Rivet analyses

title: ATLAS_2017_I1604029

ttbar + gamma at 8 TeV

Experiment: ATLAS (LHC)

Inspire ID: 1604029

Status: VALIDATED

Authors: - Yichen Li - Christian Gutschow

References: - Expt page: ATLAS-TOPQ-2015-21 - JHEP 1711 (2017) 086 - DOI: 10.1007/JHEP11(2017) - arXiv: 1706.03046

Beams: p+ p+

Beam energies: (4000.0, 4000.0)GeV

Run details: - non-allhadronic top-quark pair production associated with a photon

The cross section of a top-quark pair produced in association with a photon is measured in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 8$ TeV with 20.2 fb${}^{-1}$ of data collected by the ATLAS detector at the Large Hadron Collider in 2012. The measurement is performed by selecting events that contain a photon with transverse momentum $p_\text{T} > 15$ GeV, an isolated lepton with large transverse momentum, large missing transverse momentum, and at least four jets, where at least one is identified as originating from a $b$-quark. The production cross section is measured in a fiducial region close to the selection requirements. It is found to be $139 \pm 7$ (stat.) $\pm 17$ (syst.) fb, in good agreement with the theoretical prediction at next-to-leading order of $151 \pm 24$ fb. In addition, differential cross sections in the fiducial region are measured as a function of the transverse momentum and pseudorapidity of the photon.

Source code:ATLAS_2017_I1604029.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/FastJets.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/LeptonFinder.hh"

include "Rivet/Projections/PromptFinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief ttbar + gamma at 8 TeV class ATLAS_2017_I1604029 : public Analysis { public:

// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ATLAS_2017_I1604029);

// Book histograms and initialise projections before the run
void init() {

  const FinalState fs;

  // signal photons
  PromptFinalState prompt_ph(Cuts::abspid == PID::PHOTON && Cuts::pT > 15 * GeV && Cuts::abseta < 2.37);
  declare(prompt_ph, "photons");

  // bare leptons
  Cut base_cuts = (Cuts::abseta < 2.7) && (Cuts::pT > 10 * GeV);
  IdentifiedFinalState bare_leps(base_cuts);
  bare_leps.acceptIdPair(PID::MUON);
  bare_leps.acceptIdPair(PID::ELECTRON);
  declare(bare_leps, "bare_leptons");

  // dressed leptons
  Cut dressed_cuts = (Cuts::abseta < 2.5) && (Cuts::pT > 25 * GeV);
  PromptFinalState prompt_mu(base_cuts && Cuts::abspid == PID::MUON);
  PromptFinalState prompt_el(base_cuts && Cuts::abspid == PID::ELECTRON);
  IdentifiedFinalState all_photons(fs, PID::PHOTON);
  LeptonFinder elecs(prompt_el, all_photons, 0.1, dressed_cuts);
  declare(elecs, "elecs");
  LeptonFinder muons(prompt_mu, all_photons, 0.1, dressed_cuts);
  declare(muons, "muons");

  // auxiliary projections for 'single-lepton ttbar filter'
  PromptFinalState prompt_lep(Cuts::abspid == PID::MUON || Cuts::abspid == PID::ELECTRON);
  declare(prompt_lep, "prompt_leps");
  declare(UnstableParticles(), "ufs");

  // jets
  FastJets jets(fs, JetAlg::ANTIKT, 0.4, JetMuons::NONE, JetInvisibles::NONE);
  declare(jets, "jets");


  // BOOK HISTOGRAMS
  book(_h["pt"], 2, 1, 1);
  book(_h["eta"], 3, 1, 1);
}


// Perform the per-event analysis
void analyze(const Event& event) {

  // analysis extrapolated to 1-lepton-plus-jets channel, where "lepton" cannot be a tau
  // (i.e. contribution from dileptonic ttbar where one of the leptons is outside
  // the detector acceptance has been subtracted as a background)
  if (apply<PromptFinalState>(event, "prompt_leps").particles().size() != 1) vetoEvent;
  for (const auto& p : apply<UnstableParticles>(event, "ufs").particles()) {
    if (p.fromPromptTau()) vetoEvent;
  }

  // photon selection
  Particles photons = apply<PromptFinalState>(event, "photons").particlesByPt();
  Particles bare_leps = apply<IdentifiedFinalState>(event, "bare_leptons").particles();
  for (const Particle& lep : bare_leps) idiscard(photons, deltaRLess(lep, 0.1));
  if (photons.size() != 1) vetoEvent;
  const Particle& photon = photons[0];

  // jet selection
  Jets jets = apply<JetFinder>(event, "jets").jetsByPt(Cuts::abseta < 2.5 && Cuts::pT > 25 * GeV);

  // lepton selection
  const DressedLeptons& elecs = apply<LeptonFinder>(event, "elecs").dressedLeptons();
  const DressedLeptons& all_muons = apply<LeptonFinder>(event, "muons").dressedLeptons();

  // jet photon/electron overlap removal
  for (const DressedLepton& e : elecs) idiscard(jets, deltaRLess(e, 0.2, RAPIDITY));
  for (const Particle& ph : photons) idiscard(jets, deltaRLess(ph, 0.1, RAPIDITY));
  if (jets.size() < 4) vetoEvent;

  // photon-jet minimum deltaR
  double mindR_phjet = 999.;
  for (Jet jet : jets) {
    const double dR_phjet = deltaR(photon, jet);
    if (dR_phjet < mindR_phjet) mindR_phjet = dR_phjet;
  }
  if (mindR_phjet < 0.5) vetoEvent;

  // muon jet overlap removal
  DressedLeptons muons;
  for (const DressedLepton& mu : all_muons) {
    bool overlaps = false;
    for (const Jet& jet : jets) {
      if (deltaR(mu, jet) < 0.4) {
        overlaps = true;
        break;
      }
    }
    if (overlaps) continue;
    muons.push_back(mu);
  }

  // one electron XOR one muon
  bool isEl = elecs.size() == 1 && muons.size() == 0;
  bool isMu = muons.size() == 1 && elecs.size() == 0;
  if (!isEl && !isMu) vetoEvent;

  // photon-lepton deltaR
  double mindR_phlep = deltaR(photon, isEl ? elecs[0] : muons[0]);
  if (mindR_phlep < 0.7) vetoEvent;

  // b-tagging
  Jets bjets;
  for (const Jet& jet : jets) {
    if (jet.bTagged(Cuts::pT > 5 * GeV)) bjets += jet;
  }
  if (bjets.empty()) vetoEvent;

  _h["pt"]->fill(photon.pT() / GeV);
  _h["eta"]->fill(photon.abseta());
}


// Normalise histograms etc., after the run
void finalize() {
  const double normto(crossSection() / femtobarn / sumOfWeights());
  for (auto& hist : _h) {
    scale(hist.second, normto);
  }
}

private:

map<string, Histo1DPtr> _h;

};

RIVET_DECLARE_PLUGIN(ATLAS_2017_I1604029); } ```