Rivet analyses

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 $=13TeV.Theanalysisusesadatasamplewithanintegratedluminosityof139fb^{-1}$ collected by the ATLAS detector at the LHC from 2015 to 2018. The production cross-section for the process pp → ℓ+ℓ− + γ + X (ℓ = e, μ) 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 ℓ+ℓ−γ 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++ -*-
#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);

}