G4SingleDiffractiveExcitation.cc

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// $Id$
// ------------------------------------------------------------
//      GEANT 4 class implemetation file
//
//      ---------------- G4SingleDiffractiveExcitation --------------
//             by Gunter Folger, October 1998.
//      diffractive Excitation used by strings models
//  Take a projectile and a target
//  excite the projectile and target
// ------------------------------------------------------------

#include "G4SingleDiffractiveExcitation.hh"
#include "globals.hh"
#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"
#include "Randomize.hh"
#include "G4LorentzRotation.hh"
#include "G4ThreeVector.hh"
#include "G4ParticleDefinition.hh"
#include "G4VSplitableHadron.hh"
#include "G4ExcitedString.hh"

G4SingleDiffractiveExcitation::G4SingleDiffractiveExcitation(G4double sigmaPt, G4double minextraMass,G4double x0mass)
:
widthOfPtSquare(-2*sqr(sigmaPt)) , minExtraMass(minextraMass),
minmass(x0mass)
{}

G4SingleDiffractiveExcitation::~G4SingleDiffractiveExcitation()
{}

G4bool G4SingleDiffractiveExcitation::
ExciteParticipants(G4VSplitableHadron *projectile, G4VSplitableHadron *target) const
{

    G4LorentzVector Pprojectile=projectile->Get4Momentum();
    G4double Mprojectile2=sqr(projectile->GetDefinition()->GetPDGMass() + minExtraMass);

    G4LorentzVector Ptarget=target->Get4Momentum();
    G4double Mtarget2=sqr(target->GetDefinition()->GetPDGMass() + minExtraMass);
    //       G4cout << "E proj, target :" << Pprojectile.e() << ", " <<
    //                      Ptarget.e() << G4endl;

    G4bool KeepProjectile= G4UniformRand() > 0.5;

    //     reset the min.mass of the non diffractive particle to its value, ( minus a bit for rounding...)
    if ( KeepProjectile )
    {
        //      cout << " Projectile fix" << G4endl;
        Mprojectile2 = sqr(projectile->GetDefinition()->GetPDGMass() * (1-perCent) );
    } else {
        //      cout << " Target fix" << G4endl;
        Mtarget2=sqr(target->GetDefinition()->GetPDGMass() * (1-perCent) );
    }

    // Transform momenta to cms and then rotate parallel to z axis;

    G4LorentzVector Psum;
    Psum=Pprojectile+Ptarget;

    G4LorentzRotation toCms(-1*Psum.boostVector());

    G4LorentzVector Ptmp=toCms*Pprojectile;

    if ( Ptmp.pz() <= 0. )
    {
        // "String" moving backwards in  CMS, abort collision !!
        //         G4cout << " abort Collision!! " << G4endl;
        return false;
    }

    toCms.rotateZ(-1*Ptmp.phi());
    toCms.rotateY(-1*Ptmp.theta());

    //     G4cout << "Pprojectile  be4 boost " << Pprojectile << G4endl;
    //     G4cout << "Ptarget be4 boost : " << Ptarget << G4endl;



    G4LorentzRotation toLab(toCms.inverse());

    Pprojectile.transform(toCms);
    Ptarget.transform(toCms);

    G4LorentzVector Qmomentum;
    G4int whilecount=0;
    do {
        //  Generate pt

        G4double maxPtSquare=sqr(Ptarget.pz());
        if (whilecount++ >= 500 && (whilecount%100)==0)
            //       G4cout << "G4SingleDiffractiveExcitation::ExciteParticipants possibly looping"
            //       << ", loop count/ maxPtSquare : "
            //               << whilecount << " / " << maxPtSquare << G4endl;
            if (whilecount > 1000 )
            {
                Qmomentum=G4LorentzVector(0.,0.,0.,0.);
                //       G4cout << "G4SingleDiffractiveExcitation::ExciteParticipants: Aborting loop!" << G4endl;
                return false;     //  Ignore this interaction
            }
        Qmomentum=G4LorentzVector(GaussianPt(widthOfPtSquare,maxPtSquare),0);


        //  Momentum transfer
        G4double Xmin = minmass / ( Pprojectile.e() + Ptarget.e() );
        G4double Xmax=1.;
        G4double Xplus =ChooseX(Xmin,Xmax);
        G4double Xminus=ChooseX(Xmin,Xmax);

        G4double pt2=G4ThreeVector(Qmomentum.vect()).mag2();
        G4double Qplus =-1 * pt2 / Xminus/Ptarget.minus();
        G4double Qminus=     pt2 / Xplus /Pprojectile.plus();

        if ( KeepProjectile )
        {
            Qminus = (sqr(projectile->GetDefinition()->GetPDGMass()) + pt2 )
                            / (Pprojectile.plus() + Qplus )
                            -  Pprojectile.minus();
        } else
        {
            Qplus = Ptarget.plus()
                          - (sqr(target->GetDefinition()->GetPDGMass()) + pt2 )
                          / (Ptarget.minus() - Qminus );
        }           

        Qmomentum.setPz( (Qplus-Qminus)/2 );
        Qmomentum.setE(  (Qplus+Qminus)/2 );

        //   G4cout << "Qplus / Qminus " << Qplus << " / " << Qminus<<G4endl;
        //   G4cout << "pt2 " << pt2 << G4endl;
        //   G4cout << "Qmomentum " << Qmomentum << G4endl;
        //   G4cout << " Masses (P/T) : " << (Pprojectile+Qmomentum).mag() <<
        //             " / " << (Ptarget-Qmomentum).mag() << G4endl;

    } while (  (Ptarget-Qmomentum).mag2() <= Mtarget2
            || (Pprojectile+Qmomentum).mag2() <= Mprojectile2
            || (Ptarget-Qmomentum).e() < 0.
            || (Pprojectile+Qmomentum).e() < 0. );


    //     G4double Ecms=Pprojectile.e() + Ptarget.e();

    Pprojectile += Qmomentum;

    Ptarget     -= Qmomentum;

    //     G4cout << "Pprojectile.e()  : " << Pprojectile.e() << G4endl;
    //     G4cout << "Ptarget.e()      : " << Ptarget.e() << G4endl;

    //     G4cout << "end event_______________________________________________"<<G4endl;
    //


    //     G4cout << "Pprojectile with Q : " << Pprojectile << G4endl;
    //     G4cout << "Ptarget with Q : " << Ptarget << G4endl;
    //     G4cout << "Projectile back: " << toLab * Pprojectile << G4endl;
    //     G4cout << "Target back: " << toLab * Ptarget << G4endl;

    // Transform back and update SplitableHadron Participant.
    Pprojectile.transform(toLab);
    Ptarget.transform(toLab);

    //     G4cout << "G4SingleDiffractiveExcitation- Target mass      " <<  Ptarget.mag() << G4endl;
    //     G4cout << "G4SingleDiffractiveExcitation- Projectile mass  " <<  Pprojectile.mag() << G4endl;

    target->Set4Momentum(Ptarget);
    projectile->Set4Momentum(Pprojectile);


    return true;
}




// --------- private methods ----------------------

G4double G4SingleDiffractiveExcitation::ChooseX(G4double Xmin, G4double Xmax) const
{
    // choose an x between Xmin and Xmax with P(x) ~ 1/x

    //  to be improved...

    G4double range=Xmax-Xmin;

    if ( Xmin <= 0. || range <=0. ) 
    {
        G4cout << " Xmin, range : " << Xmin << " , " << range << G4endl;
        throw G4HadronicException(__FILE__, __LINE__, "G4SingleDiffractiveExcitation::ChooseX : Invalid arguments ");
    }

    G4double x;
    do {
        x=Xmin + G4UniformRand() * range;
    }  while ( Xmin/x < G4UniformRand() );

    //  cout << "DiffractiveX "<<x<<G4endl;
    return x;
}

G4ThreeVector G4SingleDiffractiveExcitation::GaussianPt(G4double widthSquare, G4double maxPtSquare) const
{            //  @@ this method is used in FTFModel as well. Should go somewhere common!

    G4double pt2;

    do {
        pt2=widthSquare * std::log( G4UniformRand() );
    } while ( pt2 > maxPtSquare);

    pt2=std::sqrt(pt2);

    G4double phi=G4UniformRand() * twopi;

    return G4ThreeVector (pt2*std::cos(phi), pt2*std::sin(phi), 0.);    
}