Helicity projection techniques for evaluating cross sections of heavy-fermion production and decay viaWbosons

We discuss and illustrate the application of density-matrix techniques in the helicity basis for the tree-level production and decay of heavy fermions, via real or virtual W bosons. Both Drell-Yan and O(${\alpha }_s$) QCD production processes for W production are considered, with W→Lν${¯}_L$ and subsequent L and ν${¯}_L$ decays, where L and ν${¯}_L$ denote heavy fermions with no strong interactions (such as lepton and neutrino or W gaugino and photino). Compact expressions are given for the complete production/decay cross sections at the parton level, including possible multistage cascade decays.