Multiple Interactions and Beam Remnants -- New Model

A new multiple interactions scenario is introduced with PYTHIA 6.3 [Sjö03a]. The early versions still represent a tryout phase, where 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 hadron-hadron collisions. It cannot be used with pile-up, for example, nor with SUSY events where baryon number is violated.

The basic scheme for the generation of the multiple interactions is kept from the old models. That is, based on the standard $2 \to 2$ QCD matrix elements convoluted with standard parton densities, a sequence of $p_{\perp}$-ordered interactions is generated: $p_{\perp 1} > p_{\perp2} > p_{\perp3} > \ldots$ . The sequence is stopped at some lower cut-off scale $p_{\perp min}$, or alternatively the matrix elements are damped smoothly around and below a scale $p_{\perp 0}$. An impact-parameter-dependent picture of hadron-hadron interactions can lead to further elements of variability between events.

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 $p_{\perp}$, had to be substantially simplified. The introduction of junction fragmentation in [Sjö03] allowed this restriction to be lifted.