  In this paper we study the problem of neutral electro-weak interactions in a
de Sitter geometry. We develop the formalism of reduction for the Proca field
with the help of the solutions for the interacting fields and by using
perturbative methods we obtain the definition of the transition amplitudes in
the first order of perturbation theory. As an application to our formalism we
study the generation of massive fermions and Z bosons from vacuum in the
expanding de Sitter universe. Our results are the generalization to the curved
geometry of the Weinberg-Salam electro-weak theory for the case of Z boson
interaction with leptons. The probability is found to be a quantity that
depends on the Hubble parameter and we prove that such perturbative processes
are possible only for large expansion regime of the early Universe. The total
probability and rate of transition are obtained for the case of large expansion
and we use the dimensional regularization for extract finite results from the
momenta integrals. In the Minkowski limit we obtain that the probability of
particle generation from vacuum is vanishing recovering the well known result
that forbids particle production in flat space-time due to the momentum-energy
conservation.
