  Production of Z bosons in emission processes by neutrinos in the expanding de
Sitter universe is studied by using perturbative methods. The total probability
and transition rate for the spontaneous emission of a Z boson by a neutrino is
computed analytically, then we perform a graphical analysis in terms of the
expansion parameter. Our results prove that this process is possible only for
large expansion conditions of the early Universe. Finally the density number of
Z bosons is defined and we obtain a quantitative estimation of this quantity in
terms of the density number of neutrinos.
