SU(5) Unified Theories from Intersecting Branes

TL;DR

The work demonstrates how to realize three-generation, non-supersymmetric SU(5) and flipped SU(5) GUTs within string theory using intersecting D6-branes on a Type IIA $Z_3$ orientifold. The models satisfy RR tadpole cancellation, fix complex-structure moduli, and realize electroweak Higgses and a neutrino see-saw while avoiding extra low-energy matter; proton decay is suppressed by a high GUT scale and a non-supersymmetric doublet–triplet splitting mechanism. A core achievement is the explicit identification of GUT- and EW-Higgs sectors as bifundamentals arising from specific brane sectors, with masses set by worldsheet areas and brane angles, and the realization of Yukawa structures that yield hierarchical quark and lepton masses. The results indicate that consistent, non-supersymmetric string GUTs can reproduce the SM at low energy and accommodate neutrino masses and proton-stability considerations, though a complete resolution of the gauge hierarchy remains open.

Abstract

We discuss the first string theory examples of three generation non-supersymmetric SU(5) and {\em flipped} SU(5) GUTS, which break to the Standard model at low energy, without extra matter and/or gauge group factors. Our GUT examples are based on IIA $Z_3$ orientifolds with D6-branes intersecting at non-trivial angles. These theories necessarily satisfy RR tadpoles and are free of NSNS tadpoles as the complex structure moduli are frozen (even though a dilaton tadpole remains) to discrete values. We identify appropriately the bifundamental Higgses responsible for electroweak symmetry breaking. In this way, the neutrino see-saw mechanism get nicely realized in these constructions. Moreover, as baryon number is not a gauged symmetry gauge mediated dimension six operators do contribute to proton decay; however proton lifetime may be safely enhanced by appropriately choosing a high GUT scale. An accompanying natural doublet-triplet splitting guarantees the suppression of scalar mediated proton decay modes and the stability of triplet scalar masses against higher dimensional non-renormalizable operators.

Figures (5)

• Figure 1: Proton decay mode $p \rightarrow e^{+} \pi^{0}$ for flipped SU(5) GUTS from intersecting branes.
• Figure 2: Proton decay mode $p \rightarrow {\bar{\nu}}_L \ \pi^{+}$ for flipped SU(5) GUTS from intersecting branes.
• Figure 3: Proton decay mode $p \rightarrow e^{+} \pi^{0}$ for SU(5) GUTS from intersecting branes.
• Figure 4: Proton decay mode $p \rightarrow {\bar{\nu}}_L \ \pi^{+}$ for SU(5) GUTS from intersecting branes.
• Figure 5: Proton decay mode $n \rightarrow e^{+} \pi^{-}$ for SU(5) GUTS from intersecting branes.