  In the production of very heavy final states - high Mandelstam s-hat - extra
QCD radiation can play a significant role. By comparing several different
parton shower approximations to results obtained with fixed-order perturbation
theory, we quantify the degree to which these approaches agree (or disagree),
focussing on initial state radiation above pT = 50 GeV, for top pair production
at the Tevatron and at the LHC, and for SUSY pair production at the LHC.
Special attention is paid to ambiguities associated with the choice of the
maximum value of the ordering variable in parton shower models.
