PYTHIA 6.4 Physics and Manual

2 Physics OverviewIn this section we will try to give an overview of the main physics features of Pythia, andalso to introduce some terminology. The details will be discussed in subsequent sections.For the description of a typical high-energy event, an event generator should containa simulation of several physics aspects. If we try to follow the evolution of an event insome semblance of a time order, one may arrange these aspects as follows:1. Initially two beam particles are coming in towards each other. Normally each particleis characterized by a set of parton distributions, which defines the partonicsubstructure in terms of flavour composition and energy sharing.2. One shower initiator parton from each beam starts off a sequence of branchings,such as q → qg, which build up an initial-state shower.3. One incoming parton from each of the two showers enters the hard process, wherethen a number of outgoing partons are produced, usually two. It is the nature ofthis process that determines the main characteristics of the event.4. The hard process may produce a set of short-lived resonances, like the Z 0 /W ± gaugebosons, whose decay to normal partons has to be considered in close association withthe hard process itself.5. The outgoing partons may branch, just like the incoming did, to build up final-stateshowers.6. In addition to the hard process considered above, further semihard interactions mayoccur between the other partons of two incoming hadrons.7. When a shower initiator is taken out of a beam particle, a beam remnant is leftbehind. This remnant may have an internal structure, and a net colour charge thatrelates it to the rest of the final state.8. The QCD confinement mechanism ensures that the outgoing quarks and gluons arenot observable, but instead fragment to colour neutral hadrons.9. Normally the fragmentation mechanism can be seen as occurring in a set of separatecolour singlet subsystems, but interconnection effects such as colour rearrangementor Bose–Einstein may complicate the picture.10. Many of the produced hadrons are unstable and decay further.Conventionally, only quarks and gluons are counted as partons, while leptons andphotons are not. If pushed ad absurdum this may lead to some unwieldy terminology. Wewill therefore, where it does not matter, speak of an electron or a photon in the ‘partonic’substructure of an electron, lump branchings e → eγ together with other ‘parton shower’branchings such as q → qg, and so on. With this notation, the division into the aboveten points applies equally well to an interaction between two leptons, between a leptonand a hadron, and between two hadrons.In the following sections, we will survey the above ten aspects, not in the same orderas given here, but rather in the order in which they appear in the program execution, i.e.starting with the hard process.2.1 Hard Processes and Parton DistributionsIn the original Jetset code, only two hard processes were available. The first and mainone is e + e − → γ ∗ /Z 0 → qq. Here the ‘∗’ of γ ∗ is used to denote that the photon must beoff the mass shell. The distinction is of some importance, since a photon on the mass shellcannot decay. Of course also the Z 0 can be off the mass shell, but here the distinction isless relevant (strictly speaking, a Z 0 is always off the mass shell). In the following we maynot always use ‘∗’ consistently, but the rule of thumb is to use a ‘∗’ only when a process isnot kinematically possible for a particle of nominal mass. The quark q in the final stateof e + e − → γ ∗ /Z 0 → qq may be u, d, s, c, b or t; the flavour in each event is picked at10