New grand unified models with intersecting D6-branes, neutrino masses, and flipped SU(5)

We construct new supersymmetric SU(5) grand unified models based on Z4×Z2 orientifolds with intersecting D6-branes. Unlike constructions based on Z2×Z2 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 an 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.