MultipleScattering.cc

Go to the documentation of this file.

#include "FastSimulation/Utilities/interface/RandomEngineAndDistribution.h"
#include "FastSimulation/SimplifiedGeometryPropagator/interface/Particle.h"
#include "FastSimulation/SimplifiedGeometryPropagator/interface/SimplifiedGeometry.h"
#include "FastSimulation/SimplifiedGeometryPropagator/interface/Constants.h"
#include "FWCore/MessageLogger/interface/MessageLogger.h"
#include "FWCore/ParameterSet/interface/ParameterSet.h"

#include <cmath>
#include <memory>

#include <Math/AxisAngle.h>

#include "FastSimulation/SimplifiedGeometryPropagator/interface/InteractionModelFactory.h"
#include "FastSimulation/SimplifiedGeometryPropagator/interface/InteractionModel.h"
#include "DataFormats/Math/interface/LorentzVector.h"
#include "DataFormats/Math/interface/Vector3D.h"

// Author: Patrick Janot
// Date: 8-Jan-2004
//
// Revision: Class structure modified to match SimplifiedGeometryPropagator
//           Fixed a bug in which particles could no longer be on the layer after scattering
//           S. Kurz, 29 May 2017

typedef math::XYZVector XYZVector;

namespace fastsim {

  class MultipleScattering : public InteractionModel {
  public:
    MultipleScattering(const std::string& name, const edm::ParameterSet& cfg);

    ~MultipleScattering() override { ; };


    void interact(Particle& particle,
                  const SimplifiedGeometry& layer,
                  std::vector<std::unique_ptr<Particle> >& secondaries,
                  const RandomEngineAndDistribution& random) override;

  private:
    XYZVector orthogonal(const XYZVector& aVector) const;

    double minPt_;       
    double radLenInCm_;  
  };
}  // namespace fastsim

fastsim::MultipleScattering::MultipleScattering(const std::string& name, const edm::ParameterSet& cfg)
    : fastsim::InteractionModel(name) {
  // Set the minimal pT for interaction
  minPt_ = cfg.getParameter<double>("minPt");
  // Material properties
  radLenInCm_ = cfg.getParameter<double>("radLen");
}

void fastsim::MultipleScattering::interact(fastsim::Particle& particle,
                                           const SimplifiedGeometry& layer,
                                           std::vector<std::unique_ptr<fastsim::Particle> >& secondaries,
                                           const RandomEngineAndDistribution& random) {
  //
  // only charged particles
  //
  if (particle.charge() == 0) {
    return;
  }

  double radLengths = layer.getThickness(particle.position(), particle.momentum());
  //
  // no material
  //
  if (radLengths < 1E-10) {
    return;
  }

  // particle must have minimum pT
  if (particle.momentum().Pt() < minPt_) {
    return;
  }

  double p2 = particle.momentum().Vect().Mag2();
  double m2 = particle.momentum().mass2();
  double e = std::sqrt(p2 + m2);

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

  // Average multiple scattering angle from Moliere radius
  // The sqrt(2) factor is because of the *space* angle
  double theta0 = 0.0136 / pbeta * 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. * M_PI * random.flatShoot();

  // The two rotations
  ROOT::Math::AxisAngle rotation1(orthogonal(particle.momentum().Vect()), theta);
  ROOT::Math::AxisAngle rotation2(particle.momentum().Vect(), phi);
  // Rotate!
  XYZVector rotated = rotation2((rotation1(particle.momentum().Vect())));
  particle.momentum().SetXYZT(rotated.X(), rotated.Y(), rotated.Z(), particle.momentum().E());

  // 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) / sqrt(12.)) * radLengths * radLenInCm_;
  double yp = (sin(phi) * theta / 2. + random.gaussShoot(0., theta0) / sqrt(12.)) * radLengths * radLenInCm_;

  // Determine a unitary vector tangent to the surface
  XYZVector normal;
  if (layer.isForward()) {
    normal = XYZVector(0., 0., particle.momentum().Pz() > 0. ? 1. : -1.);
  } else {
    double norm = particle.position().Rho();
    normal = XYZVector(particle.position().X() / norm, particle.position().Y() / norm, 0.);
  }
  XYZVector tangent = orthogonal(normal);
  // The total displacement
  XYZVector delta = xp * tangent + yp * normal.Cross(tangent);
  // Translate!
  particle.position().SetXYZT(particle.position().X() + delta.X(),
                              particle.position().Y() + delta.Y(),
                              particle.position().Z() + delta.Z(),
                              particle.position().T());

  // Make sure particle is still physically on the layer (particle moved on a tangent but a barrel layer is bent)
  // Happens only in very very few cases
  if (!layer.isForward() && std::abs(layer.getGeomProperty() - particle.position().Rho()) > 1e-2) {
    // Scale positions so the radii agree
    double scalePos = layer.getGeomProperty() / particle.position().Rho();
    particle.position().SetXYZT(particle.position().X() * scalePos,
                                particle.position().Y() * scalePos,
                                particle.position().Z(),
                                particle.position().T());

    // Add a protection in case something goes wrong
    if (std::abs(layer.getGeomProperty() - particle.position().Rho()) > 1e-2) {
      throw cms::Exception("fastsim::MultipleScattering") << "particle no longer on layer's surface";
    }
  }
}

XYZVector fastsim::MultipleScattering::orthogonal(const XYZVector& aVector) const {
  double x = fabs(aVector.X());
  double y = fabs(aVector.Y());
  double z = fabs(aVector.Z());

  if (x < y)
    return x < z ? XYZVector(0., aVector.Z(), -aVector.Y()) : XYZVector(aVector.Y(), -aVector.X(), 0.);
  else
    return y < z ? XYZVector(-aVector.Z(), 0., aVector.X()) : XYZVector(aVector.Y(), -aVector.X(), 0.);
}

DEFINE_EDM_PLUGIN(fastsim::InteractionModelFactory, fastsim::MultipleScattering, "fastsim::MultipleScattering");