ElasticTruthTrajectoryBuilder.cxx

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/*
  Copyright (C) 2002-2025 CERN for the benefit of the ATLAS collaboration
*/
 
// Andrei Gaponenko, 2008
 
#include "TrkTruthCreatorTools/ElasticTruthTrajectoryBuilder.h"
 
#include "TrkTruthData/TruthTrajectory.h"
 
#include "AtlasHepMC/GenParticle.h"
#include "AtlasHepMC/GenVertex.h"
#include "TruthUtils/HepMCHelpers.h"
 
#include "GeneratorObjects/HepMcParticleLink.h"
#include "AthContainers/DataVector.h"
 
#include <stack>
 
namespace Trk {
 
//================================================================
ElasticTruthTrajectoryBuilder::
ElasticTruthTrajectoryBuilder(const std::string& type,
const std::string& name,
const IInterface* parent)
    :  AthAlgTool(type,name,parent)
{
    declareInterface<Trk::ITruthTrajectoryBuilder>(this);
}
 
 
//================================================================
StatusCode ElasticTruthTrajectoryBuilder::initialize() {
    return StatusCode::SUCCESS;
}
 
//================================================================
void ElasticTruthTrajectoryBuilder::
buildTruthTrajectory(TruthTrajectory *result, const HepMC::ConstGenParticlePtr& input) const
{
  result->clear();
  if (input) {
    HepMC::ConstGenParticlePtr next(nullptr);
    HepMC::ConstGenParticlePtr current = input;
 
    // Extend trajectory outwards.  The last particle should go at [0]
    // in the TruthTrajectory, so we need to use a tmp storage while
    // traversing the structure.
    std::stack<HepMC::ConstGenParticlePtr> tmp;
    while ( (next = getDaughter(current)) ) {
      tmp.push(current = next);
    }
 
    // All particles in the TruthTrajectory will be from the same GenEvent
    const int eventNumber = input->parent_event()->event_number();
 
    // copy the outer half to result
    while (!tmp.empty()) {
      result->emplace_back(tmp.top(), eventNumber, HepMcParticleLink::IS_EVENTNUM);
      tmp.pop();
    }
 
    // The input particle itself
    result->emplace_back(input, eventNumber, HepMcParticleLink::IS_EVENTNUM);
 
    // Now continue towards the interaction point
    while ( (next = getMother(current)) ) {
      result->emplace_back(current = next, eventNumber, HepMcParticleLink::IS_EVENTNUM);
    }
  }
}
 
//================================================================
ElasticTruthTrajectoryBuilder::MotherDaughter
    ElasticTruthTrajectoryBuilder::truthTrajectoryCuts(const HepMC::ConstGenVertexPtr& vtx) 
{
    HepMC::ConstGenParticlePtr mother{nullptr};
    HepMC::ConstGenParticlePtr daughter{nullptr};
    // only truth vertices with 1 incoming particle
    if(vtx && (vtx->particles_in_size() == 1)) {
        mother = vtx->particles_in().front();
        // Allow status code 1 and 2.  E.g. a pion that produced a long track can decay  outside of InDet and have status==2.
        if( mother && MC::isPhysical(mother) ) {
 
            // Restrict to quasi-elastic processes (e.g. brems, delta-rays, pi->pi+Delta).
            // 
            // Require not more than 2 outgoing particles. Note that
            // delta-rays for primary==electron is a special case, because we have two
            // outgoing particles with the same PDG id.  The "correct" one
            // is that with the higher energy (NOT pt).
            // 
            // allow 1 outgoing to cover possible vertexes from interaction in detector material
            if (vtx->particles_out_size() <= 2) {
                int num_passed_cuts = 0;
                HepMC::ConstGenParticlePtr passed_cuts{nullptr};
                for(const auto& candidate: *vtx) {
                    if(candidate->pdg_id() == mother->pdg_id()) {
 
                        if(passed_cuts && MC::isElectron(mother)) { // second negative electron is a special case
                            if(candidate->momentum().e() > passed_cuts->momentum().e()) {
                                passed_cuts = candidate; // don't increment num_passed_cuts, we are replacing chosen particle
                            }
                        } else {
                            passed_cuts = candidate;
                            ++num_passed_cuts;
                        }
                    }
                }
 
                if(num_passed_cuts==1) { // disallow hadronic pi->N*pi etc.
                    daughter = std::move(passed_cuts);
                }
 
            } 
        }
    }
 
    return std::make_pair(mother, daughter);
}
 
//================================================================
HepMC::ConstGenParticlePtr ElasticTruthTrajectoryBuilder::getDaughter(const HepMC::ConstGenParticlePtr& mother) const {
  if(mother) {
    MotherDaughter res = truthTrajectoryCuts(mother->end_vertex());
    if(res.first == mother) return res.second;
  }
  return {nullptr};
}
 
//================================================================
HepMC::ConstGenParticlePtr ElasticTruthTrajectoryBuilder::getMother(const HepMC::ConstGenParticlePtr& daughter) const {
  if(daughter) {
    MotherDaughter res = truthTrajectoryCuts(daughter->production_vertex());
    if(res.second == daughter) return res.first;
  }
  return {nullptr};
}
 
    //================================================================
 
} // namespace Trk