Discriminating MSSM families in (free-field) Gepner Orientifolds
  A complete analysis of orientifold compactifications involving Gepner models that are free fields (k=1,2) is performed. A set of tadpole solutions is found that are variants of a single chiral spectrum. The vacua found have the property that different families have different U(1) charges so that one family cannot obtain masses in perturbation theory. Its masses must come from instantons, allowing for a hierarchy of masses. The phenomenological aspects of such vacua are analyzed. 
Elias Kiritsis, Bert Schellekens, Mirian Tsulaia