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Initial- and Final-State Radiation

Starting from the hard interaction, initial- and final-state radiation corrections may be added. This is normally done by making use of the parton-shower language -- only for the $\mathrm{e}^+\mathrm{e}^-\to \mathrm{q}\overline{\mathrm{q}}$ process does PYTHIA offer a matrix-element option (described in section [*]). The algorithms used to generate initial- and final-state showers are rather different, and are therefore described separately below, starting with the conceptually easier final-state one. Before that, some common elements are introduced.

As a further doubling-up, recently new transverse-momentum-ordered showers were introduced as an alternative to the older virtuality-ordered ones. The $p_{\perp}$-ordering offers several advantages, on its own and especially in combination with the new, more sophisticated multiple interactions scenarios described in section [*]. In the long run, the new algorithms may be the only ones to survive, but they are not yet sufficiently well established that the older can be removed; in addition, comparisons between different orderings are helpful for a better understanding of the powers and limitations of the shower approach [Ple05,Ska05]. While the newer routines are quite different in many respects, they still share a lot of the philosophy of the older ones. Therefore it is feasible to give a reasonably detailed presentation of the old formalisms and only provide a brief summary of the main differences introduced by the $p_{\perp}$-ordering.

The main references for virtuality-ordered final-state showers are [Ben87a,Nor01] and for ditto initial-state ones [Sjö85,Miu99], while the transverse-momentum-ordered showers of both kinds are described in [Sjö04a].



Subsections
next up previous contents
Next: Shower Evolution Up: LaTeX2HTML Test Document Previous: Other Routines and Common   Contents
Stephen_Mrenna 2012-10-24