Considered as an extension of the two-Higgs-doublet model by a singly- charged scalar, in its general form, this model induces tree- level flavor-changing neutral currents (FCNCs) in both lepton and quark sectors. Usually, such dangerous processes can be avoided by imposing the natural flavor...
1. In addition to this, there is also another diagram whose internal particles are replaced by their charge conjugates, which is made possible by Majorana property of neutrinos. The sum of the two diagrams yields a symmetric neutrino mass matrix Mν=κ(f^MℓdiagY^T+Y^Mℓdiagf^T), ...