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## Rivet analyses reference

### ATLAS_2017_I1591327

Inclusive diphoton cross-sections at 8 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1591327
Status: VALIDATED
Authors:
• Frank Siegert
References:
Beams: p+ p+
Beam energies: (4000.0, 4000.0) GeV
Run details:
• $pp \to \gamma \gamma$ at 8 TeV

A measurement of the production cross section for two isolated photons in proton--proton collisions at a center-of-mass energy of $\sqrt{s}=8$ TeV is presented. The results are based on an integrated luminosity of 20.2 fb$^{-1}$ recorded by the ATLAS detector at the Large Hadron Collider. The measurement considers photons with pseudorapidities satisfying $|\eta^\gamma| < 1.37$ or $1.56 < |\eta^\gamma| < 2.37$ and transverse energies of respectively $E^\gamma_\text{T,1}>40$ GeV and $E^\gamma_\text{T,2}> 30$ GeV for the two leading photons ordered in transverse energy produced in the interaction. The background due to hadronic jets and electrons is subtracted using data-driven techniques. The fiducial cross sections are corrected for detector effects and measured differentially as a function of six kinematic observables. The measured cross section integrated within the fiducial volume is $16.8\pm0.8$ pb. The data are compared to fixed-order QCD calculations at next-to-leading-order and next-to-next-to-leading-order accuracy as well as next-to-leading-order computations including resummation of initial-state gluon radiation at next-to-next-to-leading logarithm or matched to a parton shower, with relative uncertainties varying from 5% to 20%.

Source code: ATLAS_2017_I1591327.cc
  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 // -*- C++ -*- #include "Rivet/Analysis.hh" #include "Rivet/Projections/FinalState.hh" #include "Rivet/Projections/IdentifiedFinalState.hh" #include "Rivet/Projections/FastJets.hh" namespace Rivet { /// Isolated diphoton + X differential cross-sections class ATLAS_2017_I1591327 : public Analysis { public: // Constructor ATLAS_2017_I1591327() : Analysis("ATLAS_2017_I1591327") { setNeedsCrossSection(true); } // Book histograms and initialise projections before the run void init() { const FinalState fs; declare(fs, "FS"); FastJets fj(fs, FastJets::KT, 0.5); _area_def = new fastjet::AreaDefinition(fastjet::VoronoiAreaSpec()); fj.useJetArea(_area_def); declare(fj, "KtJetsD05"); IdentifiedFinalState photonfs(Cuts::abseta < 2.37 && Cuts::pT > 30*GeV); photonfs.acceptId(PID::PHOTON); declare(photonfs, "Photon"); // Histograms _h_M = bookHisto1D(2, 1, 1); _h_pT = bookHisto1D(3, 1, 1); _h_at = bookHisto1D(4, 1, 1); _h_phistar = bookHisto1D(5, 1, 1); _h_costh = bookHisto1D(6, 1, 1); _h_dPhi = bookHisto1D(7, 1, 1); } // Perform the per-event analysis void analyze(const Event& event) { // Require at least 2 photons in final state const Particles photons = apply(event, "Photon").particlesByPt(); if (photons.size() < 2) vetoEvent; // Compute the median energy density _ptDensity.clear(); _sigma.clear(); _Njets.clear(); vector > ptDensities; vector emptyVec; ptDensities.assign(ETA_BINS.size()-1, emptyVec); // Get jets, and corresponding jet areas const shared_ptr clust_seq_area = applyProjection(event, "KtJetsD05").clusterSeqArea(); for (const fastjet::PseudoJet& jet : apply(event, "KtJetsD05").pseudoJets(0.0*GeV)) { const double aeta = fabs(jet.eta()); const double pt = jet.perp(); const double area = clust_seq_area->area(jet); if (area < 1e-3) continue; const int ieta = binIndex(aeta, ETA_BINS); if (ieta != -1) ptDensities[ieta].push_back(pt/area); } // Compute median jet properties over the jets in the event for (size_t b = 0; b < ETA_BINS.size()-1; ++b) { double median = 0.0, sigma = 0.0; int Njets = 0; if (ptDensities[b].size() > 0) { std::sort(ptDensities[b].begin(), ptDensities[b].end()); int nDens = ptDensities[b].size(); median = (nDens % 2 == 0) ? (ptDensities[b][nDens/2]+ptDensities[b][(nDens-2)/2])/2 : ptDensities[b][(nDens-1)/2]; sigma = ptDensities[b][(int)(.15865*nDens)]; Njets = nDens; } _ptDensity.push_back(median); _sigma.push_back(sigma); _Njets.push_back(Njets); } // Loop over photons and fill vector of isolated ones Particles isolated_photons; for (const Particle& photon : photons) { // Check if it's a prompt photon (needed for SHERPA 2->5 sample, otherwise I also get photons from hadron decays in jets) if (photon.fromDecay()) continue; // Remove photons in ECAL crack region if (inRange(photon.abseta(), 1.37, 1.56)) continue; const double eta_P = photon.eta(); const double phi_P = photon.phi(); // Compute isolation via particles within an R=0.4 cone of the photon const Particles fs = apply(event, "FS").particles(); FourMomentum mom_in_EtCone; for (const Particle& p : fs) { // Reject if not in cone if (deltaR(photon.momentum(), p.momentum()) > 0.4) continue; // Reject if in the 5x7 cell central core if (fabs(eta_P - p.eta()) < 0.025 * 5 * 0.5 && fabs(phi_P - p.phi()) < PI/128. * 7 * 0.5) continue; // Sum momentum mom_in_EtCone += p.momentum(); } // Now figure out the correction (area*density) const double EtCone_area = M_PI*sqr(0.4) - (7*.025)*(5*M_PI/128.); // cone area - central core rectangle const double correction = _ptDensity[binIndex(fabs(eta_P), ETA_BINS)] * EtCone_area; // Discard the photon if there is more than 11 GeV of cone activity // NOTE: Shouldn't need to subtract photon itself (it's in the central core) if (mom_in_EtCone.Et() - correction > 11*GeV) continue; // Add isolated photon to list isolated_photons.push_back(photon); } // Require at least two isolated photons if (isolated_photons.size() < 2) vetoEvent; // Select leading pT pair sortByPt(isolated_photons); const FourMomentum y1 = isolated_photons[0]; const FourMomentum y2 = isolated_photons[1]; // Leading photon should have pT > 40 GeV, subleading > 30 GeV if (y1.pT() < 40.*GeV) vetoEvent; if (y2.pT() < 30.*GeV) vetoEvent; // Require the two photons to be separated (dR>0.4) if (deltaR(y1,y2) < 0.4) vetoEvent; const FourMomentum yy = y1 + y2; const double Myy = yy.mass(); const double pTyy = yy.pT(); const double dPhiyy = mapAngle0ToPi(y1.phi() - y2.phi()); // phi* const double costhetastar_ = fabs(tanh(( y1.eta() - y2.eta() ) / 2.)); const double sinthetastar_ = sqrt(1. - pow(costhetastar_, 2)); const double phistar = tan(0.5 * (PI - dPhiyy)) * sinthetastar_; // a_t const Vector3 t_hat(y1.x()-y2.x(), y1.y()-y2.y(), 0.); const double factor = t_hat.mod(); const Vector3 t_hatx(t_hat.x()/factor, t_hat.y()/factor, t_hat.z()/factor); const Vector3 At(y1.x()+y2.x(), y1.y()+y2.y(), 0.); // Compute a_t transverse component with respect to t_hat const double at = At.cross(t_hatx).mod(); // Fill histograms const double weight = event.weight(); _h_M->fill(Myy, weight); _h_pT->fill(pTyy, weight); _h_dPhi->fill(dPhiyy, weight); _h_costh->fill(costhetastar_, weight); _h_phistar->fill(phistar, weight); _h_at->fill(at, weight); } // Normalise histograms etc., after the run void finalize() { const double sf = crossSection() / (femtobarn * sumOfWeights()); scale({_h_M, _h_pT, _h_dPhi, _h_costh, _h_phistar, _h_at}, sf); } private: Histo1DPtr _h_M, _h_pT, _h_dPhi, _h_costh, _h_phistar, _h_at; fastjet::AreaDefinition* _area_def; const vector ETA_BINS = {0.0, 1.5, 3.0}; vector _ptDensity, _sigma, _Njets; }; // The hook for the plugin system DECLARE_RIVET_PLUGIN(ATLAS_2017_I1591327); }