A new description of multiple interactions and beam remnants is introduced with PYTHIA 6.3 [Sjö04]. It is more sophisticated than the old one, but unfortunately as a consequence less robust. Therefore extra caution should be exercised. Currently it should only be used for the study of single minimum-bias events or events underlying hard processes in or collisions. It is not fully developed and tested for meson or photon beams, and cannot be used with pile-up events, or with baryon-number-violating SUSY events.
The basic scheme for the generation of the multiple interactions is kept from the old models. That is, based on the standard QCD matrix elements convoluted with standard parton densities, a sequence of -ordered interactions is generated: . The sequence is stopped at some lower cut-off scale , or alternatively the matrix elements are damped smoothly around and below a scale . An impact-parameter-dependent picture of hadron-hadron interactions can lead to further elements of variability between events. See above for details on the basic framework.
The main limitation of the old approach is that there was no way to handle complicated beam remnants, e.g. where two valence quarks had been kicked out. Therefore the structure of all interactions subsequent to the first one, i.e. the one with largest , had to be substantially simplified. The introduction of junction fragmentation in [Sjö03] allowed this restriction to be lifted.