We propose a new seesaw model based on a fermionic hypercharge zero weak quintuplet in conjunction with an additional scalar quadruplet which attains an induced vacuum expectation value. The model provides both tree-level seesaw ∼ v 6 /M 5 and loop-suppressed radiative ∼ (1/16π 2 ) · v 2 /M contributions to active neutrino masses. The empirical masses m ν ∼ 10 −1 eV can be achieved with M ∼ TeV new states, accessible at the LHC. For 5 fb −1 of accumulated integrated luminosity at the LHC, there could be ∼ 500 doubly-charged Σ ++ or Σ ++ fermions with mass M Σ = 400 GeV, leading to interesting multilepton signatures. The neutral component of the fermion quintuplet, previously identified as a minimal dark matter candidate, becomes unstable in the proposed seesaw setup. The stability can be restored by introducing a Z 2 symmetry, in which case neutrinos get mass only from radiative contributions.

We propose novel tree level seesaw mechanism with TeV-scale vectorlike Dirac mediators that produce Majorana masses of the known neutrinos. The gauge quantum number assignment to the Dirac mediators allows them to belong to a weak triplet and a five-plet of nonzero hypercharge. The latter leads to new seesaw formula m_\nu ~ v^6/M^5, so that the empirical masses m_\nu ~ 10^{-1} eV can be achieved by M ~ TeV new states. There is a limited range of the parameter space with M < a few 100 GeV where the tree level contribution dominates over the respective loop contributions and the proposed mechanism is testable at the LHC. We discuss specific signatures for Dirac type heavy leptons produced by Drell-Yan fusion at the LHC.Comment: 10 pages, 1 figure, version corresponding to PL

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