fastsim::LayerNavigator Class Reference

Handles/tracks (possible) intersections of particle's trajectory and tracker layers. More...

#include <LayerNavigator.h>

Public Member Functions
	LayerNavigator (const Geometry &geometry)
	Constructor. More...
bool	moveParticleToNextLayer (Particle &particle, const SimplifiedGeometry *&layer)
	Move particle along its trajectory to the next intersection with any of the tracker layers. More...

Private Attributes
const Geometry *const	geometry_
	The geometry of the tracker material. More...
const BarrelSimplifiedGeometry *	nextBarrelLayer_
	Pointer to the next (direction of the particle's momentum) barrel layer. More...
const ForwardSimplifiedGeometry *	nextForwardLayer_
	Pointer to the next (direction of the particle's momentum) forward layer. More...
const BarrelSimplifiedGeometry *	previousBarrelLayer_
	Pointer to the previous (opposite direction of the particle's momentum) barrel layer. More...
const ForwardSimplifiedGeometry *	previousForwardLayer_
	Pointer to the previous (opposite direction of the particle's momentum) forward layer. More...

Static Private Attributes
static const std::string	MESSAGECATEGORY = "FastSimulation"

Detailed Description

Handles/tracks (possible) intersections of particle's trajectory and tracker layers.

the geometry is described by 2 sets of layers:

• forward layers: flat layers, perpendicular to the z-axis, positioned at a given z these layers have material / instruments between a given materialMinR and materialMaxR no 2 forward layers should have the same z-position
• barrel layers: cylindrically shaped layers, with the z-axis as axis, infinitely long these layers have material / instruments for |z| < materialMaxAbsZ no 2 barrel layers should have the same radius
• forward (barrel) layers are ordered according to increasing z (r)

principle

• neutral particles follow a straight trajectory
• charged particles follow a helix-shaped trajectory: constant speed along the z-axis circular path in the x-y plane => the next layer that the particle will cross is among the following 3 layers
  • closest forward layer with
  • z >(<) particle.z() for particles moving in the positive(negative) direction
  • closest barrel layer with r < particle.r
  • closest barrel layer with r > particle.r

algorithm

• find the 3 candidate layers
• find the earliest positive intersection time for each of the 3 candidate layers
• move the particle to the earliest intersection time
• select and return the layer with the earliest positive intersection time

Definition at line 48 of file LayerNavigator.h.

Constructor & Destructor Documentation

fastsim::LayerNavigator::LayerNavigator

(

const Geometry &

geometry

)

Constructor.

Parameters

geometry

The geometry of the tracker material.

Definition at line 60 of file LayerNavigator.cc.

    : geometry_(&geometry),
      nextBarrelLayer_(nullptr),
      previousBarrelLayer_(nullptr),
      nextForwardLayer_(nullptr),
      previousForwardLayer_(nullptr) {
  ;
}

Member Function Documentation

bool fastsim::LayerNavigator::moveParticleToNextLayer	(	fastsim::Particle &	particle,
		const SimplifiedGeometry *&	layer
	)

Move particle along its trajectory to the next intersection with any of the tracker layers.

Parameters

particle	The particle that has to be moved to the next layer.
layer	The layer to which the particle was moved in the previous call of this function (0 if first call). Returns the layer this particle was then moved to.

Returns

true / false if propagation succeeded / failed.

Definition at line 69 of file LayerNavigator.cc.

References funct::abs(), fastsim::Trajectory::createTrajectory(), alignCSCRings::e, Exception, fastsim::Particle::gamma(), fastsim::SimplifiedGeometry::getMagneticFieldZ(), fastsim::SimplifiedGeometry::index(), fastsim::SimplifiedGeometry::isForward(), fastsim::Particle::isOnLayer(), fastsim::Particle::isStable(), phase1PixelTopology::layer, LayerTriplets::layers(), LogDebug, fastsim::Particle::momentum(), fastsim::Particle::position(), fastsim::Particle::remainingProperLifeTimeC(), fastsim::Particle::resetOnLayer(), fastsim::Particle::setOnLayer(), and fastsim::Particle::setRemainingProperLifeTimeC().

Referenced by FastSimProducer::createFSimTrack(), and FastSimProducer::produce().

                                                                                               {
  LogDebug(MESSAGECATEGORY) << "   moveToNextLayer called";

  // if the layer is provided, the particle must be on it
  if (layer) {
    if (!particle.isOnLayer(layer->isForward(), layer->index())) {
      throw cms::Exception("FastSimulation") << "If layer is provided, particle must be on layer."
                                             << "\n   Layer: " << *layer << "\n   Particle: " << particle;
    }
  }

  // magnetic field at the current position of the particle
  // considered constant between the layers
  double magneticFieldZ =
      layer ? layer->getMagneticFieldZ(particle.position()) : geometry_->getMagneticFieldZ(particle.position());
  LogDebug(MESSAGECATEGORY) << "   magnetic field z component:" << magneticFieldZ;

  // particle moves inwards?
  bool particleMovesInwards =
      particle.momentum().X() * particle.position().X() + particle.momentum().Y() * particle.position().Y() < 0;

  //
  //  update nextBarrelLayer and nextForwardLayer
  //

  // first time method is called
  if (!layer) {
    LogDebug(MESSAGECATEGORY) << "      called for first time";

    // find the narrowest barrel layers with
    // layer.r > particle.r (the closest layer with layer.r < particle.r will then be considered, too)
    // assume barrel layers are ordered with increasing r
    for (const auto& layer : geometry_->barrelLayers()) {
      if (particle.isOnLayer(false, layer->index()) ||
          std::abs(layer->getRadius() - particle.position().Rho()) < 1e-2) {
        if (particleMovesInwards) {
          nextBarrelLayer_ = layer.get();
          break;
        } else {
          continue;
        }
      }

      if (particle.position().Pt() < layer->getRadius()) {
        nextBarrelLayer_ = layer.get();
        break;
      }

      previousBarrelLayer_ = layer.get();
    }

    //  find the forward layer with smallest z with
    //  layer.z > particle z (the closest layer with layer.z < particle.z will then be considered, too)
    for (const auto& layer : geometry_->forwardLayers()) {
      if (particle.isOnLayer(true, layer->index()) || std::abs(layer->getZ() - particle.position().Z()) < 1e-3) {
        if (particle.momentum().Z() < 0) {
          nextForwardLayer_ = layer.get();
          break;
        } else {
          continue;
        }
      }

      if (particle.position().Z() < layer->getZ()) {
        nextForwardLayer_ = layer.get();
        break;
      }

      previousForwardLayer_ = layer.get();
    }
  }
  // last move worked, let's update
  else {
    LogDebug(MESSAGECATEGORY) << "      ordinary call";

    // barrel layer was hit
    if (layer == nextBarrelLayer_) {
      if (!particleMovesInwards) {
        previousBarrelLayer_ = nextBarrelLayer_;
        nextBarrelLayer_ = geometry_->nextLayer(nextBarrelLayer_);
      }
    } else if (layer == previousBarrelLayer_) {
      if (particleMovesInwards) {
        nextBarrelLayer_ = previousBarrelLayer_;
        previousBarrelLayer_ = geometry_->previousLayer(previousBarrelLayer_);
      }
    }
    // forward layer was hit
    else if (layer == nextForwardLayer_) {
      if (particle.momentum().Z() > 0) {
        previousForwardLayer_ = nextForwardLayer_;
        nextForwardLayer_ = geometry_->nextLayer(nextForwardLayer_);
      }
    } else if (layer == previousForwardLayer_) {
      if (particle.momentum().Z() < 0) {
        nextForwardLayer_ = previousForwardLayer_;
        previousForwardLayer_ = geometry_->previousLayer(previousForwardLayer_);
      }
    }

    // reset layer
    layer = nullptr;
  }

  // move particle to first hit with one of the enclosing layers

  LogDebug(MESSAGECATEGORY) << "   particle between BarrelLayers: "
                            << (previousBarrelLayer_ ? previousBarrelLayer_->index() : -1) << "/"
                            << (nextBarrelLayer_ ? nextBarrelLayer_->index() : -1)
                            << " (total: " << geometry_->barrelLayers().size() << ")"
                            << "\n   particle between ForwardLayers: "
                            << (previousForwardLayer_ ? previousForwardLayer_->index() : -1) << "/"
                            << (nextForwardLayer_ ? nextForwardLayer_->index() : -1)
                            << " (total: " << geometry_->forwardLayers().size() << ")";

  // calculate and store some variables related to the particle's trajectory
  std::unique_ptr<fastsim::Trajectory> trajectory = Trajectory::createTrajectory(particle, magneticFieldZ);

  // push back all possible candidates
  std::vector<const fastsim::SimplifiedGeometry*> layers;
  if (nextBarrelLayer_) {
    layers.push_back(nextBarrelLayer_);
  }
  if (previousBarrelLayer_) {
    layers.push_back(previousBarrelLayer_);
  }

  if (particle.momentum().Z() > 0) {
    if (nextForwardLayer_) {
      layers.push_back(nextForwardLayer_);
    }
  } else {
    if (previousForwardLayer_) {
      layers.push_back(previousForwardLayer_);
    }
  }

  // calculate time until each possible intersection
  // -> pick layer that is hit first
  double deltaTimeC = -1;
  for (auto _layer : layers) {
    double tempDeltaTime =
        trajectory->nextCrossingTimeC(*_layer, particle.isOnLayer(_layer->isForward(), _layer->index()));
    LogDebug(MESSAGECATEGORY) << "   particle crosses layer " << *_layer << " in time " << tempDeltaTime;
    if (tempDeltaTime > 0 && (layer == nullptr || tempDeltaTime < deltaTimeC || deltaTimeC < 0)) {
      layer = _layer;
      deltaTimeC = tempDeltaTime;
    }
  }

  // if particle decays on the way to the next layer, stop propagation there and return
  double properDeltaTimeC = deltaTimeC / particle.gamma();
  if (!particle.isStable() && properDeltaTimeC > particle.remainingProperLifeTimeC()) {
    // move particle in space, time and momentum until it decays
    deltaTimeC = particle.remainingProperLifeTimeC() * particle.gamma();

    trajectory->move(deltaTimeC);
    particle.position() = trajectory->getPosition();
    particle.momentum() = trajectory->getMomentum();

    particle.setRemainingProperLifeTimeC(0.);

    // particle no longer is on a layer
    particle.resetOnLayer();
    LogDebug(MESSAGECATEGORY) << "    particle about to decay. Will not be moved all the way to the next layer.";
    return false;
  }

  if (layer) {
    // move particle in space, time and momentum so it is on the next layer
    trajectory->move(deltaTimeC);
    particle.position() = trajectory->getPosition();
    particle.momentum() = trajectory->getMomentum();

    if (!particle.isStable())
      particle.setRemainingProperLifeTimeC(particle.remainingProperLifeTimeC() - properDeltaTimeC);

    // link the particle to the layer
    particle.setOnLayer(layer->isForward(), layer->index());
    LogDebug(MESSAGECATEGORY) << "    moved particle to layer: " << *layer;
  } else {
    // particle no longer is on a layer
    particle.resetOnLayer();
  }

  // return true / false if propagations succeeded /failed
  LogDebug(MESSAGECATEGORY) << "    success: " << bool(layer);
  return layer;
}

Member Data Documentation

const Geometry* const fastsim::LayerNavigator::geometry_

private

The geometry of the tracker material.

Definition at line 65 of file LayerNavigator.h.

const std::string fastsim::LayerNavigator::MESSAGECATEGORY = "FastSimulation"

staticprivate

find the next layer that the particle will cross

motivation for a new algorithm

• old algorithm is flawed
• the new algorithm allows to put material and instruments on any plane perpendicular to z, or on any cylinder with the z-axis as axis
• while the old algorith, with the requirement of nested layers, forbids the introduction of long narrow cylinders, required for a decent simulation of material in front of HF

definitions the geometry is described by 2 sets of layers:

• forward layers: flat layers, perpendicular to the z-axis, positioned at a given z these layers have material / instruments between a given materialMinR and materialMaxR no 2 forward layers should have the same z-position
• barrel layers: cylindrically shaped layers, with the z-axis as axis, infinitely long these layers have material / instruments for |z| < materialMaxAbsZ no 2 barrel layers should have the same radius
• forward(barrel) layers are ordered according to increasing z (r)

principle

• neutral particles follow a straight trajectory
• charged particles follow a helix-shaped trajectory: constant speed along the z-axis circular path in the x-y plane => the next layer that the particle will cross is among the following 3 layers
• closest forward layer with
• z >(<) particle.z() for particles moving in the positive(negative) direction
• closest barrel layer with r < particle.r
• closest barrel layer with r > particle.r

algorithm

• find the 3 candidate layers
• find the earliest positive intersection time for each of the 3 candidate layers
• move the particle to the earliest intersection time
• select and return the layer with the earliest positive intersection time

notes

• the implementation of the algorithm can probably be optimised, e.g.
• one can probably gain time in moveToNextLayer if LayerNavigator is aware of the candidate layers of the previous call to moveToNextLayer
• for straight tracks, the optimal strategy to find the next layer might be very different

Definition at line 74 of file LayerNavigator.h.

const BarrelSimplifiedGeometry* fastsim::LayerNavigator::nextBarrelLayer_

private

Pointer to the next (direction of the particle's momentum) barrel layer.

Definition at line 67 of file LayerNavigator.h.

const ForwardSimplifiedGeometry* fastsim::LayerNavigator::nextForwardLayer_

private

Pointer to the next (direction of the particle's momentum) forward layer.

Definition at line 71 of file LayerNavigator.h.

const BarrelSimplifiedGeometry* fastsim::LayerNavigator::previousBarrelLayer_

private

Pointer to the previous (opposite direction of the particle's momentum) barrel layer.

Definition at line 69 of file LayerNavigator.h.

const ForwardSimplifiedGeometry* fastsim::LayerNavigator::previousForwardLayer_

private

Pointer to the previous (opposite direction of the particle's momentum) forward layer.

Definition at line 73 of file LayerNavigator.h.