Rivet analyses

title: ALICE_2025_I2895564

Measurement of isolated prompt photon production in pp and pPb collisions at the LHC

Experiment: ALICE (LHC)

Inspire ID: 2895564

Status: VALIDATED

Authors: - Alexander Puck Neuwirth

References: - Eur.Phys.J.C 85 (2025) 12, 1407 - arXiv: 2502.18054 - Expt page: ALICE-11890

Beams: p+ p+, 1000822080 p+, 1000822080 p+

Beam energies: (4000.0, 4000.0); (328000.0, 4000.0); (533000.0, 6500.0)GeV

Run details: - Inclusive photon+X events at $\sqrt{s} = 5.02$~TeV, $8$~TeV and, $8.16$~TeV at low transverse momentum.

The measurement is performed in pPb collisions at centre-of-mass energies per nucleon pair of $\sqrt{s_{\text{NN}}}=5.02$~TeV and $8.16$~TeV, as well as in pp collisions at $\sqrt{s}=5.02$~TeV and $8$~TeV. The cross section is obtained at midrapidity $(|y|<0.7)$ using a charged-track based isolation momentum $p_{\rm T}^{\text{iso,~ch}}<1.5$~GeV/c in a cone with radius $R=0.4$.

Source code:ALICE_2025_I2895564.cc

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

include "Rivet/Analysis.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 and pPb collisions class ALICE_2025_I2895564 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ALICE_2025_I2895564);


/// @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;

  if (!_iso_charged)
    MSG_WARNING("LIMITED VALIDITY - measured data was obtained for ISO_CHARGED=true case only.");

  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["pp_8_p_t"], 1, 1, 1);
  book(_h["pPb_8_16_p_t"], 2, 1, 1);
  book(_h["pPb_5_02_p_t"], 3, 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) {
    // 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().pT();
  }

  // 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.pT();
  }

  if (mom_in_EtCone > _iso_et_cut) vetoEvent;

  if (beamsMatch(PID::PROTON, PID::PROTON, 4000 * GeV, 4000 * GeV)) {
    _h["pp_8_p_t"]->fill(photon.pT() / GeV);
  }
  else if (beamsMatch(PID::LEAD, PID::PROTON, 533000 * GeV, 6500 * GeV)) {
    _h["pPb_8_16_p_t"]->fill(photon.pT() / GeV);
  }
  else if (beamsMatch(PID::LEAD, PID::PROTON, 328000 * GeV, 4000 * GeV)) {
    _h["pPb_5_02_p_t"]->fill(photon.pT() / GeV);
  }
}

/// Normalise histograms etc., after the run
void finalize() {
  scale(_h, crossSection() / nanobarn / sumOfWeights() / (2 * _abseta)); // also divide by |eta| range
}

/// @}

// Analysis parameters

bool _iso_charged = true;
const double _abseta = 0.7;
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_2025_I2895564);

} ```