$\mathrm{e}^+\mathrm{e}^-$ continuum event generation

The only routine a normal user will call to generate $\mathrm{e}^+\mathrm{e}^-$ continuum events is PYEEVT. The other routines listed below, as well as PYSHOW (see section ), are called by PYEEVT.

Purpose:

to generate a complete event $\mathrm{e}^+\mathrm{e}^-\to \gamma^* / \mathrm{Z}^0\to \mathrm{q}\overline{\mathrm{q}}\to$ parton shower $\to$ hadrons according to QFD and QCD cross sections. As an alternative to parton showers, second-order matrix elements are available for $\mathrm{q}\overline{\mathrm{q}}+ \mathrm{q}\overline{\mathrm{q}}\mathrm{g}+ \ma... ...{g}\mathrm{g}+ \mathrm{q}\overline{\mathrm{q}}\mathrm{q}'\overline{\mathrm{q}}'$ production.

KFL :

flavour of events generated.

= 0 :

mixture of all allowed flavours according to relevant probabilities.

= 1 - 8 :

primary quarks are only of the specified flavour KFL.

ECM :

total c.m. energy of system.

Remark:

Each call generates one event, which is independent of preceding ones, with one exception, as follows. If radiative corrections are included, the shape of the hard photon spectrum is recalculated only with each PYXTEE call, which normally is done only if KFL, ECM or MSTJ(102) is changed. A change of e.g. the $\mathrm{Z}^0$ mass in mid-run has to be followed either by a user call to PYXTEE or by an internal call forced e.g. by putting MSTJ(116)=3.

SUBROUTINE PYXTEE(KFL,ECM,XTOT) :

to calculate the total hadronic cross section, including quark thresholds, weak, beam polarization, and QCD effects and radiative corrections. In the process, variables necessary for the treatment of hard photon radiation are calculated and stored.

KFL, ECM :

as for PYEEVT.

XTOT :

the calculated total cross section in nb.

SUBROUTINE PYRADK(ECM,MK,PAK,THEK,PHIK,ALPK) :

to describe initial-state hard $\gamma$ radiation.

SUBROUTINE PYXKFL(KFL,ECM,ECMC,KFLC) :

to generate the primary quark flavour in case this is not specified by you.

SUBROUTINE PYXJET(ECM,NJET,CUT) :

to determine the number of jets (2, 3 or 4) to be generated within the kinematically allowed region (characterized by CUT $= y_{\mathrm{cut}}$) in the matrix-element approach; to be chosen such that all probabilities are between 0 and 1.

SUBROUTINE PYX3JT(NJET,CUT,KFL,ECM,X1,X2) :

to generate the internal momentum variables of a 3-jet event, $\mathrm{q}\overline{\mathrm{q}}\mathrm{g}$, according to first- or second-order QCD matrix elements.

SUBROUTINE PYX4JT(NJET,CUT,KFL,ECM,KFLN,X1,X2,X4,X12,X14) :

to generate the internal momentum variables for a 4-jet event, $\mathrm{q}\overline{\mathrm{q}}\mathrm{g}\mathrm{g}$ or $\mathrm{q}\overline{\mathrm{q}}\mathrm{q}' \overline{\mathrm{q}}'$, according to second-order QCD matrix elements.

SUBROUTINE PYXDIF(NC,NJET,KFL,ECM,CHI,THE,PHI) :

to describe the angular orientation of the jets. In first-order QCD the complete QED or QFD formulae are used; in second order 3-jets are assumed to have the same orientation as in first, and 4-jets are approximated by 3-jets.