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Initial-State Showers

The initial-state shower algorithms in PYTHIA are not quite as sophisticated as the final-state ones. This is partly because initial-state radiation is less well understood theoretically, and partly because the programming task is more complicated and ambiguous. Still, the program at disposal is known to do a reasonably good job of describing existing data, such as $\mathrm{Z}^0$ production properties at hadron colliders [Sjö85]. It can be used both for QCD showers and for photon emission off leptons ($\mathrm{e}$, $\mu$ or $\tau$; relative to earlier versions, the description of incoming $\mu$ and $\tau$ are better geared to represent the differences in lepton mass, and the lepton-inside-lepton parton distributions are properly defined).

Again we begin with a fairly model-independent overview before zooming in on the old virtuality-ordered algorithm implemented in PYSSPA. The new transverse-momentum-ordered formalism in PYPTIS, described at the end, shares much of the same philosophy, apart from the quite important choice of evolution variable, of course.



Subsections
next up previous contents
Next: The shower structure Up: Initial- and Final-State Radiation Previous: A new -ordered final-state   Contents
Stephen Mrenna 2007-10-30