Rivet analyses

title: ATLAS_2019_I1764342

Z(ll)y cross-section at 13 TeV

Experiment: ATLAS (LHC)

Inspire ID: 1764342

Status: VALIDATED

Authors: - Ruchi Gupta

References: - Expt page: ATLAS-STDM-2018-04 - arXiv: 1911.04813

Beams: p+ p+

Beam energies: (6500.0, 6500.0)GeV

Run details: - pp -> Z(ll)+y at 13 TeV

The production of a prompt photon in association with a Z boson is studied in proton-proton collisions at a centre-of-mass energy $\sqrt{s}=$13 TeV. The analysis uses a data sample with an integrated luminosity of 139 fb$^{-1}$ collected by the ATLAS detector at the LHC from 2015 to 2018. The production cross-section for the process $pp \rightarrow \ell^+ \ell^- + \gamma + X$ ($\ell=e,\mu$) is measured within a fiducial phase-space region defined by kinematic requirements on the photon and the leptons, and by isolation requirements on the photon. An experimental precision of $2.9\%$ is achieved for the fiducial cross-section. Differential cross-sections are measured as a function of each of six kinematic variables characterising the $\ell^+\ell^-\gamma$ system. The data are compared with theoretical predictions based on next-to-leading-order and next-to-next-to-leading-order perturbative QCD calculations. The impact of next-to-leading-order electroweak corrections is also considered.

Source code:ATLAS_2019_I1764342.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/InvisibleFinalState.hh"

include "Rivet/Projections/LeptonFinder.hh"

include "Rivet/Projections/PromptFinalState.hh"

include "Rivet/Projections/VetoedFinalState.hh"

namespace Rivet {

/// @brief Z(ll)y cross-section at 13 TeV class ATLAS_2019_I1764342 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ATLAS_2019_I1764342);

/// @name Analysis methods
/// @{

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

  // get option
  _mode = 0;
  if (getOption("LMODE") == "EL") _mode = 1;
  if (getOption("LMODE") == "MU") _mode = 2;

  // Prompt photons
  const PromptFinalState photon_fs(Cuts::abspid == PID::PHOTON && Cuts::pT > 30 * GeV
                                   && Cuts::abseta < 2.37);
  declare(photon_fs, "Photons");

  // Prompt leptons
  const PromptFinalState bareelectron_fs = Cuts::abspid == PID::ELECTRON;
  const PromptFinalState baremuon_fs = Cuts::abspid == PID::MUON;

  // Dressed leptons
  const FinalState allphoton_fs(Cuts::abspid == PID::PHOTON); // photons used for lepton dressing
  const Cut leptoncut = Cuts::pT > 25 * GeV && Cuts::abseta < 2.47;
  const LeptonFinder dressedelectron_fs(bareelectron_fs, allphoton_fs, 0.1, leptoncut);
  const LeptonFinder dressedmuon_fs(baremuon_fs, allphoton_fs, 0.1, leptoncut);

  declare(dressedelectron_fs, "Electrons");
  declare(dressedmuon_fs, "Muons");

  // FS excluding the leading photon
  VetoedFinalState vfs;
  vfs.addVetoOnThisFinalState(photon_fs);
  vfs.addVetoOnThisFinalState(dressedmuon_fs);
  vfs.addVetoOnThisFinalState(InvisibleFinalState());
  declare(vfs, "isolatedFS");

  // Histograms
  book(_h["EgammaT"], 2, 1, 1); // dSigma / dE^gamma_T
  book(_h["etagamma"], 3, 1, 1);
  book(_h["mZgamma"], 4, 1, 1); // dSigma / dm^{Zgamma}
  book(_h["EZgammaT"], 5, 1, 1);
  book(_h["dPhiZgamma"], 6, 1, 1);
  book(_h["ETbyMZgamma"], 7, 1, 1);
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  // Get objects
  Particles electrons = apply<LeptonFinder>(event, "Electrons").particlesByPt();
  Particles muons = apply<LeptonFinder>(event, "Muons").particlesByPt();
  const Particles& photons = apply<PromptFinalState>(event, "Photons").particlesByPt();

  if (photons.empty()) vetoEvent;
  if (electrons.size() < 2 && muons.size() < 2) vetoEvent;

  if (_mode == 1 && muons.size()) vetoEvent;
  if (_mode == 2 && electrons.size()) vetoEvent;

  Particles lep;
  // Sort the dressed leptons by pt
  if (electrons.size() >= 2) {
    lep.push_back(electrons[0]);
    lep.push_back(electrons[1]);
  }
  else {
    lep.push_back(muons[0]);
    lep.push_back(muons[1]);
  }
  if (lep[0].Et() < 30 * GeV) vetoEvent;
  double mll = (lep[0].momentum() + lep[1].momentum()).mass();
  if (mll < 40 * GeV) vetoEvent;

  Particles selectedPh;
  Particles fs = apply<VetoedFinalState>(event, "isolatedFS").particles();
  for (const Particle& ph : photons) {
    // check photon isolation
    double coneEnergy(0.0);
    for (const Particle& p : fs) {
      if (deltaR(ph, p) < 0.2) coneEnergy += p.Et();
    }
    if (coneEnergy / ph.Et() > 0.07) continue;
    if (deltaR(ph, lep[0]) < 0.4) continue;
    if (deltaR(ph, lep[1]) < 0.4) continue;
    selectedPh.push_back(ph);
  }

  if (selectedPh.size() < 1) vetoEvent;
  double mlly = (lep[0].momentum() + lep[1].momentum() + selectedPh[0].momentum()).mass();
  if (mll + mlly <= 182 * GeV) vetoEvent;

  double ptlly = (lep[0].momentum() + lep[1].momentum() + selectedPh[0].momentum()).pT();
  double dphilly = deltaPhi((lep[0].momentum() + lep[1].momentum()).phi(),
                            selectedPh[0].momentum().phi());

  // Fill plots
  _h["EgammaT"]->fill(selectedPh[0].pT() / GeV);
  _h["etagamma"]->fill(selectedPh[0].abseta());
  _h["mZgamma"]->fill(mlly / GeV);
  _h["EZgammaT"]->fill(ptlly / GeV);
  _h["dPhiZgamma"]->fill(dphilly / pi);
  _h["ETbyMZgamma"]->fill(ptlly / mlly);
}


/// Normalise histograms etc., after the run
void finalize() {
  double sf = crossSection() / femtobarn / sumOfWeights();
  if (_mode == 0) sf *= 0.5;
  scale(_h, sf);
  scale(_h["dPhiZgamma"], 1.0 / pi);
}

/// @}

private:

/// Mode flag
size_t _mode;

/// Histograms
map<string, Histo1DPtr> _h;

};

RIVET_DECLARE_PLUGIN(ATLAS_2019_I1764342);

} ```