EvtBLLNuL.cpp

Go to the documentation of this file.

/***********************************************************************
* Copyright 1998-2020 CERN for the benefit of the EvtGen authors       *
*                                                                      *
* This file is part of EvtGen.                                         *
*                                                                      *
* EvtGen is free software: you can redistribute it and/or modify       *
* it under the terms of the GNU General Public License as published by *
* the Free Software Foundation, either version 3 of the License, or    *
* (at your option) any later version.                                  *
*                                                                      *
* EvtGen is distributed in the hope that it will be useful,            *
* but WITHOUT ANY WARRANTY; without even the implied warranty of       *
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the        *
* GNU General Public License for more details.                         *
*                                                                      *
* You should have received a copy of the GNU General Public License    *
* along with EvtGen.  If not, see <https://www.gnu.org/licenses/>.     *
***********************************************************************/

#include "EvtGenModels/EvtBLLNuL.hh"

#include "EvtGenBase/EvtIdSet.hh"
#include "EvtGenBase/EvtPDL.hh"
#include "EvtGenBase/EvtParticle.hh"
#include "EvtGenBase/EvtReport.hh"
#include "EvtGenBase/EvtSpinType.hh"

EvtBLLNuL::EvtBLLNuL() : calcAmp_()
{
}

std::string EvtBLLNuL::getName()
{
    // The model name
    return "BLLNUL";
}

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

void EvtBLLNuL::init()
{
    // check that there are 4 daughters
    checkNDaug( 4 );

    // We expect that the parent to be a scalar (B meson) and
    // the daughters to be ell+ (k1), ell- (k2), neutrino (k3)
    // and the last lepton ell- (k4)

    // Check spin types
    checkSpinParent( EvtSpinType::SCALAR );

    checkSpinDaughter( 0, EvtSpinType::DIRAC );       // ell+(k_1)
    checkSpinDaughter( 1, EvtSpinType::DIRAC );       // ell-(k_2)
    checkSpinDaughter( 2, EvtSpinType::NEUTRINO );    // neu (k_3)
    checkSpinDaughter( 3, EvtSpinType::DIRAC );       // ell-(k_4)

    // Check that we have a charged B parent
    static EvtIdSet BMesons( "B-", "B+" );
    if ( !BMesons.contains( getParentId() ) ) {
        EvtGenReport( EVTGEN_ERROR, "EvtBLLNuL" )
            << "Expecting the parent to be a charged B. Found PDG = "
            << EvtPDL::getStdHep( getParentId() ) << std::endl;
        ::abort();
    }

    // Make sure the first two leptons are charged conjugates of each other
    int id1 = EvtPDL::getStdHep( getDaug( 0 ) );
    int id2 = EvtPDL::getStdHep( getDaug( 1 ) );
    if ( id1 != -id2 ) {
        EvtGenReport( EVTGEN_ERROR, "EvtBLLNuL" )
            << "Expecting the first 2 leptons, with PDG codes " << id1 << " and "
            << id2 << ", to be charged conjugates of each other" << std::endl;
        ::abort();
    }

    // Check that the last lepton has the same charge as the B parent
    int q3 = EvtPDL::chg3( getDaug( 3 ) ) / 3;
    int qB = EvtPDL::chg3( getParentId() ) / 3;
    if ( q3 != qB ) {
        EvtGenReport( EVTGEN_ERROR, "EvtBLLNuL" )
            << "The 3rd lepton charge " << q3 << " does not match the B charge "
            << qB << std::endl;
        ::abort();
    }

    // Also check that the 2nd lepton has the same charge as the 3rd one
    int q2 = EvtPDL::chg3( getDaug( 1 ) ) / 3;
    if ( q2 != q3 ) {
        EvtGenReport( EVTGEN_ERROR, "EvtBLLNuL" )
            << "The 2nd lepton charge " << q2
            << " does not match the 3rd lepton charge " << q3 << std::endl;
        ::abort();
    }

    // Identify if the decay has 3 charged leptons with the same flavour.
    // If so, then we need to include amplitude terms where the 2nd and 3rd
    // same-sign leptons are interchanged: k2 <-> k4
    bool symmetry( false );
    int id3 = EvtPDL::getStdHep( getDaug( 3 ) );

    if ( abs( id1 ) == abs( id2 ) && abs( id1 ) == abs( id3 ) ) {
        symmetry = true;
    }

    // Specify the qSq minimum cut-off as 4*(muon mass)^2 = 0.044655 and the
    // kSq minimum cut off as 4*(electron mass)^2 = 1.044e-6
    double muMass = EvtPDL::getMeanMass( EvtPDL::getId( "mu+" ) );
    double eMass = EvtPDL::getMeanMass( EvtPDL::getId( "e+" ) );
    double qSqMin = 4.0 * muMass * muMass;
    double kSqMin = 4.0 * eMass * eMass;

    // Optionally set these cut-offs using two decay file parameters. We may
    // have a 3rd parameter (max prob), so check for at least 2 parameters
    if ( getNArg() >= 2 ) {
        qSqMin = getArg( 0 );
        kSqMin = getArg( 1 );
    }

    // Define the amplitude qSq and kSq cut-offs, also
    // specifying if the decay mode has flavour symmetry
    calcAmp_.setParameters( qSqMin, kSqMin, symmetry );
}

void EvtBLLNuL::initProbMax()
{
    // Set the maximum probability of the decay
    double maxProb( 3.2 );

    // Optional 3rd decay file parameter, e.g. if qSq and/or kSq min have changed.
    // Note that both qSq and kSq parameters must still be specified in the decay
    // file to ensure that the maximum probability value is the 3rd parameter!
    if ( getNArg() == 3 ) {
        maxProb = getArg( 2 );
    }

    setProbMax( maxProb );
}

void EvtBLLNuL::decay( EvtParticle* p )
{
    p->initializePhaseSpace( getNDaug(), getDaugs() );

    calcAmp_.CalcAmp( p, _amp2 );
}