EvtSemiLeptonicAmp.cpp

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#include "EvtGenBase/EvtSemiLeptonicAmp.hh"

#include "EvtGenBase/EvtAmp.hh"
#include "EvtGenBase/EvtDiracSpinor.hh"
#include "EvtGenBase/EvtGenKine.hh"
#include "EvtGenBase/EvtId.hh"
#include "EvtGenBase/EvtPDL.hh"
#include "EvtGenBase/EvtParticle.hh"
#include "EvtGenBase/EvtPatches.hh"
#include "EvtGenBase/EvtReport.hh"
#include "EvtGenBase/EvtScalarParticle.hh"
#include "EvtGenBase/EvtTensor4C.hh"
#include "EvtGenBase/EvtTensorParticle.hh"
#include "EvtGenBase/EvtVector4C.hh"
#include "EvtGenBase/EvtVectorParticle.hh"

double EvtSemiLeptonicAmp::CalcMaxProb( EvtId parent, EvtId meson, EvtId lepton,
                                        EvtId nudaug,
                                        EvtSemiLeptonicFF* FormFactors,
                                        int nQ2Bins )
{
    //This routine takes the arguements parent, meson, and lepton
    //number, and a form factor model, and returns a maximum
    //probability for this semileptonic form factor model.  A
    //brute force method is used.  The 2D cos theta lepton and
    //q2 phase space is probed.

    //Start by declaring a particle at rest.

    //It only makes sense to have a scalar parent.  For now.
    //This should be generalized later.

    EvtScalarParticle* scalar_part;
    EvtParticle* root_part;

    scalar_part = new EvtScalarParticle;

    //cludge to avoid generating random numbers!
    scalar_part->noLifeTime();

    EvtVector4R p_init;

    p_init.set( EvtPDL::getMass( parent ), 0.0, 0.0, 0.0 );
    scalar_part->init( parent, p_init );
    root_part = (EvtParticle*)scalar_part;
    root_part->setDiagonalSpinDensity();

    EvtParticle *daughter, *lep, *trino;

    EvtAmp amp;

    EvtId listdaug[3];
    listdaug[0] = meson;
    listdaug[1] = lepton;
    listdaug[2] = nudaug;

    amp.init( parent, 3, listdaug );

    root_part->makeDaughters( 3, listdaug );
    daughter = root_part->getDaug( 0 );
    lep = root_part->getDaug( 1 );
    trino = root_part->getDaug( 2 );

    //cludge to avoid generating random numbers!
    daughter->noLifeTime();
    lep->noLifeTime();
    trino->noLifeTime();

    //Initial particle is unpolarized, well it is a scalar so it is
    //trivial
    EvtSpinDensity rho;
    rho.setDiag( root_part->getSpinStates() );

    double mass[3];

    double m = root_part->mass();

    EvtVector4R p4meson, p4lepton, p4nu, p4w;
    double q2min;
    double q2max;

    double q2, elepton, plepton;
    int i, j;
    double erho, prho, costl;

    double maxfoundprob = 0.0;
    double prob = -10.0;
    int massiter;

    for ( massiter = 0; massiter < 3; massiter++ ) {
        mass[0] = EvtPDL::getMeanMass( meson );
        mass[1] = EvtPDL::getMeanMass( lepton );
        mass[2] = EvtPDL::getMeanMass( nudaug );
        if ( massiter == 1 ) {
            mass[0] = EvtPDL::getMinMass( meson );
        }
        if ( massiter == 2 ) {
            mass[0] = EvtPDL::getMaxMass( meson );
            if ( ( mass[0] + mass[1] + mass[2] ) > m )
                mass[0] = m - mass[1] - mass[2] - 0.00001;
        }

        q2min = mass[1] * mass[1];
        q2max = ( m - mass[0] ) * ( m - mass[0] );

        //loop over q2 (default = 25 bins)

        for ( i = 0; i < nQ2Bins; i++ ) {
            q2 = q2min + ( ( i + 0.5 ) * ( q2max - q2min ) ) / nQ2Bins;

            erho = ( m * m + mass[0] * mass[0] - q2 ) / ( 2.0 * m );

            prho = sqrt( erho * erho - mass[0] * mass[0] );

            p4meson.set( erho, 0.0, 0.0, -1.0 * prho );
            p4w.set( m - erho, 0.0, 0.0, prho );

            //This is in the W rest frame
            elepton = ( q2 + mass[1] * mass[1] ) / ( 2.0 * sqrt( q2 ) );
            plepton = sqrt( elepton * elepton - mass[1] * mass[1] );

            double probctl[3];

            for ( j = 0; j < 3; j++ ) {
                costl = 0.99 * ( j - 1.0 );

                //These are in the W rest frame. Need to boost out into
                //the B frame.
                p4lepton.set( elepton, 0.0, plepton * sqrt( 1.0 - costl * costl ),
                              plepton * costl );
                p4nu.set( plepton, 0.0,
                          -1.0 * plepton * sqrt( 1.0 - costl * costl ),
                          -1.0 * plepton * costl );

                EvtVector4R boost( ( m - erho ), 0.0, 0.0, 1.0 * prho );
                p4lepton = boostTo( p4lepton, boost );
                p4nu = boostTo( p4nu, boost );

                //Now initialize the daughters...

                daughter->init( meson, p4meson );
                lep->init( lepton, p4lepton );
                trino->init( nudaug, p4nu );

                CalcAmp( root_part, amp, FormFactors );

                //Now find the probability at this q2 and cos theta lepton point
                //and compare to maxfoundprob.

                //Do a little magic to get the probability!!
                prob = rho.normalizedProb( amp.getSpinDensity() );

                probctl[j] = prob;
            }

            //probclt contains prob at ctl=-1,0,1.
            //prob=a+b*ctl+c*ctl^2

            double a = probctl[1];
            double b = 0.5 * ( probctl[2] - probctl[0] );
            double c = 0.5 * ( probctl[2] + probctl[0] ) - probctl[1];

            prob = probctl[0];
            if ( probctl[1] > prob )
                prob = probctl[1];
            if ( probctl[2] > prob )
                prob = probctl[2];

            if ( fabs( c ) > 1e-20 ) {
                double ctlx = -0.5 * b / c;
                if ( fabs( ctlx ) < 1.0 ) {
                    double probtmp = a + b * ctlx + c * ctlx * ctlx;
                    if ( probtmp > prob )
                        prob = probtmp;
                }
            }

            if ( prob > maxfoundprob ) {
                maxfoundprob = prob;
            }
        }
        if ( EvtPDL::getWidth( meson ) <= 0.0 ) {
            //if the particle is narrow dont bother with changing the mass.
            massiter = 4;
        }
    }
    root_part->deleteTree();

    maxfoundprob *= 1.1;
    return maxfoundprob;
}