example_MyPythia.cxx

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// Matt.Dobbs@Cern.CH, December 1999
// November 2000, updated to use Pythia 6.1
// example of generating events with Pythia
// using HepMC/PythiaWrapper.h 
// Events are read into the HepMC event record from the FORTRAN HEPEVT 
// common block using the IO_HEPEVT strategy and then output to file in
// ascii format using the IO_Ascii strategy.
// To Compile: go to the HepMC directory and type:
// gmake examples/example_MyPythia.exe
//
// In this example the precision and number of entries for the HEPEVT 
// fortran common block are explicitly defined to correspond to those 
// used in the Pythia version of the HEPEVT common block. 
//
// If you get funny output from HEPEVT in your own code, probably you have
// set these values incorrectly!
//

#include <iostream>
#include "HepMC/PythiaWrapper.h"
#include "HepMC/IO_HEPEVT.h"
#include "HepMC/IO_GenEvent.h"
#include "HepMC/GenEvent.h"
    
int main() { 
    //
    //........................................HEPEVT
    // Pythia 6.1 uses HEPEVT with 4000 entries and 8-byte floating point
    //  numbers. We need to explicitly pass this information to the 
    //  HEPEVT_Wrapper.
    //
    HepMC::HEPEVT_Wrapper::set_max_number_entries(4000);
    HepMC::HEPEVT_Wrapper::set_sizeof_real(8);
    //
    //........................................PYTHIA INITIALIZATIONS
    // (Some platforms may require the initialization of pythia PYDATA block 
    //  data as external - if you get pythia initialization errors try 
    //  commenting in/out the below call to initpydata() )
    // initpydata();
    //
    // Select W+gamma process (process number 20) 
    // (here we have to be careful of C/F77 differences: arrays in C 
    //  start at 0, F77 at 1, so we need to subtract 1 from the process #)
    pysubs.msel=0;
    pysubs.msub[20-1] = 1;
    // set random number seed (mandatory!)
    pydatr.mrpy[0]=55122 ;
    // Tell Pythia not to write multiple copies of particles in event record.
    pypars.mstp[128-1] = 2;
    // Example of setting a Pythia parameter: set the top mass 
    pydat2.pmas[1-1][6-1]= 175;  
    //
    // Call pythia initialization
    call_pyinit( "CMS", "p", "p", 14000. );

    //........................................HepMC INITIALIZATIONS
    //
    // Instantiate an IO strategy for reading from HEPEVT.
    HepMC::IO_HEPEVT hepevtio;
    //
    // Instantial an IO strategy to write the data to file - it uses the 
    //  same ParticleDataTable
    HepMC::IO_GenEvent ascii_io("example_MyPythia.dat",std::ios::out);
    //
    //........................................EVENT LOOP
    for ( int i = 1; i <= 10; i++ ) {
        if ( i%50==1 ) std::cout << "Processing Event Number " 
                                 << i << std::endl;
        call_pyevnt();      // generate one event with Pythia
        // pythia pyhepc routine converts common PYJETS in common HEPEVT
        call_pyhepc( 1 );
        HepMC::GenEvent* evt = hepevtio.read_next_event();
        // add some information to the event
        evt->set_event_number(i);
        evt->set_signal_process_id(20);
        // write the event out to the ascii file
        ascii_io << evt;
        // we also need to delete the created event from memory
        delete evt;
    }
    //........................................TERMINATION
    // write out some information from Pythia to the screen
    call_pystat( 1 );    

    return 0;
}