The Old Electron-Positron Annihilation Routines

From the JETSET package, PYTHIA inherits routines for the dedicated simulation of two hard processes in $\mathrm{e}^+\mathrm{e}^-$ annihilation. The process of main interest is $\mathrm{e}^+\mathrm{e}^-\to \gamma^* / \mathrm{Z}^0\to \mathrm{q}\overline{\mathrm{q}}$. The description provided by the PYEEVT routine has been a main staple from PETRA days up to the LEP1 era. Nowadays it is superseded by process 1 of the main PYTHIA event generation machinery, see section . This latter process offers a better description of flavour selection, resonance shape and initial-state radiation. It can also, optionally, be used with the second-order matrix element machinery documented in this section. For backwards compatibility, however, the old routines have still been retained here. There are also a few features found in the routines in this section, and not in the other ones, such as polarized incoming beams.

For the process $\mathrm{e}^+\mathrm{e}^-\to \gamma^* / \mathrm{Z}^0\to \mathrm{q}\overline{\mathrm{q}}$, higher-order QCD corrections can be obtained either with parton showers or with second-order matrix elements. The details of the parton-shower evolution are given in section , while this section contains the matrix-element description, including a summary of the older algorithm for initial-state photon radiation used here.

The other standalone hard process in this section is $\Upsilon$ decay to $\mathrm{g}\mathrm{g}\mathrm{g}$ or $\gamma \mathrm{g}\mathrm{g}$, which is briefly commented on.

The main sources of information for this chapter are refs. [Sjö83,Sjö86,Sjö89].