*** PYTHIA in PISA ***                          5/19/2000 Y.Goto
                                       revised 11/24/2004 Y.Goto

PYTHIA has been implemented in PISA by Andrew Rose.
I describe here how to make, run and use it on Red Hat Linux machine.
We need 3 steps to use it.
The first step is to make a PYTHIA program 'pythia', the second step
is to generate a PYTHIA file 'pythia.dat', and the third step is to
read the 'pythia.dat' in PISA.

* How to make a PYTHIA program 'pythia'

Source files to make a program 'pythia' can be obtained from the
PHENIX CVS repository by;
        % cvs checkout event_gen
The event_gen directory includes;
        src/pythia/gen_pythia.f    - source file
        inc/pythia/gen_pythia.inc  - include files
        inc/pythia/gcflag.inc
        inc/pythia/guevgen.inc
        lib/pythia.mk              - makefile
        lib/linux/pythialink.linux - file used in link time
In order to make the program, extra object files, structm.o and
pdfset.o, are necessary.
They can be obtained by extracting them from CERN library,
/cern/pro/lib/libpdflib804.a.
You can do it by;
        % ar x /cern/pro/lib/libpdflib804.a structm.o
        % ar x /cern/pro/lib/libpdflib804.a pdfset.o
They need to be put as;
        lib/linux/structm.o
        lib/linux/pdfset.o
After setting some environment variables to point;
        EVENT_GEN -> your 'event_gen' directory
        CERNLIB   -> /cern/pro/lib
you are ready to make the program.
At the 'lib' directory, you can type;
        % make -f pythia.mk
and you will get;
        bin/linux/pythia
You can execute the program at the 'bin' directory, and generate a
PYTHIA file, 'pythia.dat'.
The program file 'gen_pythia.f' is a very simple example to generate
PYTHIA event without any cut.
You can modify this file and make your own program to include some
kinematical and/or detector cut, etc.

* How to generate a PYTHIA file 'pythia.dat'

To generate the file 'pythia.dat', you need to prepare a file;
        bin/pythia.par
to specify running conditions of the PYTHIA simulation.
This is one example;
----
 $pyth_par
  tnumevt =  250,
  imsel   =  1,
  imstp   =  52,2,
             51,5005,
  ickin   =  3,10,
             4,20,
  imrlu   =  1,500,
  sqrts   =  200.,
  verbose =.true.,
  xverbose=.false.,
  zebrastore=.false.,
  hist_file='pythia.hist'
 $end
----
Each condition specifies;
  tnumevt =  250,        - number of event
  imsel   =  1,          - MSEL process ID - MSEL=1
  imstp   =  52,2,       - MSTP variables  - MSTP(52)=2
             51,5005,                      - MSTP(51)=5005
  ickin   =  3,10,       - CKIN variables  - CKIN(3)=10
             4,20,                         - CKIN(4)=20
  imrlu   =  1,500,      - MRLU variables  - MRLU(1)=500
  sqrts   =  200.,       - SQRT(s)
MSEL, MSTP, CKIN, MRLU are variables in PYTHIA.
Please refer to the official PYTHIA manual in detail.
You can get it from the official web site;
        http://www.thep.lu.se/~torbjorn/Pythia.html
For instance, we have interest in;
        MSEL=1  - QCD jets
        MSEL=2  - minimum bias
        MSEL=10 - prompt photon production
MSTP(52)=2 specifies to use libpdflib804.a as a parton distribution function
library instead of internal one in the PYTHIA library and
MSTP(51)=5005 specifies the GRV94LO parton distribution function.
CKIN(3) and CKIN(4) specify minimum and maximum parton p_T values.
MRLU(1) specifies a seed of the random number generator.
Please refer to PYTHIA and PDFLIB manual in detail.

* How to read the PYTHIA file 'pythia.dat' in PISA

I assume you know how to run the PISA program.
Place your 'pythia.dat' file in your current directory and execute
PISA.
At the PISA prompt, for example, you can do;
        GEANT> pythia 100
        GEANT> ptrig  250