## main291-python

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    # main291.py is a part of the PYTHIA event generator.
    # Copyright (C) 2025 Torbjorn Sjostrand.
    # PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
    # Please respect the MCnet Guidelines, see GUIDELINES for details.

    # Keywords:
    #            Basic usage
    #            Charged multiplicity
    #            Python

    # This is a simple test program, equivalent with main101.cc,
    # but written in Python. It fits on one slide in a talk.
    # It studies the charged multiplicity distribution at the LHC.

    # To set the path to the Pythia 8 Python interface do either
    # (in a shell prompt):
    #      export PYTHONPATH=$(PREFIX_LIB):$PYTHONPATH
    # or the following which sets the path from within Python.
    import sys
    cfg = open("Makefile.inc")
    lib = "../lib"
    for line in cfg:
        if line.startswith("PREFIX_LIB="): lib = line[11:-1]; break
    sys.path.insert(0, lib)

    #==========================================================================

    # Import the Pythia module.
    import pythia8
    pythia = pythia8.Pythia()
    pythia.readString("Beams:eCM = 8000.")
    pythia.readString("HardQCD:all = on")
    pythia.readString("PhaseSpace:pTHatMin = 20.")
    pythia.init()
    mult = pythia8.Hist("charged multiplicity", 100, -0.5, 799.5)
    # Begin event loop. Generate event. Skip if error. List first one.
    for iEvent in range(0, 100):
        if not pythia.next(): continue
        # Find number of all final charged particles and fill histogram.
        nCharged = 0
        for prt in pythia.event:
            if prt.isFinal() and prt.isCharged(): nCharged += 1
        mult.fill(nCharged)
    # End of event loop. Statistics. Histogram. Done.
    pythia.stat();
    print(mult)