Bounds on Heavy Chiral Fermions

Abstract

We derive the low-energy electroweak effective lagrangian for the case of additional heavy, unmixed, sequential fermions. Present data still allow for the presence of a new quark and/or lepton doublet with masses greater than $M_Z/2$, provided that these multiplets are sufficiently degenerate. Deviations of the effective lagrangian predictions from a full one-loop computation are sizeable only for fermion masses close to the threshold $M_Z/2$. Some of the constraints on new sequential fermions coming from accelerator results and cosmological considerations are presented. We point out that the new fermions can significantly affect the production and decay rate into $γγ$ of the intermediate Higgs at LHC.