Doublet-Triplet Splitting in Realistic Heterotic String Derived models

There has been recently a surge of interest in Grand Unified Theories on orbifolds of higher dimensional spaces. In particular, the higher dimensional doublet-triplet splitting mechanism has been of much interest. I revisit the superstring doublet-triplet splitting mechanism in which the color triplets are projected out by the GSO projections, while leaving the electroweak doublets in the physical spectrum. The connection with the higher dimensional theories is elucidated. It is shown that the doublet-triplet splitting depends crucially on the assignment of boundary conditions in the compactified directions.