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

Abstract

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, 〈 n〉, satisfies, the inequality 〈 n〉 ⩽ n, where 〈 n〉 is defined by Γ( Z→neutrinos) ≡ 〈 n〉 Γ 0 and Γ 0 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.