Complete Helicity Decomposition of the B-t-tbar Vertex including Higher Order QCD Corrections and Applications to e+e- --> t tbar

The complete density matrix for all polarization configurations in the process B--> t tbar, where B is an off--shell Z or photon and t is the top quark, is calculated numerically including oneloop QCD corrections, i.e. virtual and real gluon contributions in O(alphas). The analysis is done in the framework of the helicity formalism. The results are particularly suited for top quark production at the Linear Collider, but may be useful in other circumstances as well. Relations to LEP and Tevatron physics are pointed out.