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
In the latter case the existing processes would act as obvious
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
- QCD Processes
- Electroweak Processes
- Onia Processes
- Top Processes
- Higgs Processes
- SUSY Processes
- Leptoquark Processes
- Compositeness Processes
- Technicolor Processes
- Hidden Valley Processes
- Dark Matter Processes
QCD processes fall in two main categories: soft and hard.
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
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.
Prompt-photon, gamma^*/Z^0 and W^+- production,
plus a few processes with t-channel boson exchange.
Reserved code range: 201 - 299.
Colour singlet and octet production of charmonium and bottomonium.
Reserved code range: 401 - 499 for charmonium and
501 - 599 for bottomonium.
Top production, singly or doubly.
Reserved code range: 601 - 699.
Production of hypothetical fourth-generation fermions.
Reserved code range: 801 - 899.
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.
Production of supersymmetric particles, currently barely begun.
Reserved code range: 1001 - 2999. (Whereof 1001 - 1199
for Higgs bosons; see above.)
Production of new gauge bosons such as Z' and W'.
Reserved code range: 3001 - 3099.
Production of righthanded Z_R and W_R bosons and of
doubly charged Higgs bosons.
Reserved code range: 3101 - 3199.
Production of a simple scalar leptoquark state.
Reserved code range: 3201 - 3299.
Production of excited fermion states and contact-interaction modification
to interactions between fermions (excluding technicolor; see below).
Reserved code range: 4001 - 4099.
Production of technicolor particles and modifications of QCD processes
by technicolor interactions. Does not exist yet.
Reserved code range: 4101 - 4199.
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
Reserved code range: 4901 - 4999.
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
Reserved code range: 5001 - 5099.
An area of increasing interest. Currently only represented by
a few basic processes.
Reserved code range: 6001 - 6099.