/data/refman/pasoursint/CMSSW_5_3_1/src/FastSimulation/MaterialEffects/src/EnergyLossSimulator.cc

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#include "FastSimulation/MaterialEffects/interface/EnergyLossSimulator.h"
//#include "FastSimulation/Utilities/interface/RandomEngine.h"
#include "FastSimulation/Utilities/interface/LandauFluctuationGenerator.h"

#include <cmath>

EnergyLossSimulator::EnergyLossSimulator(const RandomEngine* engine,
                                         double A, double Z, double density, double radLen) :
    MaterialEffectsSimulator(engine,A,Z,density,radLen) 
{

  theGenerator = new LandauFluctuationGenerator(engine);

}

EnergyLossSimulator::~EnergyLossSimulator() {

  delete theGenerator;

}

void 
EnergyLossSimulator::compute(ParticlePropagator &Particle)
{

  //  FamosHistos* myHistos = FamosHistos::instance();

  // double gamma_e = 0.577215664901532861;  // Euler constant
  
  // The thickness in cm
  double thick = radLengths * radLenIncm();
  
  // This is a simple version (a la PDG) of a dE/dx generator.
  // It replaces the buggy GEANT3 -> C++ former version.
  // Author : Patrick Janot - 8-Jan-2004

  double p2  = Particle.Vect().Mag2();
  double verySmallP2 = 0.0001;
  if (p2<=verySmallP2) {
    deltaP.SetXYZT(0.,0.,0.,0.);
    return;
  }
  double m2  = Particle.mass() * Particle.mass();
  double e2  = p2+m2;

  double beta2 = p2/e2;
  double gama2 = e2/m2;
  
  double charge2 = Particle.charge() * Particle.charge();
  
  // Energy loss spread in GeV
  double eSpread  = 0.1536E-3*charge2*(theZ()/theA())*rho()*thick/beta2;
 
  // Most probable energy loss (from the integrated Bethe-Bloch equation)
  mostProbableLoss = eSpread * ( log ( 2.*eMass()*beta2*gama2*eSpread
                                     / (excitE()*excitE()) )
                                 - beta2 + 0.200 );

  // This one can be needed on output (but is not used internally)
  // meanEnergyLoss = 2.*eSpread * ( log ( 2.*eMass()*beta2*gama2 /excitE() ) - beta2 );

  // Generate the energy loss with Landau fluctuations
  double dedx = mostProbableLoss + eSpread * theGenerator->landau();

  // Compute the new energy and momentum
  double aBitAboveMass = Particle.mass()*1.0001;
  double newE = std::max(aBitAboveMass,Particle.e()-dedx);
  //  double newE = std::max(Particle.mass(),Particle.e()-dedx);
  double fac  = std::sqrt((newE*newE-m2)/p2);

  
  // Update the momentum
  deltaP.SetXYZT(Particle.Px()*(1.-fac),Particle.Py()*(1.-fac),
                 Particle.Pz()*(1.-fac),Particle.E()-newE);
  Particle.SetXYZT(Particle.Px()*fac,Particle.Py()*fac, 
                   Particle.Pz()*fac,newE);
  
}