Neutrino counting at the Z-peak and right-handed neutrinos

We consider the implications of extending the minimal standard model, with n families of quarks and leptons, by introducing an arbitrary number of right-handed neutrinos, for neutrino-counting via the ''invisible width'' of the Z. It is shown that the effective number of neutrinos, , satisfies, the inequality ≤, where is defined by Γ(Z→neutrinos)=Γ0 and Γ0 is the standard width for one massless neutrino. Thus, in the case of three families, the neutrino-counting can give a result which is less than three, if there are right-handed neutrinos. (orig.)