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Heavy lepton quintuplet and TeV scale seesaw mechanism

Two novel seesaw models based on weak-isospin quintuplet leptons are presented. In the first one, the zero hypercharge Majorana quintuplet lepton is accompanied by a scalar weak-isospin quadruplet, and in the second one, a non-zero hypercharge Dirac quintuplet lepton is accompanied by two scalar weak-isospin quadruplets. In both models scalar quadruplets attain the induced vacuum expectation values. This leads to neutrino mass contribution ~v_H^6/M^5$ arising at the tree level from a dimension-nine operator. Additional neutrino mass contribution ~ (1 / 16 \pi^2) v_H^2/M comes from a loop suppressed dimension-five operator. The empirical masses m_\nu ~ 0.1 eV can be achieved with the new states being within the reach of the Large Hadron Collider and the origin of the neutrino masses could be directly tested. The Drell-Yan production of the new fermions is abundant due to nontrivial electroweak gauge charges and their decays lead to interesting multilepton and same sign dilepton events. The charged components of new scalar multiplets contribute to loop-mediated Higgs decays h -> gamma gamma and h -> Z gamma. The neutral component of the Majorana 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.

Neutrino mass; Beyond Standard Model; Quintuplet leptons; Exotic leptons; Non-standard scalar fields; Dark matter

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