MultipleScatteringSimulator.cc

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#include "FastSimulation/MaterialEffects/interface/MultipleScatteringSimulator.h"
#include "FastSimulation/Utilities/interface/RandomEngineAndDistribution.h"

#include <cmath>

MultipleScatteringSimulator::MultipleScatteringSimulator(double A, double Z, double density, double radLen)
    : MaterialEffectsSimulator(A, Z, density, radLen) {
  sqr12 = std::sqrt(12.);
}

void MultipleScatteringSimulator::compute(ParticlePropagator& Particle, RandomEngineAndDistribution const* random) {
  double p2 = Particle.particle().Vect().Mag2();
  double m2 = Particle.particle().mass() * Particle.particle().mass();
  double e = std::sqrt(p2 + m2);

  double pbeta = p2 / e;  // This is p*beta

  // Average multiple scattering angle from Moliere radius
  // The sqrt(2) factor is because of the *space* angle
  double theta0 =
      0.0136 / pbeta * Particle.particle().charge() * std::sqrt(2. * radLengths) * (1. + 0.038 * std::log(radLengths));

  // Generate multiple scattering space angle perpendicular to the particle motion
  double theta = random->gaussShoot(0., theta0);
  // Plus a random rotation angle around the particle motion
  double phi = 2. * 3.14159265358979323 * random->flatShoot();
  // The two rotations
  RawParticle::Rotation rotation1(orthogonal(Particle.particle().Vect()), theta);
  RawParticle::Rotation rotation2(Particle.particle().Vect(), phi);
  // Rotate!
  Particle.particle().rotate(rotation1);
  Particle.particle().rotate(rotation2);

  // Generate mutiple scattering displacements in cm (assuming the detectors
  // are silicon only to determine the thickness) in the directions orthogonal
  // to the vector normal to the surface
  double xp = (cos(phi) * theta / 2. + random->gaussShoot(0., theta0) / sqr12) * radLengths * radLenIncm();
  double yp = (sin(phi) * theta / 2. + random->gaussShoot(0., theta0) / sqr12) * radLengths * radLenIncm();

  // Determine a unitary vector tangent to the surface
  // This tangent vector is unitary because "normal" is
  // either (0,0,1) in the Endcap  or (x,y,0) in the Barrel !
  XYZVector normal(theNormalVector.x(), theNormalVector.y(), theNormalVector.z());
  XYZVector tangent = orthogonal(normal);
  // The total displacement
  XYZVector delta = xp * tangent + yp * normal.Cross(tangent);
  // Translate!
  Particle.particle().translate(delta);
}