Physics Overview

In this section we will try to give an overview of the main physics features of PYTHIA, and also 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 contain a simulation of several physics aspects. If we try to follow the evolution of an event in some 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 particle is characterized by a set of parton distributions, which defines the partonic substructure in terms of flavour composition and energy sharing.

2.

One shower initiator parton from each beam starts off a sequence of branchings, such as $\mathrm{q}\to \mathrm{q}\mathrm{g}$, which build up an initial-state shower.

3.

One incoming parton from each of the two showers enters the hard process, where then a number of outgoing partons are produced, usually two. It is the nature of this process that determines the main characteristics of the event.

4.

The hard process may produce a set of short-lived resonances, like the $\mathrm{Z}^0/\mathrm{W}^{\pm}$ gauge bosons, whose decay to normal partons has to be considered in close association with the hard process itself.

5.

The outgoing partons may branch, just like the incoming did, to build up final-state showers.

6.

In addition to the hard process considered above, further semihard interactions may occur between the other partons of two incoming hadrons.

7.

When a shower initiator is taken out of a beam particle, a beam remnant is left behind. This remnant may have an internal structure, and a net colour charge that relates it to the rest of the final state.

8.

The QCD confinement mechanism ensures that the outgoing quarks and gluons are not observable, but instead fragment to colour neutral hadrons.

9.

Normally the fragmentation mechanism can be seen as occurring in a set of separate colour singlet subsystems, but interconnection effects such as colour rearrangement or 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 and photons are not. If pushed ad absurdum this may lead to some unwieldy terminology. We will therefore, where it does not matter, speak of an electron or a photon in the `partonic' substructure of an electron, lump branchings $\mathrm{e}\to \mathrm{e}\gamma$ together with other `parton shower' branchings such as $\mathrm{q}\to \mathrm{q}\mathrm{g}$, and so on. With this notation, the division into the above ten points applies equally well to an interaction between two leptons, between a lepton and a hadron, and between two hadrons.

In the following sections, we will survey the above ten aspects, not in the same order as given here, but rather in the order in which they appear in the program execution, i.e. starting with the hard process.