One-loop corrections to polarization observables

We review the physics of polarization observables in high energy reactions in general and discuss the status of \NLO one--loop corrections to these observables in specific. Many high order radiative corrections exists for rates but not many \NLO radiative corrections exist for polarization observables. The radiative correction calculations for polarization observables are somewhat more complicated than those for rates. They tend to be smaller than those for the rates. In most of the examples we discuss we include mass effects which significantly complicate the radiative correction calculations. We elaborate a general scheme which allows one to enumerate the number of independent density matrix elements in a reaction and provide explicit examples of angular decay distributions in self--analyzing decays that allow one to experimentally measure the density matrix elements. We provide examples of reactions where certain density matrix elements are only populated at \NLO or by mass effects. In our discussion we concentrate on semileptonic bottom and top quark decays which are linked to leptonic $ \mu $ and $ \tau $ decays through a Fierz transformation.