Rivet analyses
title: ATLAS_2012_I1199269
Inclusive diphoton $+ X$ events at $\sqrt{s} = 7$~TeV
Experiment: ATLAS (LHC)
Inspire ID: 1199269
Status: VALIDATED
Authors: - Giovanni Marchiori
References: - Expt page: ATLAS-STDM-2012-05 - arXiv: 1211.1913 - JHEP 1301 (2013) 086
Beams: p+ p+
Beam energies: (3500.0, 3500.0)GeV
Run details: - Inclusive diphoton $+ X$ events at $\sqrt{s} = 7$~TeV.
The ATLAS experiment at the LHC has measured the production cross section of events with two isolated photons in the final state, in proton-proton collisions at $\sqrt{s} = 7$ TeV. The full data set collected in 2011, corresponding to an integrated luminosity of 4.9 fb$^{-1}$, is used. The amount of background, from hadronic jets and isolated electrons, is estimated with data-driven techniques and subtracted. The total cross section, for two isolated photons with transverse energies above 25 GeV and 22 GeV respectively, in the acceptance of the electromagnetic calorimeter ($|\eta|<1.37$ and $1.52<|\eta|<2.37$) and with an angular separation $\Delta R>0.4$, is $44.0^{+3.2}_{-4.2}$ pb. The differential cross sections as a function of the di-photon invariant mass, transverse momentum, azimuthal separation, and cosine of the polar angle of the largest transverse energy photon in the Collins--Soper di-photon rest frame are also measured. The results are compared to the prediction of leading-order parton-shower and next-to-leading-order and next-to-next-to-leading-order parton-level generators.
Source code:ATLAS_2012_I1199269.cc
```c++ // -- C++ --
include "Rivet/Analysis.hh"
include "Rivet/Projections/FastJets.hh"
include "Rivet/Projections/IdentifiedFinalState.hh"
namespace Rivet {
/// @brief Measurement of isolated diphoton + X differential cross-sections /// /// Inclusive isolated gamma gamma cross-sections, differential in M(gg), pT(gg), /// dphi(gg), cos(theta*)_CS /// /// @author Giovanni Marchiori /// class ATLAS_2012_I1199269 : public Analysis { public:
/// Constructor
ATLAS_2012_I1199269()
: Analysis("ATLAS_2012_I1199269") { }
/// Book histograms and initialise projections before the run
void init() {
FinalState fs;
declare(fs, "FS");
FastJets fj(fs, JetAlg::KT, 0.5);
fj.useJetArea(new fastjet::AreaDefinition(fastjet::VoronoiAreaSpec()));
declare(fj, "KtJetsD05");
IdentifiedFinalState photonfs(Cuts::abseta < 2.37 && Cuts::pT > 22 * GeV);
photonfs.acceptId(PID::PHOTON);
declare(photonfs, "Photon");
book(_h_M, 1, 1, 1);
book(_h_pT, 2, 1, 1);
book(_h_dPhi, 3, 1, 1);
book(_h_cosThetaStar, 4, 1, 1);
}
/// Perform the per-event analysis
void analyze(const Event& event) {
// Require at least 2 photons in final state
const Particles photons = apply<IdentifiedFinalState>(event, "Photon").particlesByPt();
if (photons.size() < 2) vetoEvent;
// Get jets, and corresponding jet areas
vector<vector<double>> ptDensities(_eta_bins_areaoffset.size() - 1);
const auto clust_seq_area = apply<FastJets>(event, "KtJetsD05").clusterSeqArea();
for (const Jet& jet : apply<FastJets>(event, "KtJetsD05").jets()) {
const double area = clust_seq_area->area(jet); // implicit .pseudojet()
if (area < 1e-3) continue;
const int ieta = binIndex(jet.abseta(), _eta_bins_areaoffset);
if (ieta != -1) ptDensities[ieta].push_back(jet.pT() / area);
}
// Compute median jet properties over the jets in the event
vector<double> vptDensity; //, vsigma, vNjets;
for (size_t b = 0; b < _eta_bins_areaoffset.size() - 1; ++b) {
vptDensity += ptDensities[b].empty() ? 0 : median(ptDensities[b]);
}
// Loop over photons and fill vector of isolated ones
Particles isolated_photons;
for (const Particle& photon : photons) {
/// Remove photons in ECAL crack region
if (inRange(photon.abseta(), 1.37, 1.52)) continue;
// Compute isolation via particles within an R=0.4 cone of the photon
const Particles& fs = apply<FinalState>(event, "FS").particles();
FourMomentum mom_in_EtCone;
for (const Particle& p : fs) {
// Reject if not in cone
if (deltaR(photon, p) > 0.4) continue;
// Reject if in the 5x7 cell central core
if (fabs(deltaEta(photon, p)) < 0.025 * 5 * 0.5 && fabs(deltaPhi(photon, p)) < PI / 128. * 7 * 0.5)
continue;
// Sum momentum
mom_in_EtCone += p.momentum();
}
// Now figure out the correction (area*density)
const double ETCONE_AREA = PI * sqr(0.4)
- (7 * .025) * (5 * PI / 128.); // cone area - central core rectangle
const double correction = vptDensity[binIndex(photon.abseta(), _eta_bins_areaoffset)] * ETCONE_AREA;
// Discard the photon if there is more than 4 GeV of cone activity
// NOTE: Shouldn't need to subtract photon itself (it's in the central core)
// NOTE: using expected cut at hadron/particle level, not at reco level
if (mom_in_EtCone.Et() - correction > 4 * GeV) continue;
// Add isolated photon to list
isolated_photons.push_back(photon);
}
// Require at least two isolated photons and select leading pT pair
if (isolated_photons.size() < 2) vetoEvent;
isortByPt(isolated_photons);
const FourMomentum& y1 = isolated_photons[0].momentum();
const FourMomentum& y2 = isolated_photons[1].momentum();
// Leading photon should have pT > 25 GeV
if (y1.pT() < 25 * GeV) vetoEvent;
// Require the two photons to be separated by dR > 0.4
if (deltaR(y1, y2) < 0.4) vetoEvent;
// Compute diphoton vector and fill histos
FourMomentum yy = y1 + y2;
const double costhetayy = 2 * y1.pT() * y2.pT() * sinh(y1.eta() - y2.eta()) / yy.mass()
/ add_quad(yy.mass(), yy.pT());
_h_M->fill(yy.mass() / GeV);
_h_pT->fill(yy.pT() / GeV);
_h_dPhi->fill(mapAngle0ToPi(y1.phi() - y2.phi()));
_h_cosThetaStar->fill(costhetayy);
}
/// Normalise histograms etc., after the run
void finalize() {
scale(_h_M, crossSection() / picobarn / sumOfWeights());
scale(_h_pT, crossSection() / picobarn / sumOfWeights());
scale(_h_dPhi, crossSection() / picobarn / sumOfWeights());
scale(_h_cosThetaStar, crossSection() / picobarn / sumOfWeights());
}
private:
Histo1DPtr _h_M, _h_pT, _h_dPhi, _h_cosThetaStar;
const vector<double> _eta_bins_areaoffset = {0.0, 1.5, 3.0};
};
RIVET_DECLARE_PLUGIN(ATLAS_2012_I1199269);
} ```