Constraints on singlet right-handed neutrinos coming from the Z0-width

The constraints on masses and missing angles imposed by the measured Z0 invisible width, in a model in which a singlet right-handed neutrino mixes with all the Standard model neutrinos are studied. If neutrinos are massive an important question to be answered concerns the way the Z-pole observables constraint their masses and mixing parameters. In particular the measured Z-invisible width, Γinv, implies that the number of families is compatible with three. On the other hand, it is well known that this number need not to be an integer number if right-handed neutrinos transformed as singlets under SU(2)L x U(1)Y are added to the particle content of the theory. Experimental searches for sequential neutron leptons beyond the three generations exclude stable Dirac neutrinos below 41.8 GeV and stable Majorana neutrinos below 34.8 GeV. For the unstable cases these values are 46,4 and 45.1 GeV respectively. However, it is worth stressing that these limits are valid for sequential leptons and do not apply to the case of singlets of right-handed neutrinos. Here we will consider the simplest extension of the standard electroweak model with the addition of the one right-handed singlet neutral fermion, resulting in 4 physical neutrinos two of them massless and two massive ones. (author)