Hadron-Level Standalone

The Les Houches Accord allows external process-level configurations to be fed in, for subsequent parton-level and hadron-level generation to be handled internally by PYTHIA. There is no correspondingly standardized interface if you have external events that have also been generated through the parton-level stage, so that only the hadron-level remains to be handled. A non-standard way to achieve this exists, however, and can be useful both for real applications and for various tests of the hadronization model on its own.

The key trick is to set the flag ProcessLevel:all = off. When pythia.next() is called it then does not try to generate a hard process, and therefore also cannot do anything on the parton level. Instead only the HadronLevel methods are called, to take the current content of the event record stored in pythia.event as a starting point for any hadronization and decays that are allowed by the normal parameters of this step. Often the input would consist solely of partons grouped into colour singlets, but also (colour-singlet) particles are allowed.

To set up all the parameters, a pythia.init() call has to be used, without any arguments. In brief, the structure of the main program therefore should be something like

  Pythia pythia;                               // Declare generator.
  Event& event = pythia.event                  // Convenient shorthand.
  pythia.readString("ProcessLevel:all = off"); // The trick!
  pythia.init();                               // Initialization.
  for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
    // Insert filling of event here!
    pythia.next();                             // Do the hadron level.
  }
Of course this should be supplemented by analysis of events, error checks, and so on, as for a normal PYTHIA run. The unique aspect is how to fill the event inside the loop, before pythia.next() is called.

Input configuration

To set up a new configuration the first step is to throw away the current one, with event.reset(). This routine will also reserve the zeroth entry in the even record to represent the event as a whole.

With the event.append(...) methods a new entry is added at the bottom of the current record, i.e. the first time it is called entry number 1 is filled, and so on. The append method basically exists in four variants, either without or with history information, and with four-momentum provided either as a Vec4 four-vector or as four individual components:

  append( id, status, col, acol, p, m)
  append( id, status, col, acol, px, py, pz, e, m)
  append( id, status, mother1, mother2, daughter1, daughter2, col, acol, p, m)
  append( id, status, mother1, mother2, daughter1, daughter2, col, acol, px, py, pz, e, m)
The methods return the index at which the entry has been stored, but normally you would not use this feature.

You can find descriptions of the input variables here. The PDG particle code id and the Les Houches Accord colour col and anticolour acol tags must be set correctly. The four-momentum and mass have to be provided in units of GeV; if you omit the mass it defaults to 0.

The status code can normally be simplified, however; you only need to recall that positive numbers correspond to particles that are still around, while negative numbers denote ones that already hadronized or decayed, so usually +-1 is good enough. When pythia.next() is called those positive-status particles that hadronize/decay get the sign of the status code flipped to negative but the absolute value is retained. The new particles are added with normal PYTHIA status codes.

For normal hadronization/decays in pythia.next() the history encoded in the mother and daughter indices are not used. Therefore the first two append methods, which set all these indices vanishing, should suffice. The subsequent hadronization/decays will still be properly documented.

The exception is when you want to include junctions in your string topology, i.e. have three string pieces meet. Then you must insert in your event record the (decayed) particle that is the reason for the presence of a junction, e.g. a baryon beam remnant from which several valence quarks have been kicked out, or a neutralino that underwent a baryon-number-violating decay. This particle must have as daughters the three partons that together carry the baryon number.

The sample program in main21.cc illustrates how you can work with this facility, both for simple parton configurations and for more complicated ones with junctions.