Spin Effects in Processes of Single Top Quark Production at Hadron Colliders

We investigate spin properties of single top quark production at hadron colliders. Based on an analogy with single top production and polarized top quark decay, we reproduce in a simple way the results by G.Mahlon and S.Parke on the existence of preferred axes for the decomposition of the top quark spin. For the W*- and W-g fusion production modes these axes are related to the down-type quark momentum. The proposed method allows finding kinematical conditions for the observation of top quark polarization in a third process that contributes to single top production and is important at LHC energies, the tW-process, in which spin effects are smeared out by the contribution of diagrams with a QCD gtt-vertex. A simple Monte-Carlo analysis of spin correlations for the tW-process with subsequent top decay is given as an illustration.