Composite quarks and leptons in higher space-time dimensions

A new approach towards the composite structure of quarks and leptons in the context of the higher dimensional unified theories is proposed. Because of certain strong dynamics much like ordinary QCD, every possible vectorlike set of composites appears in higher dimensional bulk space-time. However, through a proper Sherk-Schwarz compactification only chiral multiplets of composite quarks and leptons survive as the massless states in four dimensions. In this scenario restrictions related to 't Hooft's anomaly matching condition turn out to be avoided and, as a result, the composite models look rather simple and economical. We demonstrate our approach by an explicit construction of a model of preons and their composites unified in the supersymmetric SU(5) GUT in five space-time dimensions. The model predicts precisely three families of composite quarks and leptons being the triplets of the chiral horizontal symmetry SU(3)h which automatically appears in the composite spectrum when going to ordinary four dimensions