Pythia Information for CDF Run II

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 version is 6_216, this is based on 6_214 with an important bug fix implemented at Fermilab. However, version 6_226 is now the default in development and will be used for releases after 6.1.4. Version 6_203 is also available, copies of the decay tables are provided in package pythia_i/doc and can be viewed from the Code Browser.

Version 6_406 is now available but has not been implemented in the development source release. It provides a new treatment for the multiple interactions which is more complete than earlier versions. However, it was felt that it could interfere with current studies, but people are encouraged to try it and a prescription to use it is provided below. Note versions of Pythia 6.3 are available but are not now recommended.

Earlier Pythia versions, 6_1xx, can only be used with releases before 4.4.0. Different decay tables and common block structures are used and they are not now recommended.

There have been a lot of changes since the versions of Pythia used in Run 1 and parameter definitions have changed. As a result tunings obtained for Run 1 are not necessarily correct. A lot of work is being carried out by the QCD group tuning the fragmentation parameters, in particular look at Rick Field's talks for details on current tunings.

A new miniguide is now available including a tutorial for new users.

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
PythiaMenu
msel set 1 Main process (default = 1)
cmEnergy set 2000. Centre of Mass energy (default = 2000.GeV)
inListLevel set 11 Initial print level (see PYLIST) (default = 11)
statListLevel set 1 Statistics print level (see PYSTAT) (default = 1)
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)
decayFileMode set read(write) Read in (write out) decay table with PYUPDA (default = no file )
decayFile set file.name Decay table file (default = none)
pygiveFile set file.name PYGIVE file - to modify parameters in commons not accessed as below(default = none)
exit

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_mdcy -masscode=443 -decay=0
set_pmas -masscode=6 -mass=175.
Details of these commands can be found by typing help from the sub-menu.

Note: PMAS (and MDCY) is accessed using the PDG masscode (KF), MDME is accessed using the array index in the same way as MSTP,etc. To find the correct index for MDME it is necessary to inspect the relevant table in the CDF pythia area page.

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. The first (Pythia_top.tcl) is a full tcl file for a sample job including the using of RandomGenManager to change seeds, the others are tcl fragments for Pythia only. Just now there is

Pythia_top1.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

Pythia_b.tcl b 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() ).

Pysubs
- int& msub(int n)
- int& kfin(int iside, int jflav)
- double& ckin(int n)
Pypars
- int& mstp(int n)
- double& parp(int n)
- int& msti(int n)
- double& pari(int n)
Pydat1
- int& mstu(int n)
- double& paru(int n)
- int& mstj(int n)
- double& parj(int n)
Pydat2
- int& kchg(int kc, int i) kc - compressed particle code (default = 19780503)
- double& pmas(int kc, int i)
- double& parf(int n)
- double& vckm(int i, int j)
Pydat3
- int& mdcy(int kc, int i)
- int& mdme(int idc, int i)
- double& brat(int idc)
- int& kfdp(int idc, int kf)
Pydatr
- int& mrpy(int n)
- double& rrpy(int n)
Pyint2
- int& iset(int isub)
- int& kfpr(int isub, int i)
- double& coef(int isub, int i)
Pyjets
partial access only
- int& njet()
- int& kjet(int jet, int var)
Pymssm
- int& imss(int n)
- double& rmss(int n)
Pyssmt
- const double& zmix(int i, int j) const
- const double& umix(int i, int j) const
- const double& vmix(int i, int j) const
- const double& smz(int i) const
- const double& smw(int i) const
- const double& sfmix(int i, int j) const
Other commons are not accessed in this manner since they should not in general be modified by the ordinary user. Parameters in many of these commons can be modified using PYGIVE. To use this method create a file setting the parameters and input it using the pygiveFile switch above, an example of such a file modifying ITCM(5) in /PYTCSM/ is
```
  MSEL=0                  ! select process via MSUB
  MSUB(381)=1             ! new equivalent to process 11
  MSUB(382)=1             ! any comment
  MSUB(383)=1
  MSUB(384)=1
  MSUB(385)=1
  MSUB(386)=1
  ITCM(5)=1               ! previously set with MSTP 
```

Using Pythia 6.406
To use Pythia version 6.406 setup cdfsoft2 development and setup the pythia package (the flavour option -f is not normally necessary)

setup pythia v6_406 -q GCC3_4_3 -f Linux+2.4-2.3.2

Create a test release, check out the package generatorMods and build the binary cdfGen as normal. If pythia is not setup the standard version of Lund using Pythia 6.226 will be used. Program versions

Pythia 6.216 (Fermi details)

Stdhep 5.05

Cernlib 2004

Pdflib 8.04

LHAPDF replacing Pdflib in development

Further reading

On Using Pythia to Model the Underlying Event in B and W events. P.Sphicas and D.Vucinic. CDF note 4097.

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

Stan Thompson

Last updated: May 29 2003