| 1 | <html> |
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| 2 | <head> |
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| 3 | <title>HepMC Interface</title> |
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| 28 | <form method='post' action='HepMCInterface.php'> |
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| 29 | |
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| 30 | <h2>HepMC Interface</h2> |
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| 31 | |
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| 32 | An interface to the HepMC [<a href="Bibliography.php" target="page">Dob01</a>] standard event record |
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| 33 | format has been provided by M. Kirsanov. To use it, the relevant |
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| 34 | libraries need to be linked, as explained in the <code>README</code> |
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| 35 | file. Only version 2 of HepMC is supported. (Version 1 requires |
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| 36 | a different interface structure, which was only supported up until |
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| 37 | Pythia 8.107.) |
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| 38 | |
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| 39 | <p/> |
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| 40 | The (simple) procedure to translate PYTHIA 8 events into HepMC ones |
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| 41 | is illustrated in the <code>main41.cc</code>, <code>main42.cc</code> |
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| 42 | <code>main61.cc</code> and <code>main62.cc</code> |
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| 43 | main programs. At the core is a call to the |
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| 44 | <pre> |
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| 45 | HepMC::I_Pythia8::fill_next_event( pythia, hepmcevt, ievnum = -1, convertGluonTo0 = false ) |
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| 46 | </pre> |
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| 47 | which takes a reference of the generator object and uses it, on the one |
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| 48 | hand, to read out and covert the event record in <code>pythia.event</code> |
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| 49 | and, on the other hand, to extract and store parton-density (PDF) |
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| 50 | information for the hard subprocess from <code>pythia.info</code>. |
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| 51 | The optional last argument, if <code>true</code>, allows you to store |
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| 52 | gluons as "PDG" code 0 rather than the normal 21; this only applies to |
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| 53 | the PDF information, not the event record. |
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| 54 | |
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| 55 | <p/> |
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| 56 | The earlier version of this routine, |
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| 57 | <pre> |
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| 58 | HepMC::I_Pythia8::fill_next_event( pythia.event, hepmcevt, ievnum = -1 ) |
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| 59 | </pre> |
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| 60 | is retained (for now) for backwards compatibility. It takes a PYTHIA event |
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| 61 | as input and returns a HepMC one, but without storing the PDF information. |
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| 62 | The latter could then instead be stored by a separate call |
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| 63 | <pre> |
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| 64 | HepMC::I_Pythia8::pdf_put_info( hepmcevt, pythia, convertGluonTo0 = false ) |
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| 65 | </pre> |
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| 66 | or not, as wished. |
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| 67 | |
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| 68 | <p/> |
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| 69 | While PYTHIA stores momenta in units of GeV, with <i>c = 1</i>, |
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| 70 | the HepMC standard originally did not specify which units to use, |
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| 71 | unfortunately. Later versions allow units to be specified either as |
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| 72 | MeV or as GeV. The dividing line goes with the introduction of the |
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| 73 | <code>HEPMC_HAS_UNITS</code> environment variable in HepMC 2.04. |
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| 74 | When PYTHIA is linked to older versions, which do not have this |
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| 75 | environment variable, the default behaviour of |
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| 76 | <code>fill_next_event</code> is to convert momenta to MeV. |
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| 77 | To store momenta in GeV instead, you must call |
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| 78 | <pre> |
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| 79 | HepMC::I_Pythia8::set_convert_to_mev( false ) |
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| 80 | </pre> |
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| 81 | beforehand. When linked to newer HepMC versions, PYTHIA will adapt |
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| 82 | to the unit specified for the HepMC event record. A default unit |
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| 83 | is chosen when HepMC is built, but this choice can be overridden |
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| 84 | in the constructor of a <code>HepMC::GenEvent</code>, to be either |
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| 85 | GeV or MeV. The <code>main41.cc</code> code illustrates these |
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| 86 | possibilities. |
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| 87 | |
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| 88 | <p/> |
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| 89 | Also for length units there could exist ambiguities, but the mm |
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| 90 | choice of PYTHIA seems to be shared by most other programs. |
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| 91 | When linked to older HepMC versions there is never a conversion, |
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| 92 | while for newer versions the PYTHIA interface will convert to the |
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| 93 | units set for the HepMC event record, as for momenta. |
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| 94 | |
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| 95 | <p/> |
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| 96 | By agreement with the LHC experimental community, support for the |
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| 97 | older behaviour will be dropped for all versions released after |
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| 98 | the end of 2012. |
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| 99 | |
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| 100 | <p/> |
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| 101 | The status code is now based on the new standard for HepMC 2.05, |
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| 102 | see the <?php $filepath = $_GET["filepath"]; |
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| 103 | echo "<a href='EventRecord.php?filepath=".$filepath."' target='page'>";?>Event::statusHepMC(...)</a> |
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| 104 | conversion routine for details. The earlier behaviour, where all |
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| 105 | final particles had status 1 and all initial or intermediate ones |
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| 106 | status 2, is available as a commented-out line in the |
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| 107 | <code>I_Pythia8::fill_next_event(...)</code> method, and so is simple |
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| 108 | to recover. |
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| 109 | |
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| 110 | <p/> |
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| 111 | In HepMC 2.05 it also becomes possible to store the generated cross |
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| 112 | section and its error. The environment variable |
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| 113 | <code>HEPMC_HAS_CROSS_SECTION</code> is used to check whether this |
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| 114 | possibility exists and, if it does the current values from |
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| 115 | <code>pythia.info.sigmaGen()</code> and |
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| 116 | <code>pythia.info.sigmaErr()</code> are stored for each event, |
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| 117 | multiplied by <i>10^9</i> to convert from mb to pb. Note that |
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| 118 | PYTHIA improves its accuracy by Monte Carlo integration in the course |
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| 119 | of the run, so the values associated with the last generated event |
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| 120 | should be the most accurate ones. (If events also come with a dimensional |
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| 121 | weight, like in some Les Houches strategies, conversion from mb to fb |
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| 122 | for that weight must be set by hand, see the last method below.) |
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| 123 | |
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| 124 | <h2>The public methods</h2> |
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| 125 | |
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| 126 | Here comes a complete list of all public methods of the |
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| 127 | <code>I_Pythia8</code> class in the <code>HepMC</code> |
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| 128 | (<i>not</i> <code>Pythia8</code>!) namespace. |
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| 129 | |
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| 130 | <a name="method1"></a> |
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| 131 | <p/><strong>I_Pythia8::I_Pythia8() </strong> <br/> |
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| 132 | |
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| 133 | <strong>virtual I_Pythia8::~I_Pythia8() </strong> <br/> |
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| 134 | the constructor and destructor take no arguments. |
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| 135 | |
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| 136 | |
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| 137 | <a name="method2"></a> |
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| 138 | <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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| 139 | convert a <code>Pythia</code> event to a <code>HepMC</code> one. |
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| 140 | Will return true if succeeded. |
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| 141 | <br/><code>argument</code><strong> pythia </strong> : |
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| 142 | the input <code>Pythia</code> generator object, from which both the |
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| 143 | event and the parton density information can be obtained. |
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| 144 | |
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| 145 | <br/><code>argument</code><strong> evt </strong> : |
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| 146 | the output <code>GenEvt</code> event, in its standard form. |
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| 147 | |
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| 148 | <br/><code>argument</code><strong> iev </strong> : |
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| 149 | set the event number of the current event. If negative then the |
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| 150 | internal event number is used, which is incremented by one for |
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| 151 | each new event. |
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| 152 | |
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| 153 | <br/><code>argument</code><strong> convertGluonTo0 </strong> : |
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| 154 | the normal gluon identity code 21 is used also when parton density |
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| 155 | information is stored, unless this optional argument is set true to |
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| 156 | have gluons represented by a 0. This choice does not affect the |
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| 157 | normal event record, where a gluon is always 21. |
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| 158 | |
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| 159 | |
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| 160 | |
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| 161 | <a name="method3"></a> |
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| 162 | <p/><strong>bool I_Pythia8::fill_next_event( Pythia8::Event& pyev, GenEvent* evt, int ievnum = -1 ) </strong> <br/> |
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| 163 | convert a <code>Pythia</code> event to a <code>HepMC</code> one. |
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| 164 | Will return true if succeeded. Do not store parton-density information. |
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| 165 | <br/><code>argument</code><strong> pyev </strong> : |
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| 166 | the input <code>Pythia</code> event, in its standard form. |
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| 167 | |
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| 168 | <br/><code>argument</code><strong> evt </strong> : |
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| 169 | the output <code>GenEvt</code> event, in its standard form. |
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| 170 | |
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| 171 | <br/><code>argument</code><strong> iev </strong> : |
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| 172 | set the event number of the current event. If negative then the |
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| 173 | internal event number is used, which is incremented by one for |
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| 174 | each new event. |
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| 175 | |
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| 176 | |
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| 177 | |
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| 178 | <a name="method4"></a> |
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| 179 | <p/><strong>bool I_Pythia8::put_pdf_info( GenEvent* evt, Pythia8::Pythia& pythia, bool convertGluonTo0 = false ) </strong> <br/> |
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| 180 | append parton-density information to an event already stored |
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| 181 | by the previous method. |
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| 182 | <br/><code>argument</code><strong> evt </strong> : |
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| 183 | the output <code>GenEvt</code> event record, in its standard form. |
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| 184 | |
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| 185 | <br/><code>argument</code><strong> pythia </strong> : |
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| 186 | the input <code>Pythia</code> generator object, from which both the |
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| 187 | event and the parton density information can be obtained. |
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| 188 | |
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| 189 | <br/><code>argument</code><strong> convertGluonTo0 </strong> : |
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| 190 | the normal gluon identity code 21 is used also when parton density |
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| 191 | information is stored, unless this optional argument is set true to |
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| 192 | have gluons represented by a 0. |
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| 193 | |
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| 194 | |
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| 195 | |
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| 196 | <p/> |
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| 197 | The following methods can be used to set, and in some cases interrogate, |
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| 198 | the status of some switches that can be used to modify the behaviour |
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| 199 | of the conversion routine. The <code>set</code> methods have the |
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| 200 | same default input values as the internal initialization ones, so |
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| 201 | that a call without an argument (re)stores the default. |
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| 202 | |
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| 203 | <a name="method5"></a> |
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| 204 | <p/><strong>void I_Pythia8::set_trust_mothers_before_daughters( bool b = true ) </strong> <br/> |
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| 205 | |
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| 206 | <strong>bool I_Pythia8::trust_mothers_before_daughters() </strong> <br/> |
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| 207 | if there is a conflict in the history information, then trust the |
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| 208 | information on mothers above that on daughters. Currently this is |
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| 209 | the only option implemented. |
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| 210 | |
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| 211 | |
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| 212 | <a name="method6"></a> |
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| 213 | <p/><strong>void I_Pythia8::set_trust_both_mothers_and_daughters( bool b = false ) </strong> <br/> |
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| 214 | |
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| 215 | <strong>bool I_Pythia8::trust_both_mothers_and_daughters() </strong> <br/> |
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| 216 | currently dummy methods intended to resolve conflicts in the event |
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| 217 | history. |
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| 218 | |
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| 219 | |
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| 220 | <a name="method7"></a> |
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| 221 | <p/><strong>void I_Pythia8::set_print_inconsistency_errors( bool b = true ) </strong> <br/> |
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| 222 | |
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| 223 | <strong>bool I_Pythia8::print_inconsistency_errors() </strong> <br/> |
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| 224 | print a warning line, on <code>cerr</code>, when inconsistent mother |
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| 225 | and daughter information is encountered. |
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| 226 | |
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| 227 | |
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| 228 | <a name="method8"></a> |
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| 229 | <p/><strong>void I_Pythia8::set_crash_on_problem( bool b = false ) </strong> <br/> |
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| 230 | if problems are encountered then the run is interrupted by an |
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| 231 | <code>exit(1)</code> command. Default is not to crash. |
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| 232 | |
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| 233 | |
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| 234 | <a name="method9"></a> |
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| 235 | <p/><strong>void I_Pythia8::set_freepartonwarnings( bool b = true ) </strong> <br/> |
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| 236 | interrupt the run by an <code>exit(1)</code> command if unhadronized |
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| 237 | gluons or quarks are encountered in the event record, unless |
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| 238 | hadronization is switched off. Default is to crash. |
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| 239 | |
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| 240 | |
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| 241 | <a name="method10"></a> |
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| 242 | <p/><strong>void I_Pythia8::set_convert_to_mev( bool b = false ) </strong> <br/> |
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| 243 | convert the normal GeV energies, momenta and masses to MeV. |
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| 244 | |
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| 245 | |
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| 246 | </body> |
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| 247 | </html> |
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| 248 | |
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| 249 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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