Rivet analyses

Measurement of the inclusive isolated photon production cross section in pp collisions at $\sqrt{s} = 7$ TeV

Experiment: ALICE (LHC)

Inspire ID: 1738300

Status: VALIDATED

Authors: - Alexander Puck Neuwirth

References: - Eur.Phys.J.C 79 (2019) 11, 896 - arXiv: 1906.01371 - Expt page: ALICE-5338

Beams: p+ p+

Beam energies: (3500.0, 3500.0)GeV

Run details: - Inclusive photon+X events at $\sqrt{s} = 7$~TeV at low transverse momentum.

The production cross section of inclusive isolated photons has been measured by the ALICE experiment at the CERN LHC in pp collisions at a centre-of-momentum energy of $\sqrt{s}= 7$ TeV. The measurement is performed with the electromagnetic calorimeter EMCal and the central tracking detectors, covering a range of |η| < 0.27 in pseudorapidity and a transverse momentum range of $ 10 < p_T^< 60$ GeV.

Source code:ALICE_2019_I1738300.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/VetoedFinalState.hh"
#include "Rivet/Projections/PromptFinalState.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/MissingMomentum.hh"
#include "Rivet/Projections/DirectFinalState.hh"

namespace Rivet {


  /// @brief Measurement of the inclusive isolated photon production cross section in pp collisions
  class ALICE_2019_I1738300 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(ALICE_2019_I1738300);


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

    /// Book histograms and initialise projections before the run
    void init() {
      const FinalState fs;

      // calorimeter particles
      VisibleFinalState visFS(fs);
      VetoedFinalState calo_fs(visFS);
      //calo_fs.addVetoPairId(PID::MUON);
      declare(calo_fs, "calo");

      // photon
      PromptFinalState photonfs(Cuts::abspid == PID::PHOTON && Cuts::abseta < _abseta);
      declare(photonfs, "photons");

      // Book histograms
      // specify custom binning
      // take binning from reference data using HEPData ID (digits in "d01-x01-y01" etc.)
      book(_h["p_t"], 1, 1, 1);
      book(_h["x_t"], 4, 1, 1);
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // Get the photon
      const Particles& photons = apply<PromptFinalState>(event, "photons").particlesByPt();
      if (photons.empty())  vetoEvent;
      // We only look at the hardest/leading photon
      const FourMomentum photon = photons[0].momentum();


      // Compute photon isolation with a standard ET cone
      double mom_in_EtCone = 0.0;
      const Particles calo_fs = apply<VetoedFinalState>(event, "calo").particles();
      for (const Particle& p : calo_fs) {
        // Check if it's in the cone of .4
        if (deltaR(p, photon) >= _iso_dr) continue;
        // Increment sum
        mom_in_EtCone += p.momentum().pT();
      }

      // Remove the photon energy from the isolation
      mom_in_EtCone -= photon.pT();

      if (mom_in_EtCone > _iso_et_cut)  vetoEvent;

      _h["p_t"]->fill(photon.pT()/GeV);
      // scale event weight by 1/(2 pi * pT ) as in ALICE_2017_I1512110 (with d eta applied later)
      _h["x_t"]->fill(2* photon.pT() / sqrtS(), 1.0 / (2. * M_PI* photon.pT()));

    }

    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_h["p_t"], crossSection()/nanobarn/sumOfWeights() / (2* _abseta)); // also divide by |eta| range
      scale(_h["x_t"], crossSection()/picobarn/sumOfWeights()* pow(sqrtS() / GeV, 4.5)/ (2* _abseta) / (sqrtS() / 2.) ); // scaled by sqrt(s)^4.5 and also divide by |eta| range. Further division by (sqrt(s)/2) to convert from xt to pT
    }

    /// @}

    // Analysis parameters
    const double _abseta = 0.27;
    const double _iso_dr = 0.4;
    const double _iso_et_cut = 2.0*GeV;

    /// @name Histograms
    /// @{
    map<string, Histo1DPtr> _h;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(ALICE_2019_I1738300);

}