EvtPythia.cpp

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#include "EvtGenExternal/EvtPythia.hh"
 
#include "EvtGenBase/EvtDecayBase.hh"
#include "EvtGenBase/EvtId.hh"
#include "EvtGenBase/EvtPDL.hh"
#include "EvtGenBase/EvtParticle.hh"
#include "EvtGenBase/EvtSpinDensity.hh"
 
#include "EvtGenModels/EvtAbsExternalGen.hh"
 
#include "EvtGenExternal/EvtExternalGenFactory.hh"
 
#include <cmath>
#include <iostream>
 
std::string EvtPythia::getName() const
{
    return "PYTHIA";
}
 
EvtDecayBase* EvtPythia::clone() const
{
    return new EvtPythia();
}
 
void EvtPythia::init()
{
    // Do not check for any arguments. The PythiaEngine will check
    // to see if there is an integer specifying the decay physics,
    // otherwise it just uses phase-space.
}
 
void EvtPythia::initProbMax()
{
    noProbMax();
}
 
void EvtPythia::decay( EvtParticle* p )
{
    // We have to initialise the Pythia engine after the decay.dec files have been read in,
    // since we will be modifying Pythia data tables, and that is only possible once we have
    // defined all Pythia-type decays we want to use.
    // We check to see if the engine has been created before doing the decay.
    // This should only create the full Pythia engine once, and all clones will point to the same engine.
 
    if ( !m_pythiaEngine ) {
        m_pythiaEngine = EvtExternalGenFactory::getInstance().getGenerator(
            EvtExternalGenFactory::GenId::PythiaGenId );
    }
 
    if ( m_pythiaEngine ) {
        m_pythiaEngine->doDecay( p );
    }
 
    this->fixPolarisations( p );
}
 
void EvtPythia::fixPolarisations( EvtParticle* p )
{
    // Special case to handle the J/psi polarisation
 
    if ( !p ) {
        return;
    }
 
    int nDaug = p->getNDaug();
    int i( 0 );
 
    static const EvtId Jpsi = EvtPDL::getId( "J/psi" );
 
    for ( i = 0; i < nDaug; i++ ) {
        EvtParticle* theDaug = p->getDaug( i );
 
        if ( theDaug ) {
            if ( theDaug->getId() == Jpsi ) {
                EvtSpinDensity rho;
 
                rho.setDim( 3 );
                rho.set( 0, 0, 0.5 );
                rho.set( 0, 1, 0.0 );
                rho.set( 0, 2, 0.0 );
 
                rho.set( 1, 0, 0.0 );
                rho.set( 1, 1, 1.0 );
                rho.set( 1, 2, 0.0 );
 
                rho.set( 2, 0, 0.0 );
                rho.set( 2, 1, 0.0 );
                rho.set( 2, 2, 0.5 );
 
                EvtVector4R p4Psi = theDaug->getP4();
 
                double alpha = atan2( p4Psi.get( 2 ), p4Psi.get( 1 ) );
                double beta = acos( p4Psi.get( 3 ) / p4Psi.d3mag() );
 
                theDaug->setSpinDensityForwardHelicityBasis( rho, alpha, beta,
                                                             0.0 );
                setDaughterSpinDensity( i );
            }
        }
    }
}
 
std::string EvtPythia::commandName()
{
    // Allow backward compatibility for decay.dec files
    // having JetSetPar parameters. They are obsolete for Pythia 8,
    // since the JetSet-type array variables do not exist.
    // Need to think about including user defined parameters in
    // EvtPythiaEngine::updatePhysicsParameters().
    return std::string( "JetSetPar" );
}
 
void EvtPythia::command( std::string cmd )
{
    // Locally store commands in a vector
    m_commandList.push_back( cmd );
}