“…\end{equation}$$Moving to the case ii), the lightest heavy leptons can be both negatively charged or neutral. The present constraints from colliders, and particularly from the L3 experiment, only put a lower bound on the masses of the charged particles that is at
, [
49 ] weaker than the consistency limit in Equation (). The neutral particles fall in the category discussed above of the HNL and the same bounds apply also here: for the lightest masses allowed by the consistency condition,
, the corresponding constraints on the couplings of Z and W with a light lepton and a heavy neutral
read
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