Flavour symmetry in the lepton sector

Masses of fermions, i.e. Yukawa couplings, extend about a range of 5 orders of magnitude and show a pronounced hierarchy. On the other hand, mixing in the quark and lepton sector show very different patterns. Motivated by the minimal (lepton) flavour hypothesis, we address these issues by promoting SM Yukawa couplings and a symmetric dim-5 neutrino mass term to scalar spurion fields. These fields transform accordingly under a global flavour symmetry. Above the electroweak scale, the spurion fields acquire VEVs in a stepwise fashion to accommodate the observed hierarchies among fermions. Especially, the possibilities of an normal, inverted or degenerate neutrino spectrum can be accounted for. This has to be understood as bottom-up approach to the flavour structure of the SM.