<chapter name="HepMC Interface"> 
 
<h2>HepMC Interface</h2> 
 
An interface to the HepMC <ref>Dob01</ref> standard event record 
format has been provided by M. Kirsanov. To use it, the relevant 
libraries need to be linked, as explained in the <code>README</code> 
and <code>README.HepMC</code> files. Only version 2.06 (or later) 
of HepMC is supported as of 1 January 2013, by agreement with the 
LHC experimental community. 
 
<p/> 
The (simple) procedure to translate PYTHIA 8 events into HepMC ones 
is illustrated in the <code>main41.cc</code>, <code>main42.cc</code> 
<code>main61.cc</code> and <code>main62.cc</code> 
main programs. At the core is a call to the 
<pre> 
HepMC::Pythia8ToHepMC::fill_next_event( pythia, hepmcevt, ievnum = -1) 
</pre> 
which takes a reference of the generator object and uses it, on the one 
hand, to read out and convert the event record in <code>pythia.event</code> 
and, on the other hand, to extract and store parton-density (PDF), 
cross section and other information for the hard subprocess from 
<code>pythia.info</code>. There is also an alternative form that 
does not requires access to the full <code>pythia</code> object, 
but only the event record, at the expense of a reduced information 
storage, see below. 
 
<p/> 
While PYTHIA always stores momenta in units of GeV, with <ei>c = 1</ei>, 
HepMC nowadays can be built either for MeV or GeV as default, a choice 
that can then be overridden on an event-by-event basis, see e.g. the 
<code>main41.cc</code> code. When filling the HepMC event record, PYTHIA 
will convert to the unit specified for the current HepMC event record. 
 
<p/> 
Also for length units there are choices, and again the PYTHIA interface 
will convert to the units set for the HepMC event record. Here the mm 
choice of PYTHIA seems to be shared by most other programs, and is 
HepMC default. 
 
<p/> 
The status code is now based on the new standard introduced for HepMC 2.05, 
see the <aloc href="EventRecord">Event::statusHepMC(...)</aloc> 
conversion routine for details. 
 
<p/> 
The current values from <code>pythia.info.sigmaGen()</code> and 
<code>pythia.info.sigmaErr()</code> are stored for each event, 
multiplied by <ei>10^9</ei> to convert from mb to pb. Note that 
PYTHIA improves its accuracy by Monte Carlo integration in the course 
of the run, so the values associated with the last generated event 
should be the most accurate ones. If events also come with a dimensional 
weight, like in some Les Houches strategies, this weight is in units of pb. 
 
<h2>The public methods</h2> 
 
Here comes a complete list of all public methods of the 
<code>Pythia8ToHepMC</code> class in the <code>HepMC</code> 
(<i>not</i> <code>Pythia8</code>!) namespace. 
 
<method name="Pythia8ToHepMC::Pythia8ToHepMC()"> 
</method> 
<methodmore name="virtual Pythia8ToHepMC::~Pythia8ToHepMC()"> 
the constructor and destructor take no arguments. 
</methodmore> 
 
<method name="bool Pythia8ToHepMC::fill_next_event( Pythia8::Pythia& pythia, 
GenEvent* evt, int ievnum = -1)"> 
convert a <code>Pythia</code> event to a <code>HepMC</code> one. 
Will return true if succeeded. 
<argument name="pythia"> 
the input <code>Pythia</code> generator object, from which both the 
event and other information can be obtained. 
</argument> 
<argument name="evt"> 
the output <code>GenEvt</code> event, in its standard form. 
</argument> 
<argument name="iev"> 
set the event number of the current event. If negative then the 
internal event number is used, which is incremented by one for 
each new event. 
</argument> 
</method> 
 
<method name="bool Pythia8ToHepMC::fill_next_event( Pythia8::Event& pyev, 
GenEvent* evt, int ievnum = -1, Pythia8::Info* pyinfo = 0, 
Pythia8::Settings* pyset = 0)"> 
convert a <code>Pythia</code> event to a <code>HepMC</code> one. 
Will return true if succeeded. 
<argument name="pyev"> 
the input <code>Pythia</code> event that is to be converted to HepMC 
format. 
</argument> 
<argument name="evt"> 
the output <code>GenEvt</code> event, in its standard form. 
</argument> 
<argument name="iev"> 
set the event number of the current event. If negative then the 
internal event number is used, which is incremented by one for 
each new event. 
</argument> 
<argument name="pyinfo"> 
pointer to the <code>Pythia Info</code> object, which is used to 
extract PFD values, and process and cross section information. 
Without such a pointer this information therefore cannot be stored, 
i.e. it is equivalent to the three <code>set_store</code> methods 
below being set false. 
</argument> 
<argument name="pyset"> 
pointer to the <code>Pythia Settings</code> object, which is used to 
decide whether hadronization is carried out, and therefore whether 
to warn about unhadronized partons. Without such a pointer the 
<code>set_free_parton_warnings</code> method below uniquely controls 
the behaviour. 
</argument> 
</method> 
 
<p/> 
The following paired methods can be used to set and get the status of 
some switches that modify the behaviour of the conversion routine. 
The <code>set</code> methods have the same default input values as 
the internal initialization ones, so that a call without an argument 
(re)stores the default. 
 
<method name="void Pythia8ToHepMC::set_print_inconsistency(bool b = true)"> 
</method> 
<methodmore name="bool Pythia8ToHepMC::print_inconsistency()"> 
print a warning line, on <code>cerr</code>, when inconsistent mother 
and daughter information is encountered. 
</methodmore> 
 
<method name="void Pythia8ToHepMC::set_free_parton_warnings(bool b = true)"> 
</method> 
<methodmore name="bool Pythia8ToHepMC::free_parton_warnings()"> 
check and print a warning line when unhadronized gluons or quarks are 
encountered in the event record. Does not apply when Pythia hadronization 
is switched off. Default is to do this check. 
</methodmore> 
 
<method name="void Pythia8ToHepMC::set_crash_on_problem(bool b = false)"> 
</method> 
<methodmore name="bool Pythia8ToHepMC::crash_on_problem()"> 
if problems (like the free partons above) are encountered then the run 
is interrupted by an <code>exit(1)</code> command. Default is not to crash. 
</methodmore> 
 
<method name="void Pythia8ToHepMC::set_convert_gluon_to_0(bool b = false)"> 
</method> 
<methodmore name="bool Pythia8ToHepMC::convert_gluon_to_0()"> 
the normal gluon identity code 21 is used also when parton density 
information is stored, unless this optional argument is set true to 
have gluons represented by a 0. This choice does not affect the 
normal event record, where a gluon is always 21. 
</methodmore> 
 
<method name="void Pythia8ToHepMC::set_store_pdf(bool b = true)"> 
</method> 
<methodmore name="bool Pythia8ToHepMC::store_pdf()"> 
for each event store information on the two incoming flavours, their 
x values and common factorization scale, and the values of the two 
parton distributions, <ei>xf(x,Q)</ei>. 
</methodmore> 
 
<method name="void Pythia8ToHepMC::set_store_proc(bool b = true)"> 
</method> 
<methodmore name="bool Pythia8ToHepMC::store_proc()"> 
for each event store information on the Pythia process code, the 
renormalization scale, and <ei>alpha_em</ei> and <ei>alpha_s</ei> 
values used for the hard process. 
</methodmore> 
 
<method name="void Pythia8ToHepMC::set_store_xsec(bool b = true)"> 
</method> 
<methodmore name="bool Pythia8ToHepMC::store_xsec()"> 
for each event store information on the Pythia cross section and its error, 
in pb, and the event weight. If events also come with a dimensional weight, 
like in some Les Houches strategies, this weight is in units of pb. 
</methodmore> 
 
</chapter> 
 
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