Nonlinear Realization of the SU(5) Georgi-Glashow Model
  The formulation of a grand unified field theory based on the nonlinearly realized SU(5) gauge group is presented. The tree-level action is constructed by requiring invariance under local left SU(5) gauge transformations, the existence of a weak power-counting bound and by imposing also an additional invariance under local right SU_C(3) X U_Q(1) transformations. The local functional equation associated to this latter invariance allows one to prove the absence of physical scalar modes with the quantum numbers of massless gauge bosons from the perturbative spectrum. Two independent mass invariants for the electroweak gauge bosons exist, although the gauge group of our model is simple. The flavour sector of the model is analyzed and compared with the one of the minimal Georgi-Glashow grand unified theory. 
Daniele Bettinelli, Ruggero Ferrari, Alessandro Sanzeni
hep-ph hep-th