Role of Higher Fermion Representations in TeV-scale Seesaw

We consider a scenario in which additional vectorlike TeV-scale fermions belonging to higher weak-isospin multiplets provide new seesaw mediators. If these fermions have non-zero hypercharge, their tree-level exchange produces novel seesaw mechanism different from type I and III seesaw. In order to produce Majorana masses for light neutrinos, new Dirac seesaw mediators are constrained by the SM gauge symmetry to belong to a weak triplet and a five-plet. The latter, in conjunction with two isospin 3/2 scalar multiplets, leads to new seesaw formula m_\nu ~ v^6/M^5. It reproduces the empirical masses m_\nu ~ 10^{-1} eV by M < TeV new states, testable at the LHC.