Radiative seesaw model with degenerate Majorana dark matter

We study a three-loop-induced neutrino mass model with exotic vectorlike isospin doublet leptons which contain a dark matter candidate. Then we explore lepton flavor violations and dark matter physics in a coannihilation system. In this paper, the nearly degenerate Majorana fermion dark matter can naturally be achieved at the two-loop level, while the mass splitting can be larger than O (200 ) keV which is required from the constraint of the direct detection search with spin-independent inelastic scattering through the Z -boson portal. As a result, a monochromatic photon excess, with threshold energy greater than O (200 ) keV , is predicted in our model and could be measured through indirect detection experiments such as INTEGRAL.