GEMSignalModel.cc

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#include "SimMuon/GEMDigitizer/interface/GEMSignalModel.h"
#include "Geometry/GEMGeometry/interface/GEMEtaPartitionSpecs.h"
#include "Geometry/CommonTopologies/interface/GEMStripTopology.h"
#include "Geometry/GEMGeometry/interface/GEMGeometry.h"
#include "FWCore/MessageLogger/interface/MessageLogger.h"
#include "FWCore/Utilities/interface/Exception.h"
#include "CLHEP/Random/RandFlat.h"
#include "CLHEP/Random/RandPoissonQ.h"
#include "CLHEP/Random/RandGaussQ.h"
#include "CLHEP/Units/GlobalPhysicalConstants.h"
#include "DataFormats/Math/interface/GeantUnits.h"
#include <cmath>
#include <utility>
#include <map>

GEMSignalModel::GEMSignalModel(const edm::ParameterSet& config)
    : GEMDigiModel(config),
      averageEfficiency_(config.getParameter<double>("averageEfficiency")),
      averageShapingTime_(config.getParameter<double>("averageShapingTime")),
      timeResolution_(config.getParameter<double>("timeResolution")),
      timeJitter_(config.getParameter<double>("timeJitter")),
      signalPropagationSpeed_(config.getParameter<double>("signalPropagationSpeed")),
      resolutionX_(config.getParameter<double>("resolutionX")),
      cspeed(geant_units::operators::convertMmToCm(CLHEP::c_light)),
      // average energy required to remove an electron due to ionization for an Ar/CO2 gas mixture (in the ratio of 70/30) is 28.1 eV
      energyMinCut(28.1e-09) {}

GEMSignalModel::~GEMSignalModel() {}

void GEMSignalModel::simulate(const GEMEtaPartition* roll,
                              const edm::PSimHitContainer& simHits,
                              CLHEP::HepRandomEngine* engine,
                              Strips& strips_,
                              DetectorHitMap& detectorHitMap_) {
  const GEMStripTopology* top(dynamic_cast<const GEMStripTopology*>(&(roll->topology())));
  for (const auto& hit : simHits) {
    if (hit.energyLoss() < energyMinCut)
      continue;
    const int bx(getSimHitBx(&hit, engine));
    const std::vector<std::pair<int, int> >& cluster(simulateClustering(top, &hit, bx, engine));
    for (const auto& digi : cluster) {
      detectorHitMap_.emplace(digi, &hit);
      strips_.emplace(digi);
    }
  }
}

int GEMSignalModel::getSimHitBx(const PSimHit* simhit, CLHEP::HepRandomEngine* engine) {
  int bx = -999;
  const LocalPoint simHitPos(simhit->localPosition());
  const float tof(simhit->timeOfFlight());
  // random Gaussian time correction due to electronics jitter
  float randomJitterTime = CLHEP::RandGaussQ::shoot(engine, 0., timeJitter_);
  const GEMDetId id(simhit->detUnitId());
  const GEMEtaPartition* roll(geometry_->etaPartition(id));
  if (!roll) {
    throw cms::Exception("Geometry") << "GEMSignalModel::getSimHitBx() - GEM simhit id does not match any GEM roll id: "
                                     << id << "\n";
    return 999;
  }
  if (roll->id().region() == 0) {
    throw cms::Exception("Geometry")
        << "GEMSignalModel::getSimHitBx() - this GEM id is from barrel, which cannot happen: " << roll->id() << "\n";
  }
  const int nstrips = roll->nstrips();
  float middleStrip = nstrips / 2.;
  const LocalPoint& middleOfRoll = roll->centreOfStrip(middleStrip);
  const GlobalPoint& globMiddleRol = roll->toGlobal(middleOfRoll);
  double muRadius = sqrt(globMiddleRol.x() * globMiddleRol.x() + globMiddleRol.y() * globMiddleRol.y() +
                         globMiddleRol.z() * globMiddleRol.z());
  double timeCalibrationOffset_ = muRadius / cspeed;  //[ns]

  const GEMStripTopology* top(dynamic_cast<const GEMStripTopology*>(&(roll->topology())));
  const float halfStripLength(0.5 * top->stripLength());
  const float distanceFromEdge(halfStripLength - simHitPos.y());

  // signal propagation speed in material in [cm/ns]
  double signalPropagationSpeedTrue = signalPropagationSpeed_ * cspeed;

  // average time for the signal to propagate from the SimHit to the top of a strip
  const float averagePropagationTime(distanceFromEdge / signalPropagationSpeedTrue);
  // random Gaussian time correction due to the finite timing resolution of the detector
  float randomResolutionTime = CLHEP::RandGaussQ::shoot(engine, 0., timeResolution_);
  const float simhitTime(tof + averageShapingTime_ + randomResolutionTime + averagePropagationTime + randomJitterTime);
  float referenceTime = 0.;
  referenceTime = timeCalibrationOffset_ + halfStripLength / signalPropagationSpeedTrue + averageShapingTime_;
  const float timeDifference(simhitTime - referenceTime);
  // assign the bunch crossing
  bx = static_cast<int>(std::round((timeDifference) / 25.));

  // check time
  LogDebug("GEMDigiProducer") << "checktime "
                              << "bx = " << bx << "\tdeltaT = " << timeDifference << "\tsimT =  " << simhitTime
                              << "\trefT =  " << referenceTime << "\ttof = " << tof
                              << "\tavePropT =  " << averagePropagationTime
                              << "\taveRefPropT = " << halfStripLength / signalPropagationSpeedTrue << "\n";
  return bx;
}

std::vector<std::pair<int, int> > GEMSignalModel::simulateClustering(const GEMStripTopology* top,
                                                                     const PSimHit* simHit,
                                                                     const int bx,
                                                                     CLHEP::HepRandomEngine* engine) {
  const LocalPoint& hit_entry(simHit->entryPoint());
  const LocalPoint& hit_exit(simHit->exitPoint());

  LocalPoint start_point, end_point;
  if (hit_entry.x() < hit_exit.x()) {
    start_point = hit_entry;
    end_point = hit_exit;
  } else {
    start_point = hit_exit;
    end_point = hit_entry;
  }

  // Add Gaussian noise to the points towards outside.
  float smeared_start_x = start_point.x() - std::abs(CLHEP::RandGaussQ::shoot(engine, 0, resolutionX_));
  float smeared_end_x = end_point.x() + std::abs(CLHEP::RandGaussQ::shoot(engine, 0, resolutionX_));

  LocalPoint smeared_start_point(smeared_start_x, start_point.y(), start_point.z());
  LocalPoint smeared_end_point(smeared_end_x, end_point.y(), end_point.z());

  int cluster_start = top->channel(smeared_start_point);
  int cluster_end = top->channel(smeared_end_point);

  std::vector<std::pair<int, int> > cluster;
  for (int strip = cluster_start; strip <= cluster_end; strip++) {
    cluster.emplace_back(strip, bx);
  }

  return cluster;
}