In the original JETSET code, only two hard processes were available. The first and main one is . Here the `' of is used to denote that the photon must be off the mass shell. The distinction is of some importance, since a photon on the mass shell cannot decay. Of course also the can be off the mass shell, but here the distinction is less relevant (strictly speaking, a is always off the mass shell). In the following we may not always use `' consistently, but the rule of thumb is to use a `' only when a process is not kinematically possible for a particle of nominal mass. The quark in the final state of may be , , , , or ; the flavour in each event is picked at random, according to the relative couplings, evaluated at the hadronic c.m. energy. Also the angular distribution of the final pair is included. No parton-distribution functions are needed.
The other original JETSET process is a routine to generate and final states, as expected in onium 1 decays such as . Given the large top mass, toponium decays weakly much too fast for these processes to be of any interest, so therefore no new applications are expected.