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ATLAS_2019_I1764342

Z(ll)y cross-section at 13 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1764342
Status: VALIDATED
Authors:
  • Ruchi Gupta
References: 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
  1// -*- C++ -*-
  2#include "Rivet/Analysis.hh"
  3#include "Rivet/Projections/FinalState.hh"
  4#include "Rivet/Projections/VetoedFinalState.hh"
  5#include "Rivet/Projections/DressedLeptons.hh"
  6#include "Rivet/Projections/PromptFinalState.hh"
  7#include "Rivet/Projections/InvisibleFinalState.hh"
  8
  9namespace Rivet {
 10
 11
 12  /// @brief Z(ll)y cross-section at 13 TeV
 13  class ATLAS_2019_I1764342 : public Analysis {
 14  public:
 15
 16    /// Constructor
 17    RIVET_DEFAULT_ANALYSIS_CTOR(ATLAS_2019_I1764342);
 18
 19    /// @name Analysis methods
 20    /// @{
 21
 22    /// Book histograms and initialise projections before the run
 23    void init() {
 24
 25      // Prompt photons
 26      const PromptFinalState photon_fs(Cuts::abspid == PID::PHOTON && Cuts::pT > 30*GeV && Cuts::abseta < 2.37);
 27      declare(photon_fs, "Photons");
 28
 29      // Prompt leptons
 30      const PromptFinalState bareelectron_fs = Cuts::abspid == PID::ELECTRON;
 31      const PromptFinalState baremuon_fs = Cuts::abspid == PID::MUON;
 32
 33      // Dressed leptons
 34      const FinalState allphoton_fs(Cuts::abspid == PID::PHOTON); // photons used for lepton dressing
 35      const Cut leptoncut = Cuts::pT > 25*GeV && Cuts::abseta < 2.47;
 36      const DressedLeptons dressedelectron_fs(allphoton_fs, bareelectron_fs, 0.1, leptoncut, true); // use *all* photons for lepton dressing
 37      const DressedLeptons dressedmuon_fs(allphoton_fs, baremuon_fs, 0.1, leptoncut, true); // use *all* photons for lepton dressing
 38
 39      declare(dressedelectron_fs, "Electrons");
 40      declare(dressedmuon_fs, "Muons");
 41
 42      // FS excluding the leading photon
 43      VetoedFinalState vfs;
 44      vfs.addVetoOnThisFinalState(photon_fs);
 45      vfs.addVetoOnThisFinalState(dressedmuon_fs);
 46      vfs.addVetoOnThisFinalState(InvisibleFinalState());
 47      declare(vfs, "isolatedFS");
 48
 49      // Histograms
 50      book(_hist_EgammaT,     2, 1, 1); // dSigma / dE^gamma_T
 51      book(_hist_etagamma,    3, 1, 1);
 52      book(_hist_mZgamma,     4, 1, 1); // dSigma / dm^{Zgamma}
 53      book(_hist_EZgammaT,    5, 1, 1);
 54      book(_hist_dPhiZgamma,  6, 1, 1);
 55      book(_hist_ETbyMZgamma, 7, 1, 1);
 56    }
 57
 58
 59   /// Perform the per-event analysis
 60   void analyze(const Event& event) {
 61     // Get objects
 62     Particles electrons = apply<DressedLeptons>(event, "Electrons").particlesByPt();
 63     Particles muons = apply<DressedLeptons>(event, "Muons").particlesByPt();
 64     const Particles& photons = apply<PromptFinalState>(event, "Photons").particlesByPt();
 65
 66     if (photons.empty())  vetoEvent;
 67     if (electrons.size() < 2 && muons.size() < 2)  vetoEvent;
 68     Particles lep;
 69     // Sort the dressed leptons by pt
 70     if (electrons.size() >= 2) {
 71       lep.push_back(electrons[0]);
 72       lep.push_back(electrons[1]);
 73     } else {
 74       lep.push_back(muons[0]);
 75       lep.push_back(muons[1]);
 76     }
 77     if (lep[0].Et() < 30*GeV)  vetoEvent;
 78     double mll = (lep[0].momentum() + lep[1].momentum()).mass();
 79     if (mll < 40*GeV) vetoEvent;
 80
 81     Particles selectedPh;
 82     Particles fs = apply<VetoedFinalState>(event, "isolatedFS").particles();
 83     for (const Particle& ph : photons) {
 84       // check photon isolation
 85       double coneEnergy(0.0);
 86       for (const Particle& p : fs) {
 87         if ( deltaR(ph, p) < 0.2 )  coneEnergy += p.Et();
 88       }
 89       if (coneEnergy / ph.Et() > 0.07 )  continue;
 90       if (deltaR(ph, lep[0]) < 0.4) continue;
 91       if (deltaR(ph, lep[1]) < 0.4) continue;
 92       selectedPh.push_back(ph);
 93     }
 94
 95     if (selectedPh.size()<1) vetoEvent;
 96     double mlly = (lep[0].momentum() + lep[1].momentum() + selectedPh[0].momentum()).mass();
 97     if (mll + mlly <= 182*GeV) vetoEvent;
 98
 99     double ptlly = (lep[0].momentum() + lep[1].momentum() + selectedPh[0].momentum()).pT();
100     double dphilly = deltaPhi((lep[0].momentum() + lep[1].momentum()).phi(), selectedPh[0].momentum().phi());
101
102     // Fill plots
103     _hist_EgammaT->fill(selectedPh[0].pT()/GeV);
104     _hist_etagamma->fill(selectedPh[0].abseta());
105     _hist_mZgamma->fill(mlly/GeV);
106     _hist_EZgammaT->fill(ptlly/GeV);
107     _hist_dPhiZgamma->fill(dphilly/pi);
108     _hist_ETbyMZgamma->fill(ptlly/mlly);
109   }
110
111
112   /// Normalise histograms etc., after the run
113   void finalize() {
114      const double sf = crossSection()/femtobarn/sumOfWeights();
115      scale(_hist_EgammaT, sf);
116      scale(_hist_etagamma, sf);
117      scale(_hist_mZgamma, sf);
118      scale(_hist_EZgammaT, sf);
119      scale(_hist_dPhiZgamma, sf/pi);
120      scale(_hist_ETbyMZgamma, sf);
121   }
122
123   /// @}
124
125
126  private:
127
128    // /// Mode flag
129    // size_t _mode;
130
131    /// Histograms
132    Histo1DPtr _hist_EgammaT;
133    Histo1DPtr _hist_etagamma;
134    Histo1DPtr _hist_mZgamma;
135    Histo1DPtr _hist_EZgammaT;
136    Histo1DPtr _hist_dPhiZgamma;
137    Histo1DPtr _hist_ETbyMZgamma;
138
139  };
140
141
142  RIVET_DECLARE_PLUGIN(ATLAS_2019_I1764342);
143
144}