InvariantMassTool.cxx

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/*
  Copyright (C) 2002-2025 CERN for the benefit of the ATLAS collaboration
*/
 
// Author: James Catmore (james.catmore@cern.ch)
 
#include "DerivationFrameworkTools/InvariantMassTool.h"
#include <utility> //for std::pair
#include <cmath> //for std::hypot
 
namespace DerivationFramework {
 
  StatusCode InvariantMassTool::initialize()
  {
    ATH_CHECK(m_sgName.initialize());
 
    if (m_expression2.empty()) {
      ATH_CHECK(initializeParser( {m_expression.value(), m_expression.value()} ));
    }
    else {
      ATH_CHECK(initializeParser( {m_expression.value(),m_expression2.value()} ));
    }
 
    ATH_CHECK(m_containerName.initialize());
    ATH_CHECK(m_containerName2.initialize(SG::AllowEmpty));
    ATH_CHECK(m_inputDecorNames.initialize(SG::AllowEmpty));
 
    return StatusCode::SUCCESS;
  }
 
  StatusCode InvariantMassTool::finalize()
  {
    ATH_CHECK( finalizeParser());
    return StatusCode::SUCCESS;
  }
 
  StatusCode InvariantMassTool::addBranches(const EventContext& ctx) const
  {
    // Write masses to SG for access by downstream algs
    if (evtStore()->contains<std::vector<float> >(m_sgName.key())) {
      ATH_MSG_ERROR("Tool is attempting to write a StoreGate key " << m_sgName << " which already exists. Please use a different key");
      return StatusCode::FAILURE;
    }
    std::unique_ptr<std::vector<float> > masses(new std::vector<float>());
    ATH_CHECK(getInvariantMasses(masses.get(), ctx));
    SG::WriteHandle<std::vector<float> > writeHandle(m_sgName, ctx);
    ATH_CHECK(writeHandle.record(std::move(masses)));
    return StatusCode::SUCCESS;
  }
 
  StatusCode InvariantMassTool::getInvariantMasses(std::vector<float>* masses, const EventContext& ctx) const
  {
 
    // check the relevant information is available
    if (m_containerName.key().empty()) {
      ATH_MSG_WARNING("Input container missing - returning zero");
      masses->push_back(0.0);
      return StatusCode::FAILURE;
    }
 
    SG::ReadHandle<xAOD::IParticleContainer> particles{m_containerName, ctx};
 
    bool from2Collections(false);
    const xAOD::IParticleContainer* particles2{nullptr};
    if (!m_containerName2.key().empty() && m_containerName2.key()!=m_containerName.key()) {
      SG::ReadHandle<xAOD::IParticleContainer> particleHdl2{m_containerName2, ctx};
      particles2=particleHdl2.cptr();
      from2Collections = true;
    }
 
    // get the positions of the elements which pass the requirement
    std::vector<int> entries =  m_parser[kInvariantMassToolParser1]->evaluateAsVector();
    std::vector<int> entries2 =  m_parser[kInvariantMassToolParser2]->evaluateAsVector();
    unsigned int nEntries = entries.size();
    unsigned int nEntries2 = entries2.size();
 
    // check the sizes are compatible
    if (!from2Collections) {
      if ( (particles->size() != nEntries) || (particles->size() != nEntries2) || (nEntries!=nEntries2) ) {
        ATH_MSG_ERROR("Branch sizes incompatible - returning zero. Check your selection strings.");
        masses->push_back(0.0);
        return StatusCode::FAILURE;
      }
    }
    if (from2Collections) {
      if ( (particles->size() != nEntries) || (particles2->size() != nEntries2) ) {
        ATH_MSG_ERROR("Branch sizes incompatible - returning zero. Check your selection strings.");
        masses->push_back(0.0);
        return StatusCode::FAILURE;
      }
    }
 
    // Double loop to get all possible index pairs
    unsigned int outerIt, innerIt;
    std::vector<std::pair<int, int> > pairs;
    // Loop for case where both legs are from the same container
    if (!from2Collections) {
      for (outerIt=0; outerIt<nEntries; ++outerIt) {
        for (innerIt=outerIt+1; innerIt<nEntries; ++innerIt) {
          pairs.push_back({static_cast<int>(outerIt),static_cast<int>(innerIt)});
        }
      }
      // Select the pairs for which the mass should be calculated, and then calculate it
      for (const auto & [first, second]: pairs) {
        if ( (entries[first]==1 && entries2[second]==1) || (entries2[first]==1 && entries[second]==1) ) {
          const float mass = calculateInvariantMass( ((*particles)[first])->p4().Vect(),
                                                     ((*particles)[second])->p4().Vect(),
                                                     m_massHypothesis,
                                                     m_massHypothesis);
          masses->push_back(mass);
        }
      }
    }
 
    // Loop for case where both legs are from different containers
    if (from2Collections) {
      for (outerIt=0; outerIt<nEntries; ++outerIt) {
        if (entries[outerIt]==0) continue;
        for (innerIt=0; innerIt<nEntries2; ++innerIt) {
          if (entries2[innerIt]==0) continue;
          pairs.push_back({static_cast<int>(outerIt),static_cast<int>(innerIt)});
        }
      }
      // Select the pairs for which the mass should be calculated, and then calculate it
      for (const auto & [first, second]: pairs) {
        const float mass = calculateInvariantMass( ((*particles)[first])->p4().Vect(),
                                                   ((*particles2)[second])->p4().Vect(),
                                                   m_massHypothesis,
                                                   m_massHypothesis2);
        masses->push_back(mass);
      }
    }
 
    return StatusCode::SUCCESS;
 
  }
 
  float InvariantMassTool::calculateInvariantMass(const TVector3& v1, const TVector3&v2,float M1,float M2) {
      TLorentzVector p1(v1, M1 > 0 ? std::hypot(M1, v1.Mag()) : v1.Mag());
      TLorentzVector p2(v2, M2 > 0 ? std::hypot(M2, v2.Mag()) : v2.Mag());
      return (p1+p2).M();
 
  }
 
}