When I only allow a subset of the Z0 decay channels the cross section for Z0 production is automatically rescaled accordingly. Why is the cross section not rescaled when I study prompt J/Psi production, and only allow it to decay to leptons?

A J/Psi can be produced many different ways. For instance, you could start out with a normal QCD jet process, where the showers give a g -> b bbar branching. After hadronization a B meson is formed that then decays to the J/Psi. Here it is NOT possible to know beforehand that a J/Psi will be produced, and therefore it is not possible to take the choice of allowed J/Psi decay modes into account in cross-secion calculations. Therefore there is a clear dividing line in the program: - all particles up to and including Upsilon systems can occur "at random", and therefore a change of branching ratios is NOT propagated to the process cross section; - all particles above this, i.e. W, Z, top, Higgs, SUSY?, ..., occur only as part of the hard process (with perturbatively calculable branching ratios) and so changes in the choice of allowed channels IS propagated to the process cross section.

For a specific process like g g -> g J/Psi it would still have been possible to let the program rescale the cross section based on allowed J/Psi decay channels, but then one would be faced with the J/Psi having different consequences when produced in different channels. This would be even more confusing than the current rule.