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.);    
}

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