Partial Generation

Sometimes it may be convenient to omit certain aspects of the event generation chain. This cannot be motivated in a full-blown production run, but can often be convenient for own understanding and for debug purposes. The flags on this page allow just that.

The event generation is subdivided into three levels: the process level, the parton level and the hadron level, and flags are grouped accordingly.

Process Level

The ProcessLevel class administrates the initial step of the event generation, wherein the basic process is selected. Currently this is done either using some of the internal processes, or with Les Houches Accord input, or from the Fortran PYTHIA 6.4 code, the latter also using the Les Houches Accord standard. Since there cannot be any event at all without an initial process, there is no possibility to switch off this part of the story. It is possible, however, to stop the generation immediately after the basic process has been selected:

flag name="Pythia:afterProcessLevel" default="on"
If off then stop the generation after the hard process has been generated, but before the parton-level and hadron-level steps. The process record is filled, but the event one is not.

PartonLevel

The PartonLevel class administrates the middle step of the event generation, i.e. the evolution from an input (hard) process from ProcessLevel, containing a few partons only, to a complete parton-level configuration to be handed on to HadronLevel. This step involves the application of initial- and final-state radiation, multiple interactions and the structure of beam remnants.

Some parts of the event generation on this level may be switched off individually:

flag name="PartonLevel:MI" default="on"
Master switch for multiple interactions; on/off = true/false. Further options are found here.

flag name="PartonLevel:ISR" default="on"
Master switch for initial-state radiation; on/off = true/false. Further options are found here.

flag name="PartonLevel:FSRinProcess" default="on"
Master switch for final-state radiation in association with the hard process itself; on/off = true/false. Further options are found here.

flag name="PartonLevel:FSRinResonances" default="on"
Master switch for final-state radiation in any resonance decays subsequent to the hard process itself; on/off = true/false. Further options are found here.

It is also possible to stop the generation immediately after this step:

flag name="Pythia:afterPartonLevel" default="on"
If off then stop the generation after the hard process and parton-level activity has been generated, but before the hadron-level steps.

HadronLevel

The HadronLevel class administrates the final step of the event generation, wherein the partonic configuration from PartonLevel is hadronized, including string fragmentation and secondary decays.

Most of the code in this class deals with subdividing the partonic content of the event into separate colour singlets, that can be treated individually by the string fragmentation machinery. When a junction and an antijunction are directly connected, it also breaks the string between the two, so that the topology can be reduced back to two separate one-junction systems, while still preserving the expected particle flow in the junction-junction string region(s).

Some parts of the event generation on this level may be switched off individually:

flag name="HadronLevel:Hadronize" default="on"
Master switch for hadronization; on/off = true/false. Further options are found here.

flag name="HadronLevel:Decay" default="on"
Master switch for decays; on/off = true/false. Further options are found here.