New Grand Unified Models with Intersecting D6-branes, Neutrino Masses, and Flipped SU(5)

TL;DR

This work develops new supersymmetric SU(5) GUT constructions from intersecting D6-branes on a Z4 x Z2 orientifold, leveraging orbifold images to introduce non-chiral N=2 sectors with vector-like 15 and 10 multiplets in addition to the usual chiral 10 and 5̄. The authors provide a detailed global-consistency framework (RR tadpoles, K-theory, and SUSY) and present an explicit globally consistent four-family model with gauge group U(5) x U(1) x Sp(4), two Higgs pairs, and a rich spectrum including 5 vector pairs of 15+15̄ and a 1 vector pair of 10+10̄ in the N=2 sector. They show that these vector pairs can become massive through parallel D-brane splitting on one two-torus, and discuss potential roles for the 15s in generating Majorana-type neutrino masses, though explicit Yukawa couplings implementing this are not realized in the model. An interpretation as flipped SU(5) x U(1)_X is explored, with U(1)_X shown to be non-anomalous, but phenomenological challenges remain; the work opens a geometric path to spectrum-control in string GUTs and motivates future exploration of fractional branes and other orientifolds.

Abstract

We construct new supersymmetric SU(5) Grand Unified Models based on Z4 x Z2 orientifolds with intersecting D6-branes. Unlike constructions based on Z2 x 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-bar representations of SU(5), only, there can be new sectors with (15 + 15-bar) and (10 + 10-bar) 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) x U(1) x Sp(4). In a N = 2 sector, there are 5 x (15 + 15-bar) and 1 x (10 + 10-bar) 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 particu- lar 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. Similarly, the 10- vector-pairs can play the role of Higgs fields of the flipped SU(5), though again there are phenomenological difficulties for the specific construction.

Figures (2)

• Figure 1: The locations of O6-planes, fixed under the orientifold actions $\Omega R$, $\Omega R \omega$, $\Omega R \omega \theta^2$, and $\Omega R \theta^2$ (denoted by bold solid lines) for a factorized six-torus with the third two-torus tilted.
• Figure 2: The locations of O6-planes fixed under the orientifold actions $\Omega R\theta$, $\Omega R \omega\theta$, $\Omega R \omega \theta^3$, and $\Omega R \theta^3$ (denoted by bold solid lines) for the factorized six-torus with the third two-torus tilted.