BremsstrahlungSimulator.cc

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//FAMOS Headers
#include "FastSimulation/MaterialEffects/interface/BremsstrahlungSimulator.h"
#include "FastSimulation/Utilities/interface/RandomEngineAndDistribution.h"

#include <cmath>

BremsstrahlungSimulator::BremsstrahlungSimulator(double photonEnergyCut, 
                         double photonFractECut)
{
  // Set the minimal photon energy for a Brem from e+/-
  photonEnergy = photonEnergyCut;
  photonFractE = photonFractECut;
}


void 
BremsstrahlungSimulator::compute(ParticlePropagator &Particle, RandomEngineAndDistribution const* random)
{

  // Protection : Just stop the electron if more than 1 radiation lengths.
  // This case corresponds to an electron entering the layer parallel to 
  // the layer axis - no reliable simulation can be done in that case...
  // 08/02/06 - pv: increase protection from 1 to 4 X0 for eta>4.8 region
  // if ( radLengths > 1. ) Particle.SetXYZT(0.,0.,0.,0.);
  if ( radLengths > 4. ) Particle.SetXYZT(0.,0.,0.,0.);

  // Hard brem probability with a photon Energy above photonEnergy.
  if (Particle.e()<photonEnergy) return;
  xmin = std::max(photonEnergy/Particle.e(),photonFractE);
  if ( xmin >=1. || xmin <=0. ) return;

  double bremProba = radLengths * ( 4./3. * std::log(1./xmin)
                  - 4./3. * (1.-xmin)
                  + 1./2. * (1.-xmin*xmin) );

  
  // Number of photons to be radiated.
  unsigned int nPhotons = poisson(bremProba, random);
  _theUpdatedState.reserve(nPhotons);

  if ( !nPhotons ) return;

  //Rotate to the lab frame
  double chi = Particle.theta();
  double psi = Particle.phi();
  RawParticle::RotationZ rotZ(psi);
  RawParticle::RotationY rotY(chi);
    
  // Energy of these photons
  for ( unsigned int i=0; i<nPhotons; ++i ) {

    // Check that there is enough energy left.
    if ( Particle.e() < photonEnergy ) break;

    // Add a photon
    RawParticle thePhoton(22,brem(Particle, random));
    thePhoton.rotate(rotY);
    thePhoton.rotate(rotZ);
    _theUpdatedState.push_back(thePhoton);
    
    // Update the original e+/-
    Particle -= thePhoton.momentum();

  } 
}

XYZTLorentzVector 
BremsstrahlungSimulator::brem(ParticlePropagator& pp, RandomEngineAndDistribution const* random) const {

  // This is a simple version (a la PDG) of a Brem generator.
  // It replaces the buggy GEANT3 -> C++ former version.
  // Author : Patrick Janot - 25-Dec-2003
  double emass = 0.0005109990615;
  double xp=0;
  double weight = 0.;
  
  do {
    xp = xmin * std::exp ( -std::log(xmin) * random->flatShoot() );
    weight = 1. - xp + 3./4.*xp*xp;
  } while ( weight < random->flatShoot() );
  
  
  // Have photon energy. Now generate angles with respect to the z axis 
  // defined by the incoming particle's momentum.

  // Isotropic in phi
  const double phi = random->flatShoot()*2*M_PI;
  // theta from universal distribution
  const double theta = gbteth(pp.e(),emass,xp,random)*emass/pp.e();
  
  // Make momentum components
  double stheta = std::sin(theta);
  double ctheta = std::cos(theta);
  double sphi   = std::sin(phi);
  double cphi   = std::cos(phi);
  
  return xp * pp.e() * XYZTLorentzVector(stheta*cphi,stheta*sphi,ctheta,1.);
  
}

double
BremsstrahlungSimulator::gbteth(const double ener,
                const double partm,
                const double efrac,
                                RandomEngineAndDistribution const* random) const {
  const double alfa = 0.625;

  const double d = 0.13*(0.8+1.3/theZ())*(100.0+(1.0/ener))*(1.0+efrac);
  const double w1 = 9.0/(9.0+d);
  const double umax = ener*M_PI/partm;
  double u;
  
  do {
    double beta = (random->flatShoot()<=w1) ? alfa : 3.0*alfa;
    u = -std::log(random->flatShoot()*random->flatShoot())/beta;
  } while (u>=umax);

  return u;
}


unsigned int 
BremsstrahlungSimulator::poisson(double ymu, RandomEngineAndDistribution const* random) {

  unsigned int n = 0;
  double prob = std::exp(-ymu);
  double proba = prob;
  double x = random->flatShoot();
  
  while ( proba <= x ) {
    prob *= ymu / double(++n);
    proba += prob;
  }
  
  return n;                                                        
  
}