Two-loop snail diagrams: relating neutrino masses to dark matter
  Various mechanisms have been developed to explain the origin of Majorana neutrino masses. One of them is radiative mass generation. Two-loop mass generation is of particular interest because the masses and couplings of new particles propagating in the loop can be in the range testable by other experiments and observations. In order for the radiative mass suppression to be reliable, it should be guaranteed that lower loop contributions are suppressed. Based on loop topology and the form of electroweak presentation of the particles propagating in the loop, one can determine whether a lower---and therefore dominant---loop contribution is possible. We present a model based on these general considerations which leads to neutrino masses via a two-loop diagram which we dub as "snail-diagram". The model has two natural candidates for dark matter one of them being a neutral Dirac fermion which can satisfy the conditions of the thermal freeze-out scenario by annihilation to lepton pairs. We comment on the possibility of explaining the GeV gamma ray excess observed by Fermi-LAT from the region close to the Galaxy Center. We also discuss possible signals at the LHC and at experiments searching for lepton flavor violating rare decays. 
Yasaman Farzan
hep-ph