TtDecayChannelSelector.cc

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#include "FWCore/Utilities/interface/EDMException.h"
#include "FWCore/MessageLogger/interface/MessageLogger.h"
#include "AnalysisDataFormats/TopObjects/interface/TopGenEvent.h"
#include "TopQuarkAnalysis/TopSkimming/interface/TtDecayChannelSelector.h"
#include "DataFormats/Candidate/interface/Candidate.h"

// static const string for status check in
// TtDecayChannelSelector::search functions
static const std::string kGenParticles = "genParticles";

// maximal number of possible leptonic decay
// channels
static const unsigned int kDecayChannels = 3;

TtDecayChannelSelector::TtDecayChannelSelector(const edm::ParameterSet& cfg)
    : invert_(cfg.getParameter<bool>("invert")),
      allowElectron_(false),
      allowMuon_(false),
      allow1Prong_(false),
      allow3Prong_(false) {
  // tau decays are not restricted if this PSet does not exist at all
  restrictTauDecays_ = cfg.existsAs<edm::ParameterSet>("restrictTauDecays");
  // determine allowed tau decays
  if (restrictTauDecays_) {
    edm::ParameterSet allowedTauDecays = cfg.getParameter<edm::ParameterSet>("restrictTauDecays");
    // tau decays are not restricted if none of the following parameters exists
    restrictTauDecays_ = (allowedTauDecays.existsAs<bool>("electron") || allowedTauDecays.existsAs<bool>("muon") ||
                          allowedTauDecays.existsAs<bool>("oneProng") || allowedTauDecays.existsAs<bool>("threeProng"));
    // specify the different possible restrictions of the tau decay channels
    allowElectron_ =
        (allowedTauDecays.existsAs<bool>("electron") ? allowedTauDecays.getParameter<bool>("electron") : false);
    allowMuon_ = (allowedTauDecays.existsAs<bool>("muon") ? allowedTauDecays.getParameter<bool>("muon") : false);
    allow1Prong_ =
        (allowedTauDecays.existsAs<bool>("oneProng") ? allowedTauDecays.getParameter<bool>("oneProng") : false);
    allow3Prong_ =
        (allowedTauDecays.existsAs<bool>("threeProng") ? allowedTauDecays.getParameter<bool>("threeProng") : false);
  }
  // allowed top decays PSet
  edm::ParameterSet allowedTopDecays = cfg.getParameter<edm::ParameterSet>("allowedTopDecays");

  // fill decayBranchA_
  edm::ParameterSet decayBranchA = allowedTopDecays.getParameter<edm::ParameterSet>("decayBranchA");
  decayBranchA_.push_back(decayBranchA.getParameter<bool>("electron"));
  decayBranchA_.push_back(decayBranchA.getParameter<bool>("muon"));
  decayBranchA_.push_back(decayBranchA.getParameter<bool>("tau"));

  // fill decay branchB_
  edm::ParameterSet decayBranchB = allowedTopDecays.getParameter<edm::ParameterSet>("decayBranchB");
  decayBranchB_.push_back(decayBranchB.getParameter<bool>("electron"));
  decayBranchB_.push_back(decayBranchB.getParameter<bool>("muon"));
  decayBranchB_.push_back(decayBranchB.getParameter<bool>("tau"));

  // fill allowedDecays_
  for (unsigned int d = 0; d < kDecayChannels; ++d) {
    allowedDecays_.push_back(decayBranchA_[d] + decayBranchB_[d]);
  }
}

bool TtDecayChannelSelector::operator()(const reco::GenParticleCollection& parts, std::string_view inputType) const {
  bool verbose =
      false;  // set this to true for debugging and add TtDecayChannelSelector category to the MessageLogger in your cfg file
  unsigned int iLep = 0;
  unsigned int iTop = 0, iBeauty = 0, iElec = 0, iMuon = 0, iTau = 0;
  for (reco::GenParticleCollection::const_iterator top = parts.begin(); top != parts.end(); ++top) {
    if (search(top, TopDecayID::tID, inputType)) {
      ++iTop;
      for (reco::GenParticle::const_iterator td = top->begin(); td != top->end(); ++td) {
        if (search(td, TopDecayID::bID, inputType)) {
          ++iBeauty;
        }
        if (search(td, TopDecayID::WID, inputType)) {
          for (reco::GenParticle::const_iterator wd = td->begin(); wd != td->end(); ++wd) {
            if (std::abs(wd->pdgId()) == TopDecayID::elecID) {
              ++iElec;
            }
            if (std::abs(wd->pdgId()) == TopDecayID::muonID) {
              ++iMuon;
            }
            if (std::abs(wd->pdgId()) == TopDecayID::tauID) {
              if (restrictTauDecays_) {
                // count as iTau if it is leptonic, one-prong
                // or three-prong and ignore increasing iLep
                // though else
                if (tauDecay(*wd)) {
                  ++iTau;
                } else {
                  ++iLep;
                }
              } else {
                ++iTau;
              }
            }
          }
        }
      }
    }
  }
  if (verbose) {
    edm::LogVerbatim log("TtDecayChannelSelector");
    log << "----------------------"
        << "\n"
        << " iTop    : " << iTop << "\n"
        << " iBeauty : " << iBeauty << "\n"
        << " iElec   : " << iElec << "\n"
        << " iMuon   : " << iMuon << "\n"
        << " iTau    : " << iTau + iLep;
    if (restrictTauDecays_ && (iTau + iLep) > 0) {
      log << " (" << iTau << ")\n";
    } else {
      log << "\n";
    }
    log << "- - - - - - - - - - - "
        << "\n";
  }
  iLep += iElec + iMuon + iTau;

  bool accept = false;
  unsigned int channel = decayChannel();
  if ((iTop == 2) && (iBeauty == 2)) {
    if (channel == iLep) {
      if (channel == 0) {
        // no lepton: accept without restriction we already
        // know that the number of leptons is correct
        accept = true;
      }
      if (channel == 1) {
        // one lepton: check that this one is allowed
        accept = (iElec && allowedDecays_[Elec]) || (iMuon && allowedDecays_[Muon]) || (iTau && allowedDecays_[Tau]);
      }
      if (channel == 2) {
        if (checkSum(allowedDecays_) == channel) {
          // no redundancy
          accept = (allowedDecays_[Elec] == (int)iElec) && (allowedDecays_[Muon] == (int)iMuon) &&
                   (allowedDecays_[Tau] == (int)iTau);
        } else {
          // reject events with wrong tau decays
          if (iElec + iMuon + iTau != channel) {
            accept = false;
          } else {
            if ((iElec == 2) || (iMuon == 2) || (iTau == 2)) {
              // same lepton twice: check that this is allowed.
              accept = (allowedDecays_[Elec] == (int)iElec) || (allowedDecays_[Muon] == (int)iMuon) ||
                       (allowedDecays_[Tau] == (int)iTau);
            } else {
              // two different leptons: look if there is a possible combination
              accept = (((iElec && decayBranchA_[Elec]) &&
                         ((iMuon && decayBranchB_[Muon]) || (iTau && decayBranchB_[Tau]))) ||
                        ((iMuon && decayBranchA_[Muon]) &&
                         ((iElec && decayBranchB_[Elec]) || (iTau && decayBranchB_[Tau]))) ||
                        ((iTau && decayBranchA_[Tau]) &&
                         ((iElec && decayBranchB_[Elec]) || (iMuon && decayBranchB_[Muon]))));
            }
          }
        }
      }
    }
    accept = ((!invert_ && accept) || (!(!invert_) && !accept));
  } else {
    edm::LogWarning("NoVtbDecay") << "Decay is not via Vtb";
  }
  if (verbose)
    edm::LogVerbatim("TtDecayChannelSelector") << " accept  : " << accept;
  return accept;
}

bool TtDecayChannelSelector::search(reco::GenParticleCollection::const_iterator& part,
                                    int pdgId,
                                    std::string_view inputType) const {
  if (inputType == kGenParticles) {
    return (std::abs(part->pdgId()) == pdgId && part->status() == TopDecayID::unfrag) ? true : false;
  } else {
    return (std::abs(part->pdgId()) == pdgId) ? true : false;
  }
}

bool TtDecayChannelSelector::search(reco::GenParticle::const_iterator& part,
                                    int pdgId,
                                    std::string_view inputType) const {
  if (inputType == kGenParticles) {
    return (std::abs(part->pdgId()) == pdgId && part->status() == TopDecayID::unfrag) ? true : false;
  } else {
    return (std::abs(part->pdgId()) == pdgId) ? true : false;
  }
}

unsigned int TtDecayChannelSelector::countProngs(const reco::Candidate& part) const {
  // if stable, return 1 or 0
  if (part.status() == 1) {
    return (part.charge() != 0);
  }
  // if unstable, call recursively on daughters
  int prong = 0;
  for (reco::Candidate::const_iterator daughter = part.begin(); daughter != part.end(); ++daughter) {
    prong += countProngs(*daughter);
  }
  return prong;
}

bool TtDecayChannelSelector::tauDecay(const reco::Candidate& tau) const {
  bool electronTau = false;
  bool muonTau = false;
  unsigned int nch = 0;
  // loop on tau decays, check for an elec
  // or muon and count charged particles
  for (reco::Candidate::const_iterator daughter = tau.begin(); daughter != tau.end(); ++daughter) {
    // if the tau daughter is again a tau, this means that the particle has
    // still to be propagated; in that case, return the result of the same
    // method applied on the daughter of the current particle
    if (daughter->pdgId() == tau.pdgId()) {
      return tauDecay(*daughter);
    }
    // check for electron from tau decay
    electronTau |= (std::abs(daughter->pdgId()) == TopDecayID::elecID);
    // check for muon from tau decay
    muonTau |= (std::abs(daughter->pdgId()) == TopDecayID::muonID);
    // count charged particles
    nch += countProngs(*daughter);
  }
  return ((allowElectron_ && electronTau) || (allowMuon_ && muonTau) ||
          (allow1Prong_ && !electronTau && !muonTau && nch == 1) ||
          (allow3Prong_ && !electronTau && !muonTau && nch == 3));
}