Initial-State Showers

The initial-state shower algorithm in PYTHIA is not quite as sophisticated as the final-state one. This is partly because initial-state radiation is less well understood theoretically, 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).