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It is well known that a Weinberg dimension-5 operator for small neutrinomasses can be realized at tree level in three types of renormalizable models:(i) the type-I seesaw mediated by fermion singlets, (ii) the type-II seesawmediated by Higgs triplets, (iii) the type-III seesaw mediated by fermiontriplets. We here point out such operator can be also induced at tree level byvector-like lepton doublets in association with unusual fermion singlets, Higgstriplets or fermion triplets. If these unusual fermion singlets, Higgs tripletsor fermion triplets are heavy enough, their decays can generate a leptonasymmetry to explain the cosmic baryon asymmetry, meanwhile, the vector-likelepton doublets can lead to a novel inverse or linear seesaw with richobservable phenomena. We further specify our scenario can be naturally embeddedinto a grand unification theory without the conventional type-I, type-II ortype-III seesaw.

Weinberg dimension-5 operator by vector-like lepton doublets