Rivet analyses

title: ALICE_2024_I2803487

Measurement of the inclusive isolated photon production cross section in pp collisions at $\sqrt{s} = 13$ TeV

Experiment: ALICE (LHC)

Inspire ID: 2803487

Status: VALIDATED

Authors: - Alexander Puck Neuwirth

References: - Eur. Phys. J. C 85 (2025) 98 - arXiv: 2407.01165 - Expt page: ALICE-11198

Beams: p+ p+

Beam energies: (6500.0, 6500.0)GeV

Run details: - Inclusive photon+X events at $\sqrt{s} = 13$~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 $\sqrt{s}= 13$ TeV. The measurement is performed with the electromagnetic calorimeter EMCal and the central tracking detectors, covering a range of $|\eta|<0.67$ in pseudorapidity and a transverse momentum range of $ 7 < p_T^\gamma < 200$ GeV.

Source code:ALICE_2024_I2803487.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/FastJets.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/PromptFinalState.hh"

include "Rivet/Projections/VetoedFinalState.hh"

namespace Rivet {

/// @brief Measurement of the inclusive isolated photon production cross section in pp collisions class ALICE_2024_I2803487 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ALICE_2024_I2803487);


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

/// Book histograms and initialise projections before the run
void init() {
  // This option exists because JetPhox can not do isolation on charged particles only
  _iso_charged = getOption("ISO_CHARGED", true);
  _iso_et_cut = _iso_charged ? _iso_et_cut_charged : _iso_et_cut_all;
  const FinalState fs;

  // calorimeter particles
  VisibleFinalState visFS(fs);
  VetoedFinalState calo_fs(visFS);
  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"], 5, 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
  FourMomentum mom_in_EtCone;
  const Particles calo_fs = apply<VetoedFinalState>(event, "calo").particles();
  for (const Particle& p : calo_fs) {
    // Possibly skip non-charged particles
    if (_iso_charged && !p.isCharged()) continue;
    // Check if it's in the cone of .4
    if (deltaR(p, photon) >= _iso_dr) continue;
    // Increment sum
    mom_in_EtCone += p.momentum();
  }

  // photon itself is not charged, so only remove it if we consider all particles
  if (!_iso_charged) {
    // Remove the photon energy from the isolation
    mom_in_EtCone -= photon;
  }

  if (mom_in_EtCone.pT() > _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
bool _iso_charged = true;
const double _abseta = 0.67;
const double _iso_dr = 0.4;
double _iso_et_cut;
const double _iso_et_cut_charged = 1.5 * GeV;
const double _iso_et_cut_all = 2.0 * GeV;

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

};

RIVET_DECLARE_PLUGIN(ALICE_2024_I2803487);

} ```