Dark matter from the SU(4) model

The left-right symmetric Pati-Salam model of the unification of quarks and leptons is based on the SU(4) and SU(2) x SU(2) symmetry groups. These groups are naturally extended to include the classification of families of quarks and leptons. We assume that the family group (the group which unites the families) is also the SU(4) group. The properties of the fourth generation of fermions are the same as those of the ordinary-matter fermions in the first three generations except for the family charge of the SU(4)(F) group: F = (1/3, 1/3, 1/3, -1), where F = 1/3 for fermions of ordinary matter and F = -1 for the fourth-generation fermions. The difference in F does not allow mixing between ordinary and fourth-generation fermions. Because of the conservation of the F charge, the creation of baryons and leptons in the process of electroweak baryogenesis must be accompanied by the creation of fermions of the fourth generation. As a result, the excess n(B) of baryons over antibaryons leads to the excess n(4nu) = N - (N) over bar of neutrinos over antineutrinos in the fourth generation with n(4nu) = n(B). This massive neutrino may form nonbaryonic dark matter. In principle, the mass density of the fourth neutrino n(4nu)m(N) in the Universe can make the main contribution to dark matter, since the lower bound on the neutrino mass m(N) from the data on decay of the Z bosons is m(N) > m(Z)/2. The straightforward prediction of this model leads to the amount of cold dark matter relative to baryons, which is an order of magnitude higher than allowed by observations. This inconsistency may be avoided by nonconservation of the F charge. (C) 2003 MAIK Nauka / Interperiodica.