Heterotic Orbifold Models

Abstract

We review efforts in string model building, focusing on the heterotic orbifold compactifications. We survey how one can, starting from an explicit string theory, obtain models which resemble Nature. These models exhibit the standard model gauge group, three generations of standard model matter and an appropriate Higgs sector. Unlike many unified models, these models do not suffer from problems such as doublet-triplet splitting, too rapid proton decay and the $\mu$ problem. Realistic patterns of fermion masses emerge, which are partly explained by flavor symmetries, including their modular variants. We comment on challenges and open questions.