Extended double seesaw model and low scale leptogenesis

We propose a variant of seesaw mechanism by introducing extra singlet neutrinos, with which we show how the low scale leptogenesis is realized without imposing the tiny mass splitting between two heavy Majorana neutrinos required in the resonant leptogenesls. Thus, the so-called gravitino problem encountered in the typical supersymmetric seesaw model can be avoided. It is showed that an introduction of the new singlet fermion leads to a new contribution which can enhance the lepton asymmetry for certain range of parameter space.