Fermion generations, masses and mixing angles from extra dimensions

We discuss a toy model in six dimensions that predicts two fermion generations, natural mass hierarchy and intergenerational mixing. Matter is described by vector-like six-dimensional fermions, one per each irreducible standard model representation. Two fermion generations arise from the compactification mechanism, through orbifold projection. They are localized in different regions of the compact space by a six-dimensional mass term. Flavour symmetry is broken via Yukawa couplings, with a Higgs vacuum expectation value not constant in the extra space. A hierarchical spectrum is obtained from order one dimensionless parameters of the six-dimensional theory. The Cabibbo angle arises from the soft breaking of six-dimensional parity symmetry. We also briefly discuss how the present model could be extended to cover the realistic case. (C) 2003 Published by Elsevier B.V.