G4INCLEtaNToPiNChannel.cc

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//
// INCL++ intra-nuclear cascade model
// Alain Boudard, CEA-Saclay, France
// Joseph Cugnon, University of Liege, Belgium
// Jean-Christophe David, CEA-Saclay, France
// Pekka Kaitaniemi, CEA-Saclay, France, and Helsinki Institute of Physics, Finland
// Sylvie Leray, CEA-Saclay, France
// Davide Mancusi, CEA-Saclay, France
//
#define INCLXX_IN_GEANT4_MODE 1

#include "globals.hh"

#include "G4INCLEtaNToPiNChannel.hh"
#include "G4INCLKinematicsUtils.hh"
#include "G4INCLBinaryCollisionAvatar.hh"
#include "G4INCLRandom.hh"
#include "G4INCLGlobals.hh"
#include "G4INCLLogger.hh"

namespace G4INCL {

    EtaNToPiNChannel::EtaNToPiNChannel(Particle *p1, Particle *p2)
    : particle1(p1), particle2(p2)
    {

    }

    EtaNToPiNChannel::~EtaNToPiNChannel(){

    }

    void EtaNToPiNChannel::fillFinalState(FinalState *fs) {
        Particle * nucleon;
        Particle * eta;
        if(particle1->isNucleon()) {
            nucleon = particle1;
            eta = particle2;
        } else {
            nucleon = particle2;
            eta = particle1;
        }

        const G4double r2 = Random::shoot();
        if (nucleon->getType() == Neutron) {
            if (r2*3. < 2.) {
                nucleon->setType(Proton);
                eta->setType(PiMinus);
            }
            else {
                nucleon->setType(Neutron);
                eta->setType(PiZero);
            }
        }
        else {
            if (r2*3. < 2.) {
                nucleon->setType(Neutron);
                eta->setType(PiPlus);
            }
            else {
                nucleon->setType(Proton);
                eta->setType(PiZero);
            }
        }
        
        G4double sh=nucleon->getEnergy()+eta->getEnergy();
        G4double mn=nucleon->getMass();
        G4double me=eta->getMass();
        G4double en=(sh*sh+mn*mn-me*me)/(2*sh);
        nucleon->setEnergy(en);
        G4double ee=std::sqrt(en*en-mn*mn+me*me);
        eta->setEnergy(ee);
        G4double pn=std::sqrt(en*en-mn*mn);
/* isotropy

        ThreeVector mom_nucleon = Random::normVector(pn);

 nucleon->setMomentum(mom_nucleon);
 eta->setMomentum(-mom_nucleon);
*/

// real distribution (from PRC 78, 025204 (2008))  --- detailed balance from pi nucleon --> eta nucleon 
     
     
     G4double ECM=G4INCL::KinematicsUtils::totalEnergyInCM(particle1,particle2);
     
     const G4double pi=std::acos(-1.0);     
     G4double x1;
     G4double u1;
     G4double fteta;
     G4double teta;
     G4double fi;
     
     if (ECM < 1650.) {
      // below 1650 MeV - angular distribution (x=cos(theta): ax^2+bx+c     
      
      G4double f1= -0.0000288627*ECM*ECM+0.09155289*ECM-72.25436;  // f(1) that is the maximum (fit on experimental data)
      G4double b1=(f1-(f1/(1.5-0.5*std::pow((ECM-1580.)/95.,2))))/2.; // ideas: 1) f(-1)=0.5f(1); 2) "power term" flattens the distribution away from ECM=1580 MeV
      G4double a1=2.5*b1; // minimum at cos(theta) = -0.2
      G4double c1=f1-3.5*b1;
      
      G4double interg1=2.*a1/3. +2.*c1; // (integral to normalize)   
      
      G4int passe1=0;
      while (passe1==0) {
       // Sample x from -1 to 1
       x1=Random::shoot();
       if (Random::shoot() > 0.5) x1=-x1;
       
       // Sample u from 0 to 1
       u1=Random::shoot();
       fteta=(a1*x1*x1+b1*x1+c1)/interg1;
       // The condition
       if (u1*f1/interg1 < fteta) {
        teta=std::acos(x1);
        passe1=1;
       }
      }
     }
     else {        
      // above 1650 MeV - angular distribution (x=cos(theta): (ax^2+bx+c)*(0.5+(arctan(10*(x+dev)))/pi) + vert
      
      G4double a2=-0.29;
      G4double b2=0.348;    // ax^2+bx+c: around cos(theta)=0.6 with maximum at 0.644963 (value = 0.1872666)
      G4double c2=0.0546;
      G4double dev=-0.2;  // tail close to zero from "dev" down to -1
      G4double vert=0.04; // to avoid negative differential cross sections
      
      G4double interg2=0.1716182902205207; // with the above given parameters! (integral to normalize)
      const G4double f2=1.09118088; // maximum (integral taken into account)
      
      G4int passe2=0;
      while (passe2==0) {
       // Sample x from -1 to 1
       x1=Random::shoot();
       if (Random::shoot() > 0.5) x1=-x1;
       
       // Sample u from 0 to 1
       u1=Random::shoot();
       fteta=((a2*x1*x1+b2*x1+c2)*(0.5+(std::atan(10*(x1+dev)))/pi) + vert)/interg2;
       // The condition
       if (u1*f2 < fteta) {
        teta=std::acos(x1);
        passe2=1;
       }
      }
     }
     
     fi=(2.0*pi)*Random::shoot();       
     
     ThreeVector mom_nucleon(
                             pn*std::sin(teta)*std::cos(fi),
                             pn*std::sin(teta)*std::sin(fi),
                             pn*std::cos(teta)
                             );
// end real distribution            
     
        nucleon->setMomentum(-mom_nucleon);
        eta->setMomentum(mom_nucleon);
        
        fs->addModifiedParticle(nucleon);
        fs->addModifiedParticle(eta);
    }

}