Process Selection

There is no way PYTHIA could contain all processes of interest, neither in terms of potential physics topics nor in terms of high-multiplicity final states. What exists is a reasonably complete setup of all 2 -> 1 and 2 -> 2 processes within the Standard Model, plus a few examples of processes beyond that, again for low multiplicities. Combined with the PYTHIA parton showers, this should be enough to get a flying start in the study of many physics scenarios. Other processes could be fed in via the Les Houches Accord or be implemented as a Semi-Internal Process. In the latter case the existing processes would act as obvious templates.

By default all processes are switched off. You should switch on those you want to simulate. This may be done at two (occasionally three) levels, either for each individual process or for a group of processes. That is, a process is going to be generated either if its own flag or its group flag is on. There is no built-in construction to switch on a group and then switch off a few of its members.

Each process is assigned an integer code. This code is not used in the internal administration of events (so having the same code for two completely different processes would not be a problem), but only intended to allow a simpler user separation of different processes. Also the process name is available, as a string.

To ease navigation, the list of processes has been split into several separate pages, by main topic. The classification is hopefully intuitive, but by no means unambiguous. For instance, essentially all processes involve QCD, so the "QCD processes" are the ones that only involve QCD. (And also that is not completely true, once one includes all that may happen in multiple interactions.) On these separate pages also appear the settings that are completely local to that particular process class, but not the ones that have a broader usage.

QCD Processes

QCD processes fall in two main categories: soft and hard. The soft ones contain elastic, diffractive and "minimum-bias" events, together covering the total cross section. Hard processea are the normal 2 -> 2 ones, including charm and bottom production.
Reserved code range: 101 - 199.

Electroweak Processes

Prompt-photon, gamma^*/Z^0 and W^+- production, plus a few processes with t-channel boson exchange.
Reserved code range: 201 - 299.

Onia Processes

Colour singlet and octet production of charmonium and bottomonium.
Reserved code range: 401 - 499 for charmonium and 501 - 599 for bottomonium.

Top Processes

Top production, singly or doubly.
Reserved code range: 601 - 699.

Fourth-Generation Processes

Production of hypothetical fourth-generation fermions.
Reserved code range: 801 - 899.

Higgs Processes

Higgs production, within or beyond the Standard Model. The former part is finished, the latter under development.
Reserved code range: 901 - 999 for a Standard Model Higgs and 1001 - 1199 for MSSM Higgses.

SUSY Processes

Production of supersymmetric particles, currently barely begun.
Reserved code range: 1001 - 2999. (Whereof 1001 - 1199 for Higgses; see above.)

New-Gauge-Boson Processes

Production of new gauge bosons such as Z' and W'. Does not exist yet.
Reserved code range: 3001 - 3099.

Left-Right-Symmetry Processes

Production of righthanded Z_R and W_R bosons and of doubly charged Higgses. Does not exist yet.
Reserved code range: 3101 - 3199.

Leptoquark Processes

Production of a simple scalar leptoquark state.
Reserved code range: 3201 - 3299.

Compositeness Processes

Production of excited fermion states and contact-interaction modification to interactions between fermions (excluding tecnicolor; see below). Does not exist yet.
Reserved code range: 4001 - 4099.

Technicolor Processes

Production of technicolor particles and modifications of QCD processes by technicolor interactions. Does not exist yet.
Reserved code range: 4101 - 4199.

Extra-Dimensional Processes

A vast area, here represented by the production of a Randall-Sundrum excited graviton state.
Reserved code range: 5001 - 5099.

Resonance Decays and Cross Sections

In addition to the switches and parameters in the process machinery there also exists the possibility to set the allowed decay channels of resonances, as explained in the Particle Data Scheme description. For instance, if you study the process q qbar -> H^0 Z^0 you could specify that the Z^0 should decay only to lepton pairs, the H^0 only to W^+ W^-, the W^+ only to a muon and a neutrino, while the W^- can decay to anything. Unfortunately there are limits to the flexibility: you cannot set a resonance to have different properties in different places of a process, e.g. if instead H^0 -> Z^0 Z^0 in the above process then the three Z^0's would all obey the same rules.

The restrictions on the allowed final states of a process is directly reflected in the cross section of it. That is, if some final states are excluded then the cross section is reduced accordingly. Such restrictions are built up recursively in cases of sequential decay chains. The restrictions are also reflected in the compositions of those events that actually do get to be generated. For instance, the relative rates of H^0 -> W^+ W^- and H^0 -> Z^0 Z^0 are shifted when the allowed sets of W^+- and Z^0 decay channels are changed.

There is one important restriction, however: only those particles that Pythia treat as resonances enjoy this property. This includes the W^+-, gamma^*/Z^0, t/tbar, H^0 and other heavy unstable particles in scenarios of Beyond-the-Standard-Model physics. These particles are generated and let to decay as part of the "process level" processing, which is where cross sections are handled. It does not concern particle that are produced and/or decay at later stages, such as B mesons or tau leptons, or photons that branch as part of the shower evolution. There simply would be no way consistently to include the proper bias that should go with changed branching ratios. For instance, if you only are interested in a specific tau decay channel, this tau could come from the decay of a B meson that came from a b quark produced in the shower evolution, g -> b bbar, and thus be many steps removed from the hard process itself.