## Hard 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 some 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 Les Houches Event Files, 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 multiparton 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.
Soft processses contain elastic, diffractive and "minimum-bias" events, together covering the total cross section.
Reserved code range: 101 - 110.
Hard processes are the normal 2 → 2 ones, including charm and bottom production (plus an incomplete implementation of 2 → 3 processes). They are contained as a subset of the soft processes via the MultiParton Interactions (MPI) mechanism.
Reserved code range: 111 - 150.
The Low-energy processes, primarily used for collision energies below 10 GeV, closely mirror the soft processes, but without any MPIs to offer an admixture of hard processes. Other kinds, such resonant and annihilation processes, instead are needed at low energies.
Reserved code range: 151 - 160.

### 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. See section on Left-Right-Symmetry processes for doubly charged Higgs bosons.
Reserved code range: 901 - 999 for a Standard Model Higgs and 1001 - 1199 for MSSM Higgs bosons.

### SUSY Processes

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

### New-Gauge-Boson Processes

Production of new gauge bosons such as Z' and W'.
Reserved code range: 3001 - 3099.

### Left-Right-Symmetry Processes

Production of righthanded Z_R and W_R bosons and of doubly charged Higgs bosons.
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 technicolor; see below).
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.

### Hidden Valley Processes

A scenario for the pair production of new particles with couplings under a new gauge group, with invisible gauge bosons. Radiation of these gauge bosons is included in the standard final-state parton shower.
Reserved code range: 4901 - 4999.

### Extra-Dimensional Processes

A vast area, here represented by the production of a Randall-Sundrum excited graviton state and a Kaluza-Klein gluon, a Kaluza-Klein tower of gamma/Z^0 excitations in one TeV^- sized extra dimension, several Large Extra Dimension processes, and a few related Unparticle processes.
Reserved code range: 5001 - 5099.

### Dark Matter Processes

An area of increasing interest. Currently only represented by a few basic processes.
Reserved code range: 6001 - 6099.