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##

Beam Remnants (and Multiple Interactions) -- Intermediate Model

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.

**Subsections**

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Stephen Mrenna
2007-10-30