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<title>HepMC Interface</title>
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<h2>HepMC Interface</h2>
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An interface to the HepMC [<a href="Bibliography.html" target="page">Dob01</a>] standard event record
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format has been provided by M. Kirsanov. To use it, the relevant
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libraries need to be linked, as explained in the <code>README</code>
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file. Only version 2 of HepMC is supported. (Version 1 requires
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a different interface structure, which was only supported up until
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The (simple) procedure to translate PYTHIA 8 events into HepMC ones
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is illustrated in the <code>main41.cc</code>, <code>main42.cc</code>
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<code>main61.cc</code> and <code>main62.cc</code>
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main programs. At the core is a call to the
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HepMC::I_Pythia8::fill_next_event( pythia, hepmcevt, ievnum = -1, convertGluonTo0 = false )
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which takes a reference of the generator object and uses it, on the one
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hand, to read out and covert the event record in <code>pythia.event</code>
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and, on the other hand, to extract and store parton-density (PDF)
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information for the hard subprocess from <code>pythia.info</code>.
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The optional last argument, if <code>true</code>, allows you to store
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gluons as "PDG" code 0 rather than the normal 21; this only applies to
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the PDF information, not the event record.
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The earlier version of this routine,
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HepMC::I_Pythia8::fill_next_event( pythia.event, hepmcevt, ievnum = -1 )
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is retained (for now) for backwards compatibility. It takes a PYTHIA event
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as input and returns a HepMC one, but without storing the PDF information.
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The latter could then instead be stored by a separate call
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HepMC::I_Pythia8::pdf_put_info( hepmcevt, pythia, convertGluonTo0 = false )
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The translation routine stores momenta, energies and masses in units
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of GeV, and distances and times in units of mm, with <i>c = 1</i>,
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exactly as used in PYTHIA. This only works seamlessly for HepMC 2.04
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or later, where the choice of units can (and should) be specified in
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the <code>HepMC::GenEvent</code> call. To work also with older HepMC
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versions, where normally momenta are stored in MeV, the
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<code>HEPMC_HAS_UNITS</code> environment variable is tested. If not
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defined, i.e. if with an earlier HepMC version, conversion from
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GeV to MeV is done in <code>fill_next_event</code>.
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The status code is now based on the proposed standard for HepMC 2.05,
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see the <a href="EventRecord.html" target="page">Event::statusHepMC(...)</a>
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conversion routine for details. The earlier behaviour, where all
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final particles had status 1 and all initial or intermediate ones
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status 2, is available as a commented-out line in the
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<code>I_Pythia8::fill_next_event(...)</code> method, and so is simple
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In HepMC 2.05 it also becomes possible to store the generated cross
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section and its error. The environment variable
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<code>HEPMC_HAS_CROSS_SECTION</code> is used to check whether this
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possibility exists and, if it does the current values from
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<code>pythia.info.sigmaGen()</code> and
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<code>pythia.info.sigmaErr()</code> are stored for each event,
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multiplied by <i>10^9</i> to convert from mb to pb. Note that
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PYTHIA improves it accuracy by Monte Carlo integration in the course
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of the run, so the values associated with the last generated event
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should be the most accurate ones. (If events also come with a dimensional
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weight, like in some Les Houches strategies, conversion from mb to fb
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for that weight must be set by hand, see the last method below.)
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<h2>The public methods</h2>
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Here comes a complete list of all public methods of the
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<code>I_Pythia8</code> class in the <code>HepMC</code>
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(<i>not</i> <code>Pythia8</code>!) namespace.
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<a name="method1"></a>
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<p/><strong>I_Pythia8::I_Pythia8() </strong> <br/>
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<strong>virtual I_Pythia8::~I_Pythia8() </strong> <br/>
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the constructor and destructor take no arguments.
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<a name="method2"></a>
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<p/><strong>bool I_Pythia8::fill_next_event( Pythia8::Pythia& pythia, GenEvent* evt, int ievnum = -1, bool convertGluonTo0 = false) </strong> <br/>
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convert a <code>Pythia</code> event to a <code>HepMC</code> one.
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Will return true if succeeded.
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<br/><code>argument</code><strong> pythia </strong> :
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the input <code>Pythia</code> generator object, from which both the
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event and the parton density information can be obtained.
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<br/><code>argument</code><strong> evt </strong> :
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the output <code>GenEvt</code> event, in its standard form.
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<br/><code>argument</code><strong> iev </strong> :
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set the event number of the current event. If negative then the
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internal event number is used, which is incremented by one for
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<br/><code>argument</code><strong> convertGluonTo0 </strong> :
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the normal gluon identity code 21 is used also when parton density
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information is stored, unless this optional argument is set true to
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have gluons represented by a 0. This choice does not affect the
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normal event record, where a gluon is always 21.
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<a name="method3"></a>
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<p/><strong>bool I_Pythia8::fill_next_event( Pythia8::Event& pyev, GenEvent* evt, int ievnum = -1 ) </strong> <br/>
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convert a <code>Pythia</code> event to a <code>HepMC</code> one.
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Will return true if succeeded. Do not store parton-density information.
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<br/><code>argument</code><strong> pyev </strong> :
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the input <code>Pythia</code> event, in its standard form.
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<br/><code>argument</code><strong> evt </strong> :
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the output <code>GenEvt</code> event, in its standard form.
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<br/><code>argument</code><strong> iev </strong> :
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set the event number of the current event. If negative then the
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internal event number is used, which is incremented by one for
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<a name="method4"></a>
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<p/><strong>bool I_Pythia8::put_pdf_info( GenEvent* evt, Pythia8::Pythia& pythia, bool convertGluonTo0 = false ) </strong> <br/>
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append parton-density information to an event already stored
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by the previous method.
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<br/><code>argument</code><strong> evt </strong> :
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the output <code>GenEvt</code> event record, in its standard form.
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<br/><code>argument</code><strong> pythia </strong> :
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the input <code>Pythia</code> generator object, from which both the
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event and the parton density information can be obtained.
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<br/><code>argument</code><strong> convertGluonTo0 </strong> :
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the normal gluon identity code 21 is used also when parton density
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information is stored, unless this optional argument is set true to
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have gluons represented by a 0.
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The following methods can be used to set, and in some cases interrogate,
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the status of some switches that can be used to modify the behaviour
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of the conversion routine. The <code>set</code> methods have the
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same default input values as the internal initialization ones, so
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that a call without an argument (re)stores the default.
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<a name="method5"></a>
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<p/><strong>void I_Pythia8::set_trust_mothers_before_daughters( bool b = true ) </strong> <br/>
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<strong>bool I_Pythia8::trust_mothers_before_daughters() </strong> <br/>
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if there is a conflict in the history information, then trust the
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information on mothers above that on daughters. Currently this is
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the only option implemented.
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<a name="method6"></a>
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<p/><strong>void I_Pythia8::set_trust_both_mothers_and_daughters( bool b = false ) </strong> <br/>
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<strong>bool I_Pythia8::trust_both_mothers_and_daughters() </strong> <br/>
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currently dummy methods intended to resolve conflicts in the event
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<a name="method7"></a>
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<p/><strong>void I_Pythia8::set_print_inconsistency_errors( bool b = true ) </strong> <br/>
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<strong>bool I_Pythia8::print_inconsistency_errors() </strong> <br/>
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print a warning line, on <code>cerr</code>, when inconsistent mother
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and daughter information is encountered.
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<a name="method8"></a>
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<p/><strong>void I_Pythia8::set_crash_on_problem( bool b = false ) </strong> <br/>
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if problems are encountered then the run is interrupted by an
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<code>exit(1)</code> command. Default is not to crash.
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<a name="method9"></a>
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<p/><strong>void I_Pythia8::set_freepartonwarnings( bool b = true ) </strong> <br/>
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interrupt the run by an <code>exit(1)</code> command if unhadronized
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gluons or quarks are encountered in the event record, unless
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hadronization is switched off. Default is to crash.
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<a name="method10"></a>
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<p/><strong>void I_Pythia8::set_convert_to_mev( bool b = false ) </strong> <br/>
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convert the normal GeV energies, momenta and masses to MeV.
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<!-- Copyright (C) 2012 Torbjorn Sjostrand -->
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