New Grand Unified Models with Intersecting D 6 - branes , Neutrino Masses , and Flipped SU ( 5 )

We construct new supersymmetric SU (5) Grand Unified Models based on Z 4 × Z 2 orientifolds with intersecting D6-branes. Unlike constructions based on Z 2 × Z 2 orientifolds, the orbifold images of the three-cycles wrapped by D6-branes correspond to new configurations and thus allow for models in which, in addition to the chiral sector in 10 and ¯ 5 representations of SU (5), only, there can be new sectors with 15 + 15 and 10 + 10 vector-pairs. We construct an example of such a globally consistent, supersymmetric model with four-families, two Standard Model Higgs pair-candidates and the gauge symmetry U (5) × U (1) × Sp(4). In a N = 2 sector, there are 5 × 15 + 15 and 1 × 10 + 10 vector pairs, while another N = 1 sector contains one vector-pair of 15-plets. The N = 2 vector pairs can obtain a large mass dynamically by parallel D6-brane splitting in a particular two-torus. The 15-vector-pairs provide, after symmetry breaking to the Standard Model (via parallel D-brane splitting), triplet pair candidates which can in principle play a role in generating Majorana-type masses for left-handed neutrinos, though the necessary Yukawa couplings are absent in the specific construction. This model can also be interpreted as a flipped SU (5) × U (1) X Grand Unified Model where the 10-vector-pairs can play the role of Higgs fields, though again there are phenomenological difficulties for the specific construction.