EvtEta2MuMuGamma.cpp

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#include "EvtGenModels/EvtEta2MuMuGamma.hh"

#include "EvtGenBase/EvtDiracSpinor.hh"
#include "EvtGenBase/EvtGenKine.hh"
#include "EvtGenBase/EvtPDL.hh"
#include "EvtGenBase/EvtParticle.hh"
#include "EvtGenBase/EvtPatches.hh"
#include "EvtGenBase/EvtReport.hh"
#include "EvtGenBase/EvtTensor4C.hh"
#include "EvtGenBase/EvtVector4C.hh"

#include <fstream>
#include <stdio.h>
#include <stdlib.h>
#include <string>
using std::fstream;

std::string EvtEta2MuMuGamma::getName()
{
    return "ETA2MUMUGAMMA";
}

EvtDecayBase* EvtEta2MuMuGamma::clone()
{
    return new EvtEta2MuMuGamma;
}

void EvtEta2MuMuGamma::initProbMax()
{
    setProbMax( 3.5 );
}

void EvtEta2MuMuGamma::init()
{
    // check that there are 0 arguments
    checkNArg( 0 );
    checkNDaug( 3 );

    checkSpinParent( EvtSpinType::SCALAR );

    checkSpinDaughter( 0, EvtSpinType::DIRAC );
    checkSpinDaughter( 1, EvtSpinType::DIRAC );
    checkSpinDaughter( 2, EvtSpinType::PHOTON );
}

void EvtEta2MuMuGamma::decay( EvtParticle* p )
{
    EvtParticle *ep, *em, *gamma;
    setWeight( p->initializePhaseSpace( getNDaug(), getDaugs(), false,
                                        0.00000002, 0, 1 ) );

    ep = p->getDaug( 0 );
    em = p->getDaug( 1 );
    gamma = p->getDaug( 2 );

    // the next four lines generates events with a weight such that
    // the efficiency for selecting them is good. The parameter below of
    // 0.1 is the size of the peak at low q^2 (in arbitrary units).
    // The value of 0.1 is appropriate for muons.
    // when you use this remember to remove the cut on q^2!

    //ep em invariant mass^2
    double m2 = ( ep->getP4() + em->getP4() ).mass2();
    EvtVector4R q = ep->getP4() + em->getP4();
    //Just use the prob summed over spins...

    EvtTensor4C w, v;

    v = 2.0 * ( gamma->getP4() * q ) *
            EvtGenFunctions::directProd( q, gamma->getP4() ) -
        ( gamma->getP4() * q ) * ( gamma->getP4() * q ) * EvtTensor4C::g() -
        m2 * EvtGenFunctions::directProd( gamma->getP4(), gamma->getP4() );

    w = 4.0 * ( EvtGenFunctions::directProd( ep->getP4(), em->getP4() ) +
                EvtGenFunctions::directProd( em->getP4(), ep->getP4() ) -
                EvtTensor4C::g() *
                    ( ep->getP4() * em->getP4() - ep->getP4().mass2() ) );

    double prob = ( real( cont( v, w ) ) ) / ( m2 * m2 );
    prob *= ( 1.0 / ( ( 0.768 * 0.768 - m2 ) * ( 0.768 * 0.768 - m2 ) +
                      0.768 * 0.768 * 0.151 * 0.151 ) );

    //  EvtGenReport(EVTGEN_INFO,"EvtGen") << "prob is "<<prob<<endl;
    setProb( prob );

    return;
}