It is known that flavor democracy favors the existence of the fourth standard model (SM) family. In order to give nonzero masses for the first three-family fermions flavor democracy has to be slightly broken. A parametrization for democracy breaking, which gives the correct values for fundamental fermion masses and, at the same time, predicts quark and lepton Cabibbo-Kobayashi-Maskawa (CKM) matrices in a good agreement with the experimental data, is proposed. The pair productions of the fourth SM family Dirac $({\nu }_4)$ and Majorana $(N_1)$ neutrinos at future linear colliders with $\sqrt{s}=500\mathrm{GeV}$, 1 TeV, and 3 TeV are considered. The cross section for the process $e^{+}{e}^{-}\to {\nu }_4{\overline{\nu }}_4(N_1{N}_1)$ and the branching ratios for possible decay modes of the both neutrinos are determined. The decays of the fourth family neutrinos into muon channels $({\nu }_4(N_1)\to {\mu }^{\pm }{W}^{\mp })$ provide cleanest signature at $e^{+}{e}^{-}$ colliders. Meanwhile, in our parametrization this channel is dominant. $W$ bosons produced in decays of the fourth family neutrinos will be seen in detector as either di-jets or isolated leptons. As an example, we consider the production of 200 GeV mass fourth family neutrinos at $\sqrt{s}=500\mathrm{GeV}$ linear colliders by taking into account di-muon plus four jet events as signatures.

• Received 23 May 2005

DOI:https://doi.org/10.1103/PhysRevD.72.053006