fastsim::Bremsstrahlung Class Reference

Implementation of Bremsstrahlung from e+/e- in the tracker layers. More...

Inheritance diagram for fastsim::Bremsstrahlung:

Public Member Functions
	Bremsstrahlung (const std::string &name, const edm::ParameterSet &cfg)
	Constructor. More...
void	interact (Particle &particle, const SimplifiedGeometry &layer, std::vector< std::unique_ptr< Particle > > &secondaries, const RandomEngineAndDistribution &random) override
	Perform the interaction. More...
	~Bremsstrahlung () override
	Default destructor. More...
Public Member Functions inherited from fastsim::InteractionModel
const std::string	getName ()
	Return (unique) name of this interaction. More...
	InteractionModel (std::string name)
	Constructor. More...
virtual void	registerProducts (edm::ProducesCollector) const
	In case interaction produces and stores content in the event (e.g. TrackerSimHits). More...
virtual void	storeProducts (edm::Event &iEvent)
	In case interaction produces and stores content in the event (e.g. TrackerSimHits). More...
virtual	~InteractionModel ()
	Default destructor. More...

Private Member Functions
math::XYZTLorentzVector	brem (Particle &particle, double xmin, const RandomEngineAndDistribution &random) const
	Compute Brem photon energy and angles, if any. More...
double	gbteth (const double ener, const double partm, const double efrac, const RandomEngineAndDistribution &random) const
	A universal angular distribution. More...

Private Attributes
double	minPhotonEnergy_
	Cut on minimum energy of bremsstrahlung photons. More...
double	minPhotonEnergyFraction_
	Cut on minimum fraction of particle's energy which has to be carried by photon. More...
double	Z_
	Atomic number of material (usually silicon Z=14) More...

Detailed Description

Implementation of Bremsstrahlung from e+/e- in the tracker layers.

Computes the number, energy and angles of Bremsstrahlung photons emitted by electrons and positrons and modifies e+/e- particle accordingly.

Definition at line 33 of file Bremsstrahlung.cc.

Constructor & Destructor Documentation

Bremsstrahlung()

fastsim::Bremsstrahlung::Bremsstrahlung	(	const std::string &	name,
		const edm::ParameterSet &	cfg
	)

Constructor.

Definition at line 82 of file Bremsstrahlung.cc.

References looper::cfg, minPhotonEnergy_, minPhotonEnergyFraction_, and Z_.

    : fastsim::InteractionModel(name) {
  // Set the minimal photon energy for a Brem from e+/-
  minPhotonEnergy_ = cfg.getParameter<double>("minPhotonEnergy");
  minPhotonEnergyFraction_ = cfg.getParameter<double>("minPhotonEnergyFraction");
  // Material properties
  Z_ = cfg.getParameter<double>("Z");
}

~Bremsstrahlung()

fastsim::Bremsstrahlung::~Bremsstrahlung ( )

inlineoverride

Default destructor.

Definition at line 39 of file Bremsstrahlung.cc.

{ ; };

Member Function Documentation

brem()

math::XYZTLorentzVector fastsim::Bremsstrahlung::brem ( fastsim::Particle &  particle, double  xmin, const RandomEngineAndDistribution &  random  ) const

private

Compute Brem photon energy and angles, if any.

Parameters

particle	The particle that interacts with the matter.
xmin	Minimum fraction of the particle's energy that has to be converted to a photon.
random	The Random Engine.

Returns

Momentum 4-vector of a bremsstrahlung photon.

Definition at line 161 of file Bremsstrahlung.cc.

References funct::cos(), fastsim::Constants::eMass, JetChargeProducer_cfi::exp, RandomEngineAndDistribution::flatShoot(), dqm-mbProfile::log, M_PI, fastsim::Particle::momentum(), funct::sin(), theta(), and TrackerOfflineValidation_Dqm_cff::xmin.

                                                                                                       {
  double xp = 0;
  double weight = 0.;

  do {
    xp = xmin * std::exp(-std::log(xmin) * random.flatShoot());
    weight = 1. - xp + 3. / 4. * xp * xp;
  } while (weight < random.flatShoot());

  // Have photon energy. Now generate angles with respect to the z axis
  // defined by the incoming particle's momentum.

  // Isotropic in phi
  const double phi = random.flatShoot() * 2. * M_PI;
  // theta from universal distribution
  const double theta = gbteth(particle.momentum().E(), fastsim::Constants::eMass, xp, random) *
                       fastsim::Constants::eMass / particle.momentum().E();

  // Make momentum components
  double stheta = std::sin(theta);
  double ctheta = std::cos(theta);
  double sphi = std::sin(phi);
  double cphi = std::cos(phi);

  return xp * particle.momentum().E() * math::XYZTLorentzVector(stheta * cphi, stheta * sphi, ctheta, 1.);
}

gbteth()

double fastsim::Bremsstrahlung::gbteth ( const double  ener, const double  partm, const double  efrac, const RandomEngineAndDistribution &  random  ) const

private

A universal angular distribution.

Parameters

ener
partm
efrac
random	The Random Engine.

Returns

Theta from universal distribution

Definition at line 190 of file Bremsstrahlung.cc.

References HLT_2022v15_cff::beta, ztail::d, RandomEngineAndDistribution::flatShoot(), dqm-mbProfile::log, M_PI, and w1.

                                                                                        {
  // Details on implementation here
  // http://www.dnp.fmph.uniba.sk/cernlib/asdoc/geant_html3/node299.html#GBTETH
  // http://svn.cern.ch/guest/AliRoot/tags/v3-07-03/GEANT321/gphys/gbteth.F

  const double alfa = 0.625;

  const double d = 0.13 * (0.8 + 1.3 / Z_) * (100.0 + (1.0 / ener)) * (1.0 + efrac);
  const double w1 = 9.0 / (9.0 + d);
  const double umax = ener * M_PI / partm;
  double u;

  do {
    double beta = (random.flatShoot() <= w1) ? alfa : 3.0 * alfa;
    u = -std::log(random.flatShoot() * random.flatShoot()) / beta;
  } while (u >= umax);

  return u;
}

interact()

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

overridevirtual

Perform the interaction.

Parameters

particle	The particle that interacts with the matter.
layer	The detector layer that interacts with the particle.
secondaries	Particles that are produced in the interaction (if any).
random	The Random Engine.

Implements fastsim::InteractionModel.

Definition at line 91 of file Bremsstrahlung.cc.

References funct::abs(), mps_fire::i, dqm-mbProfile::log, SiStripPI::max, fastsim::Particle::momentum(), fastsim::Particle::pdgId(), RandomEngineAndDistribution::poissonShoot(), fastsim::Particle::position(), theta(), and TrackerOfflineValidation_Dqm_cff::xmin.

                                                                                  {
  // only consider electrons and positrons
  if (std::abs(particle.pdgId()) != 11) {
    return;
  }

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

  // Protection : Just stop the electron if more than 1 radiation lengths.
  // This case corresponds to an electron entering the layer parallel to
  // the layer axis - no reliable simulation can be done in that case...
  if (radLengths > 4.) {
    particle.momentum().SetXYZT(0., 0., 0., 0.);
    return;
  }

  // electron must have more energy than minimum photon energy
  if (particle.momentum().E() - particle.momentum().mass() < minPhotonEnergy_) {
    return;
  }

  // Hard brem probability with a photon Energy above threshold.
  double xmin = std::max(minPhotonEnergy_ / particle.momentum().E(), minPhotonEnergyFraction_);
  if (xmin >= 1. || xmin <= 0.) {
    return;
  }

  // probability to radiate a photon
  double bremProba =
      radLengths * (4. / 3. * std::log(1. / xmin) - 4. / 3. * (1. - xmin) + 1. / 2. * (1. - xmin * xmin));

  // Number of photons to be radiated.
  unsigned int nPhotons = random.poissonShoot(bremProba);
  if (nPhotons == 0) {
    return;
  }

  // Needed to rotate photons to the lab frame
  double theta = particle.momentum().Theta();
  double phi = particle.momentum().Phi();

  // Calculate energy of these photons and add them to the event
  for (unsigned int i = 0; i < nPhotons; ++i) {
    // Throw momentum of the photon
    math::XYZTLorentzVector photonMom = brem(particle, xmin, random);

    // Check that there is enough energy left.
    if (particle.momentum().E() - particle.momentum().mass() < photonMom.E())
      break;

    // Rotate to the lab frame
    photonMom = ROOT::Math::RotationZ(phi) * (ROOT::Math::RotationY(theta) * photonMom);

    // Add a photon
    secondaries.emplace_back(new fastsim::Particle(22, particle.position(), photonMom));

    // Update the original e+/-
    particle.momentum() -= photonMom;
  }
}

Member Data Documentation

minPhotonEnergy_

double fastsim::Bremsstrahlung::minPhotonEnergy_

private

Cut on minimum energy of bremsstrahlung photons.

Definition at line 76 of file Bremsstrahlung.cc.

Referenced by Bremsstrahlung().

minPhotonEnergyFraction_

double fastsim::Bremsstrahlung::minPhotonEnergyFraction_

private

Cut on minimum fraction of particle's energy which has to be carried by photon.

Definition at line 77 of file Bremsstrahlung.cc.

Referenced by Bremsstrahlung().

Z_

double fastsim::Bremsstrahlung::Z_

private

Atomic number of material (usually silicon Z=14)

Definition at line 78 of file Bremsstrahlung.cc.

Referenced by Bremsstrahlung().