PYTHIA - CDF Specific Information

Stan Thompson Glasgow University

Chris Green Purdue University


Pythia is a multi purpose event generator combining many different hard processes with fragmentation based principally on the string model. The current supported version is 6_129, this includes many new processes and bug fixes since the last supported version (6_115). In addition, the decay table has changed, please check this if your process involves changing decay parameters

Pythia itself is controlled by sets of switches and parameters defined in Commons, default values are loaded at program initialisation and are reset as necessary before the generation procedure is initialised. In the CDF implementation these values are set using a talk-to menu with a special command structure (CG) allowing direct access to the relevant array element in common.

The main switches in the talk-to environment are detailed below:
mod talk Pythia
MSEL set 1 Main process (default = 1)
CMEnergy set 2000. Centre of Mass energy (default = 2000.GeV)
Inlistlevel set 0 Initial print level (see LULIST) (default = 11)
Evlistlevel set 0 Event print level (see LULIST) (default = 1)
Listfirst set 1 First event to print (default = 1)
Listlast set 1 Last event to print (default = 1)
SEED set 1 Input random seed (default = 19780503)
decayFileMode set read(write) Read in (write out) decay table with PYUPDA (default = no file )
decayFile set file.name Decay table file (default = none)
output set HEPG

Other parameters are set in the sub-menu commonMenu, those stored in a one dimensional array are set giving the index in the array and the value, thus
set_mstp -index=2 -value=2
set_parj -index=55 -value=-0.006
More information is needed for those parameters accessed via 2 dimensional arrays, thus to change a decay table entry or mass the following is used
set_mdme -channelIndex=182 -decayType=1
set_pmas -masscode=6 -mass=175.
Details of these commands can be found by typing help from the sub-menu.

We are building up a set of examples in $CDFSOFT2_DIR/pythia_i/test to indicate what sort of hard processes can be simulated and including a set of tuned fragmentation parameters. Just now there is

  • Pythia_top.tcl top production including the use of structure functions.
  • Pythia_top2.tcl top production re-setting the mass and with other options.
  • Pythia_qq.tcl QCD jet production with a tuned set of fragmentation parameters.
  • Pythia_minbias.tcl Minimum bias processes
  • Pythia_w.tcl Single W production
  • Pythia_psimumu.tcl J/psi production limiting decays to mumu
  • Pythia_Zee.tcl single Z production, Z -> ee
  • Pythia_Zee_clean.tcl single Z production, Z -> ee in restricted pseudorapidity range
  • Pythia_stop.tcl SUSY s-top production

    • However, these can only give a limited indication of what is available, for a fuller understanding we recommend reading the manual.

    Access Methods 

     The Pythia commons are accessed through a corresponding class. Each class provides a set of methods giving read and write (PYSSMT is read-only) access to the arrays in the common. All accesses are FORTRAN style and are range checked , the length (and depth) of an array can be accessed by a method, for example int lenPmas() (depthPmas() ).

    Program versions 
  • Pythia 6.129
  • Stdhep 4.05
  • Cernlib 97a
  • Pdflib 7.07

    • Further reading
  • Studies of Quantum Chromodynamics with the ALEPH Detector (ps). Physics Reports 294 (1998) 1. Includes tuning of the hadronisation parameters
  • Tuning and Test of Fragmentation Models Based on Identified Particles and Precision Event Shape Data (Delphi) (ps). Zeit. Phys. C73 (1996) 11

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    Stan Thompson


    Jan  30 1999