G4QuarkExchange.cc

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//
// ------------------------------------------------------------
//      GEANT 4 class implemetation file
//
//      ---------------- G4QuarkExchange --------------
//             by V. Uzhinsky, October 2016.
//       QuarkExchange is used by strings models.
//      Take a projectile and a target.
//Simulate Q exchange with excitation of projectile or target.
// ------------------------------------------------------------

#include "G4QuarkExchange.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"

#include "G4Log.hh"
#include "G4Pow.hh"

//#define debugQuarkExchange

G4QuarkExchange::G4QuarkExchange(){}

G4QuarkExchange::~G4QuarkExchange(){}

G4bool G4QuarkExchange::
ExciteParticipants(G4VSplitableHadron *projectile, G4VSplitableHadron *target) const
{
  #ifdef debugQuarkExchange
    G4cout<<G4endl<<"G4QuarkExchange::ExciteParticipants"<<G4endl;
  #endif

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

  G4LorentzVector Ptarget = target->Get4Momentum();
  G4double Mtarget        = target->GetDefinition()->GetPDGMass();
  G4double Mtarget2       = sqr(Mtarget);

  #ifdef debugQuarkExchange
    G4cout<<"Proj Targ "<<projectile->GetDefinition()->GetPDGEncoding()<<" "<<target->GetDefinition()->GetPDGEncoding()<<G4endl;
    G4cout<<"Proj. 4-Mom "<<Pprojectile<<" "<<Pprojectile.mag()<<G4endl
          <<"Targ. 4-Mom "<<Ptarget    <<" "<<Ptarget.mag()   <<G4endl;
  #endif

  G4LorentzVector Psum=Pprojectile+Ptarget;
  G4double SqrtS=Psum.mag();
  G4double S    =Psum.mag2();

  #ifdef debugQuarkExchange
    G4cout<<"SS Mpr Mtr SqrtS-Mprojectile-Mtarget "<<SqrtS<<" "<<Mprojectile<<" "<<Mtarget
          <<" "<<SqrtS-Mprojectile-Mtarget<<G4endl;
  #endif
  if (SqrtS-Mprojectile-Mtarget <= 250.0*MeV) {
    #ifdef debugQuarkExchange
      G4cerr<<"Energy is too small for quark exchange!"<<G4endl;
      G4cerr<<"Projectile: "<<projectile->GetDefinition()->GetPDGEncoding()<<" "
            <<Pprojectile<<" "<<Pprojectile.mag()<<G4endl;
      G4cerr<<"Target:     "<<target->GetDefinition()->GetPDGEncoding()<<" "
            <<Ptarget<<" "<<Ptarget.mag()<<G4endl; 
      G4cerr<<"sqrt(S) = "<<SqrtS<<" Mp + Mt = "<<Pprojectile.mag()+Ptarget.mag()<<G4endl;
    #endif
    return true;
  }

  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());

  G4LorentzRotation toLab(toCms.inverse());

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

  #ifdef debugQuarkExchange
    G4cout << "Pprojectile  in CMS " << Pprojectile << G4endl;
    G4cout << "Ptarget      in CMS " << Ptarget     << G4endl;
  #endif
  G4double maxPtSquare=sqr(Ptarget.pz());

  G4double ProjectileMinDiffrMass = Pprojectile.mag()/GeV;
  G4double TargetMinDiffrMass     = Ptarget.mag()/GeV;

  G4double AveragePt2(0.);

  G4int    PDGcode=projectile->GetDefinition()->GetPDGEncoding();
  G4int absPDGcode=std::abs(PDGcode); 

  G4bool ProjectileDiffraction = true;

  if ( absPDGcode > 1000 )                          { ProjectileDiffraction = G4UniformRand() <= 0.5; }
  if ( (absPDGcode == 211) || (absPDGcode == 111) ) { ProjectileDiffraction = G4UniformRand() <= 0.66; }
  if ( (absPDGcode == 321) || (absPDGcode == 311)  || 
       (   PDGcode == 130) || (   PDGcode == 310) ) { ProjectileDiffraction = G4UniformRand() <= 0.5; }

  if ( ProjectileDiffraction ) {
    if ( absPDGcode > 1000 )                            //------Projectile is baryon --------
    {
      ProjectileMinDiffrMass = 1.16;              // GeV
      AveragePt2 = 0.3;                           // GeV^2
    }
    else if( absPDGcode == 211 || absPDGcode ==  111) //------Projectile is Pion -----------
    {
      ProjectileMinDiffrMass = 1.0;               // GeV
      AveragePt2 = 0.3;                           // GeV^2
    }
    else if( absPDGcode == 321 || absPDGcode == 130 || absPDGcode == 310) //Projectile is Kaon
    {
      ProjectileMinDiffrMass = 1.1;               // GeV
      AveragePt2 = 0.3;                           // GeV^2
    }
    else if( absPDGcode == 22)                        //------Projectile is Gamma -----------
    {
      ProjectileMinDiffrMass = 0.25;             // GeV
      AveragePt2 = 0.36;                         // GeV^2
    }
    else                                             //------Projectile is undefined, Nucleon assumed
    {
      ProjectileMinDiffrMass = 1.1;              // GeV
      AveragePt2 = 0.3;                          // GeV^2
    };

    ProjectileMinDiffrMass = ProjectileMinDiffrMass * GeV;
    Mprojectile2=sqr(ProjectileMinDiffrMass);

    if (G4UniformRand() <= 0.5) TargetMinDiffrMass += 0.22; 
    TargetMinDiffrMass *= GeV;
    Mtarget2 = sqr( TargetMinDiffrMass) ;
  }
  else
  {
    if (G4UniformRand() <= 0.5) ProjectileMinDiffrMass += 0.22; 
    ProjectileMinDiffrMass *=GeV;
    Mprojectile2=sqr(ProjectileMinDiffrMass);

    TargetMinDiffrMass = 1.16*GeV;                     // For target nucleon
    Mtarget2 = sqr( TargetMinDiffrMass) ;
    AveragePt2 = 0.3;                                  // GeV^2
  }   // end of if ( ProjectileDiffraction )

  AveragePt2 = AveragePt2 * GeV*GeV;

  if ( SqrtS - (ProjectileMinDiffrMass+TargetMinDiffrMass) < 220* MeV ) return false;

  //----------------------- 
  G4double Pt2, PZcms, PZcms2;
  G4double ProjMassT2, ProjMassT;
  G4double TargMassT2, TargMassT;
  G4double PMinusMin, PMinusMax,  sqrtPMinusMin, sqrtPMinusMax;
  G4double TPlusMin, TPlusMax,    sqrtTPlusMin,  sqrtTPlusMax;
  G4double PMinusNew, PPlusNew, TPlusNew(0.), TMinusNew;

  G4LorentzVector Qmomentum;
  G4double Qminus, Qplus;

  G4double x(0.), y(0.);
  G4int whilecount=0;
  do {
    whilecount++;

    if (whilecount > 1000 )
    {
      Qmomentum=G4LorentzVector(0.,0.,0.,0.);
      return false;     //  Ignore this interaction
    }
    
    // Generate pt
    Qmomentum=G4LorentzVector(GaussianPt(AveragePt2,maxPtSquare),0);

    Pt2 = G4ThreeVector( Qmomentum.vect() ).mag2();
    ProjMassT2 = Mprojectile2 + Pt2;
    ProjMassT = std::sqrt( ProjMassT2 );
    TargMassT2 = Mtarget2 + Pt2;
    TargMassT = std::sqrt( TargMassT2 );

    #ifdef debugQuarkExchange
      G4cout<<"whilecount  Pt2  ProjMassT  TargMassT  SqrtS  S  ProjectileDiffraction"<<G4endl;
      G4cout<<whilecount<<" "<<Pt2<<" "<<ProjMassT<<" "<<TargMassT<<" "<<SqrtS<<" "<<S<<" "<<ProjectileDiffraction<<G4endl;
    #endif

    if ( SqrtS < ProjMassT + TargMassT + 220.0*MeV ) continue;

    PZcms2 = ( S*S + ProjMassT2*ProjMassT2 + TargMassT2*TargMassT2
               - 2.0*S*ProjMassT2 - 2.0*S*TargMassT2 - 2.0*ProjMassT2*TargMassT2 ) / 4.0 / S;

    if ( PZcms2 < 0 ) continue;

    PZcms = std::sqrt( PZcms2 );

    if ( ProjectileDiffraction )
    { // The projectile will fragment, the target will saved.
      PMinusMin = std::sqrt( ProjMassT2 + PZcms2 ) - PZcms;
      PMinusMax = SqrtS - TargMassT;
      sqrtPMinusMin = std::sqrt(PMinusMin); sqrtPMinusMax = std::sqrt(PMinusMax);

      if ( absPDGcode > 1000 ) 
      { 
  PMinusNew = PMinusMax * (1.0 - (1.0 - PMinusMin/PMinusMax) 
                                     * G4Pow::GetInstance()->powA(G4UniformRand(),0.3333) );
      } 
      else if ( (absPDGcode == 211) || (absPDGcode == 111) ) 
      {
        while (true)
        {
          x=sqrtPMinusMax-(sqrtPMinusMax-sqrtPMinusMin)*G4UniformRand();
    y=G4UniformRand();
          if ( y < 1.0-0.7 * x/sqrtPMinusMax ) break;
  }
  PMinusNew = sqr(x);
      } 
      else if ( (absPDGcode == 321) || (absPDGcode == 311)  || 
          (   PDGcode == 130) || (   PDGcode == 310) ) 
      {  // For K-mesons it must be found !!! Uzhi
        while (true)
        {
    x=sqrtPMinusMax-(sqrtPMinusMax-sqrtPMinusMin)*G4UniformRand();
    y=G4UniformRand();
          if ( y < 1.0-0.7 * x/sqrtPMinusMax ) break;
        }
  PMinusNew = sqr(x);
      } 
      else
      { 
  PMinusNew = PMinusMax * (1.0 - (1.0 - PMinusMin/PMinusMax) 
                                 * G4Pow::GetInstance()->powA(G4UniformRand(),0.3333) );
      };     

      TMinusNew = SqrtS - PMinusNew;

      Qminus = Ptarget.minus() - TMinusNew;
      TPlusNew = TargMassT2 / TMinusNew;
      Qplus = Ptarget.plus() - TPlusNew;

    } 
    else 
    { // The target will fragment, the projectile will saved.
      TPlusMin = std::sqrt( TargMassT2 + PZcms2 ) - PZcms;
      TPlusMax = SqrtS - ProjMassT;
      sqrtTPlusMin = std::sqrt(TPlusMin); sqrtTPlusMax = std::sqrt(TPlusMax);

      if ( absPDGcode > 1000 ) 
      { 
        TPlusNew = TPlusMax * (1.0 - (1.0 - TPlusMin/TPlusMax) 
                                    * G4Pow::GetInstance()->powA(G4UniformRand(),0.3333) );
      } 
      else if ( (absPDGcode == 211) || (absPDGcode == 111) ) 
      {
        while (true)
        {
          x=sqrtTPlusMax-(sqrtTPlusMax-sqrtTPlusMin)*G4UniformRand();
          y=G4UniformRand();
          if ( y < 1.0-0.7 * x/sqrtTPlusMax ) break;
        }
  TPlusNew = sqr(x);
      } 
      else if ( (absPDGcode == 321) || (absPDGcode == 311)  || 
          (   PDGcode == 130) || (   PDGcode == 310) ) 
      { // For K-mesons it must be found !!! Uzhi
  while (true)
  {
          x=sqrtTPlusMax-(sqrtTPlusMax-sqrtTPlusMin)*G4UniformRand();
          y=G4UniformRand();
    if ( y < 1.0-0.7 * x/sqrtTPlusMax ) break;
  }
      } 
      else
      { 
        TPlusNew = TPlusMax * (1.0 - (1.0 - TPlusMin/TPlusMax) 
                                    * G4Pow::GetInstance()->powA(G4UniformRand(),0.3333) );
      };

      PPlusNew = SqrtS - TPlusNew;

      Qplus = PPlusNew - Pprojectile.plus();
      PMinusNew = ProjMassT2 / PPlusNew;
      Qminus = PMinusNew - Pprojectile.minus();
    }

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

    #ifdef debugQuarkExchange
      G4cout<<"ProjectileDiffraction (Pprojectile + Qmomentum).mag2()  Mprojectile2"<<G4endl;
      G4cout<<ProjectileDiffraction<<" "<<( Pprojectile + Qmomentum ).mag2()<<" "<< Mprojectile2<<G4endl;
      G4cout<<"TargetDiffraction     (Ptarget     - Qmomentum).mag2()  Mtarget2"<<G4endl;
      G4cout<<!ProjectileDiffraction<<" "<<( Ptarget    - Qmomentum ).mag2()<<" "<< Mtarget2<<G4endl;
    #endif

  } while ( ( ProjectileDiffraction&&( Pprojectile + Qmomentum ).mag2() <  Mprojectile2 ) ||
            (!ProjectileDiffraction&&( Ptarget     - Qmomentum ).mag2() <  Mtarget2       )   ); 
    // Repeat the sampling because there was not any excitation

  Pprojectile += Qmomentum;

  Ptarget     -= Qmomentum;

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

  #ifdef debugQuarkExchange
    G4cout << "Pprojectile  in Lab. " << Pprojectile << G4endl;
    G4cout << "Ptarget      in Lab. " << Ptarget     << G4endl;
    G4cout << "G4QuarkExchange: Projectile mass  " <<  Pprojectile.mag() << G4endl;
    G4cout << "G4QuarkExchange: Target mass      " <<  Ptarget.mag() << G4endl;
  #endif

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

  //=================================== Quark exchange ================================
  projectile->SplitUp();
  target->SplitUp();

  G4Parton* PrQuark = projectile->GetNextParton(); 
  G4Parton* TrQuark = target->GetNextParton(); 

  G4ParticleDefinition * Tmp = PrQuark->GetDefinition();
  PrQuark->SetDefinition(TrQuark->GetDefinition());
  TrQuark->SetDefinition(Tmp);

  return true;
}


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

G4ThreeVector G4QuarkExchange::GaussianPt(G4double widthSquare, G4double maxPtSquare) const
{            //  @@ this method is used in FTFModel as well. Should go somewhere common!

  G4double pt2;

  const G4int maxNumberOfLoops = 1000;
  G4int loopCounter = 0;
  do {
    pt2=-widthSquare * G4Log( G4UniformRand() );
  } while ( ( pt2 > maxPtSquare) && ++loopCounter < maxNumberOfLoops );  /* Loop checking, 07.08.2015, A.Ribon */
  if ( loopCounter >= maxNumberOfLoops ) {
    pt2 = 0.99*maxPtSquare;  // Just an acceptable value, without any physics consideration. 
  }

  pt2=std::sqrt(pt2);

  G4double phi=G4UniformRand() * twopi;

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