ∗ , G. K. Leontaris 2, ∗∗ , R. Richter 3, ∗∗∗ , and A. N. Schellekens 4,5,6, # 1 Technische Universit¨ at Wien, Institut f¨ ur Theoretische Physik, 1040 Vienna, Austria 2 Theoretical Physics Division, Ioannina University, 45110 Ioannina, Greece 3 I.N.F.N., Sezione di Roma “Tor Vergata”, 00133, Roma, Italy 4 NIKHEF, Kruislaan 409, 1009DB Amsterdam, The Netherlands 5 IMAPP, Radboud Universiteit Nijmegen, The Netherlands 6 Instituto de F´ ısica Fundamental, CSIC, Madrid, Spain Received 14 February 2011, accepted 15 February 2011 Published online 7 March 2011 Key words D-brane compactifications, GUT’s, D-instantons This proceeding is based on the paper 1010.5188 [hep-th] where we discuss SU(5) Grand Unified Theories in the context of orientifold compactifications. Specifically, we investigate three D-brane stack realizations of the Georgi-Glashow model and analyze them with respect to their Yukawa couplings. We show that most/all phenomenological problems that appear in two stack models can be resolved in three stack models. We exemplify with globally consistent Georgi-Glashow models based on RCFT that these D-brane quivers can be indeed embedded in a global setting. c  2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction and conclusions D-brane compactifications have been proven to be a promising framework for realistic string model build- ing. The basic building blocks of such constructions are D-branes which fill out the four-dimensional space-time and wrap submanifolds in the internal manifold. The gauge bosons live on the world volume of the respective D-brane while chiral matter appears at intersections of different stacks of D-branes. The mul- tiplicity of the latter is given by the number of intersections of the respective submanifolds in the internal space. Over the last decade many globally consistent semi-realistic D-brane models have been constructed (for recent reviews, see [1–3]). Here we discuss how supersymmetric SU (5) GUT’s can be realized in this framework with the empha- sis on the realization of the superpotential. The SU (5) gauge symmetry arises from a stack a of 5 D-branes giving rise to an U (5) a gauge symmetry. The abelian part U (1) a generically acquires a mass via the Green- Schwarz mechanism and survives only as global symmetry, where the latter needs to be preserved by all perturbative quantities. The 10 is localized at intersections of the U (5) stack a and its orientifold image a ′ . To accommodate for the other matter fields one needs the presence of at least one, but potentially more U (1) stacks. They might give rise to additional global U (1) symmetries. The perturbative superpotential is given by all gauge invariant couplings that are uncharged under all the global U (1)’s, the remnants of the Green-Schwarz mechanism. While the couplings 10 5 5 H , 55 H 1, and 5 H 5 H can in principle be perturbatively realized the other desired coupling 10 10 5 H is generically perturbatively forbidden since it does not preserve the global U (1) a . ∗ Corresponding author E-mail: pascal@hep.itp.tuwien.ac.at ∗∗ E-mail: leonta@uoi.gr ∗∗∗ E-mail: rrichter@roma2.infn.it # E-mail: t58@nikhef.nl c  2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim