$A_4$ Group and Tri-bimaximal Neutrino Mixing -- A Renormalizable Model
  The tetrahedron $A_4$ group has been widely used in studying neutrino mixing matrix. It provides a natural framework of model building for the tri-bimaximal mixing matrix. In this class of models, it is necessary to have two Higgs fields, $\chi$ and $\chi'$, transforming under $A_4$ as 3 with one of them having vacuum expectation values for the three components to be equal and another having only one of the components to be non-zero. These specific vev structures require separating $\chi$ and $\chi'$ from communicating with each other. The clash of the different vev structures for $\chi$ and $\chi'$ is the so called sequestering problem. In this work, I show that it is possible to construct renormalizable supersymmetric models producing the tri-bimaximal neutrino mixing with no sequestering problem. 
Xiao-Gang He (NTU)