fastsim::StraightTrajectory Class Reference

Mathematical representation of a straight trajectory. More...

#include <StraightTrajectory.h>

Inheritance diagram for fastsim::StraightTrajectory:

Public Member Functions
bool	crosses (const BarrelSimplifiedGeometry &layer) const override
	Check if an intersection of the trajectory with a barrel layer exists. More...
void	move (double deltaTimeC) override
	Move the particle along the helix trajectory for a given time. More...
double	nextCrossingTimeC (const BarrelSimplifiedGeometry &layer, bool onLayer=false) const override
	Return delta time (t*c) of the next intersection of trajectory and barrel layer. More...
	StraightTrajectory (const Particle &particle)
	Constructor. More...
	StraightTrajectory (const Trajectory &trajectory)
	Use Copy Constructor. More...
Public Member Functions inherited from fastsim::Trajectory
const math::XYZTLorentzVector &	getMomentum ()
	Simple getter: return momentum of the particle that was used to create trajectory. More...
const math::XYZTLorentzVector &	getPosition ()
	Simple getter: return position of the particle that was used to create trajectory. More...
double	nextCrossingTimeC (const ForwardSimplifiedGeometry &layer, bool onLayer=false) const
	Return delta time (t*c) of the next intersection of trajectory and forward layer. More...
double	nextCrossingTimeC (const SimplifiedGeometry &layer, bool onLayer=false) const
	Return delta time (t*c) of the next intersection of trajectory and generic layer. More...
virtual	~Trajectory ()

Additional Inherited Members
Static Public Member Functions inherited from fastsim::Trajectory
static std::unique_ptr< Trajectory >	createTrajectory (const fastsim::Particle &particle, const double magneticFieldZ)
	Calls constructor of derived classes. More...
Protected Member Functions inherited from fastsim::Trajectory
	Trajectory (const fastsim::Particle &particle)
	Constructor of base class. More...
Protected Attributes inherited from fastsim::Trajectory
math::XYZTLorentzVector	momentum_
	momentum of the particle that was used to create trajectory More...
math::XYZTLorentzVector	position_
	position of the particle that was used to create trajectory More...

Detailed Description

Mathematical representation of a straight trajectory.

Reflects trajectory of a uncharged particle.

Definition at line 16 of file StraightTrajectory.h.

Constructor & Destructor Documentation

StraightTrajectory() [1/2]

fastsim::StraightTrajectory::StraightTrajectory ( const Particle &  particle )

inline

Constructor.

Definition at line 22 of file StraightTrajectory.h.

: Trajectory(particle) { ; }

StraightTrajectory() [2/2]

fastsim::StraightTrajectory::StraightTrajectory ( const Trajectory &  trajectory )

inline

Use Copy Constructor.

Definition at line 28 of file StraightTrajectory.h.

: Trajectory(trajectory) { ; }

Member Function Documentation

crosses()

bool fastsim::StraightTrajectory::crosses ( const BarrelSimplifiedGeometry &  layer ) const

inlineoverridevirtual

Check if an intersection of the trajectory with a barrel layer exists.

There is always an intersection between a straight line and a barrel layer unless the trajectory is parallel to z axis. In this case, the particle is not propagated anyways since it will not hit any detector material.

Parameters

layer

A barrel layer.

Returns

true

Implements fastsim::Trajectory.

Definition at line 36 of file StraightTrajectory.h.

{ return true; }

move()

void fastsim::StraightTrajectory::move ( double  deltaTimeC )

overridevirtual

Move the particle along the helix trajectory for a given time.

Parameters

deltaTimeC

Time in units of t*c..

Implements fastsim::Trajectory.

Definition at line 71 of file StraightTrajectory.cc.

                                                      {
  position_.SetXYZT(position_.X() + momentum_.X() / momentum_.E() * deltaTimeC,
                    position_.Y() + momentum_.Y() / momentum_.E() * deltaTimeC,
                    position_.Z() + momentum_.Z() / momentum_.E() * deltaTimeC,
                    position_.T() + deltaTimeC / fastsim::Constants::speedOfLight);
}

References fastsim::Constants::speedOfLight.

nextCrossingTimeC()

double fastsim::StraightTrajectory::nextCrossingTimeC ( const BarrelSimplifiedGeometry &  layer, bool  onLayer = false  ) const

overridevirtual

Return delta time (t*c) of the next intersection of trajectory and barrel layer.

This function solves the quadratic equation (basically intersection a circle with a given radius and a straight line) in order to calculate the moment in time when the particle's trajectory intersects with a given barrel layer.

Parameters

layer	A barrel layer.
onLayer	Specify if the particle already is on the layer (in that case the second solution has to be picked).

Returns

t*c [ns * cm/ns] of next intersection (-1 if there is none).

Implements fastsim::Trajectory.

Definition at line 8 of file StraightTrajectory.cc.

                                                                          {
  //
  // solve the equation
  // (x^2 + y^2 = R^2),   (1)
  // x = x_0 + v_x*t,     (2)
  // y = y_0 + v_y*t      (3)
  // for t
  //
  // subsitute (2) and (3) in (1):
  // => t^2*(v_x^2 + v_y^2) + t*( 2*x_0*v_x + 2*y_0*v_y ) + x_0^2 + y_0^2 - R^2 = 0
  // => t = (-b +/- sqrt(b^2 - ac) ) / a       see https://en.wikipedia.org/wiki/Quadratic_formula, divide equation by 2
  // with a = v_x^2 + v_y^2;
  // with b = x_0*v_x + y_0*v_y
  // with c = x_0^2 + y_0^2 - R^2
  //
  // substitute: v_x = p_x / E * c  ( note: extra * c absorbed in p_x units)
  // substitute: v_y = p_y / E * c  ( note: extra * c absorbed in p_y units)
  // => t*c = (-b +/- sqrt(b^2 - ac) ) / a * E
  // with a = p_x^2 + p_y^2
  // with b = p_x*x_0 + p_y*y_0
  // with c = x_0^2 + y_0^2 - R^2
  double a = momentum_.Perp2();
  double b = (position_.X() * momentum_.X() + position_.Y() * momentum_.Y());
  double c = position_.Perp2() - layer.getRadius() * layer.getRadius();
 
  double delta = b * b - a * c;
  if (delta < 0) {
    return -1;
  }
  double sqrtDelta = sqrt(delta);
 
  //
  // return the earliest solution > 0,
  // return -1 if no positive solution exists
  // note: 'a' always positive, sqrtDelta always positive
  //
  double tc1 = (-b - sqrtDelta) / a * momentum_.E();
  double tc2 = (-b + sqrtDelta) / a * momentum_.E();
 
  if (onLayer && tc2 > 0) {
    bool particleMovesInwards = momentum_.X() * position_.X() + momentum_.Y() * position_.Y() < 0;
 
    double posX2 = position_.X() + momentum_.X() / momentum_.E() * tc2;
    double posY2 = position_.Y() + momentum_.Y() / momentum_.E() * tc2;
    bool particleMovesInwards2 = momentum_.X() * posX2 + momentum_.Y() * posY2 < 0;
 
    if (particleMovesInwards != particleMovesInwards2) {
      return tc2;
    }
 
    return -1;
  }
 
  if (tc1 > 0) {
    return tc1;
  } else if (tc2 > 0) {
    return tc2;
  }
 
  return -1.;
}

References a, b, HltBtagPostValidation_cff::c, dumpMFGeometry_cfg::delta, fastsim::BarrelSimplifiedGeometry::getRadius(), fastsim::Trajectory::momentum_, fastsim::Trajectory::position_, and mathSSE::sqrt().

Referenced by fastsim::HelixTrajectory::nextCrossingTimeC().