Rivet analyses

Centrality dependence of the charged-particle multiplicity density at mid-rapidity in Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 2.76$ TeV

Experiment: ALICE (LHC)

Inspire ID: 880049

Status: VALIDATED

Authors: - Przemyslaw Karczmarczyk - Jan Fiete Grosse-Oetringhaus - Jochen Klein

References: - arXiv: 1012.1657

Beams: 1000822080 1000822080

Beam energies: (287040.0, 287040.0)GeV

Run details: none listed

The centrality dependence of the charged-particle multiplicity density at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{NN}} = 2.76$ TeV is presented. The charged-particle density normalized per participating nucleon pair increases by about a factor 2 from peripheral (70-80%) to central (0-5%) collisions. The centrality dependence is found to be similar to that observed at lower collision energies. The data are compared with models based on different mechanisms for particle production in nuclear collisions.

Source code:ALICE_2010_I880049.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/ChargedFinalState.hh"
#include "Rivet/Tools/Cuts.hh"
#include "Rivet/Projections/SingleValueProjection.hh"
#include "Rivet/Analyses/AliceCommon.hh"
#include "Rivet/Projections/HepMCHeavyIon.hh"

namespace Rivet {

  /// @brief ALICE PbPb at 2.76 TeV multiplicity at mid-rapidity
  class ALICE_2010_I880049 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(ALICE_2010_I880049);

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

    /// Book histograms and initialise projections before the run
    void init() {

      // Declare centrality projection
      declareCentrality(ALICE::V0MMultiplicity(),
        "ALICE_2015_CENT_PBPB", "V0M", "V0M");

      // Trigger projections
      declare(ChargedFinalState((Cuts::eta > 2.8 && Cuts::eta < 5.1) &&
        Cuts::pT > 0.1*GeV), "VZERO1");
      declare(ChargedFinalState((Cuts::eta > -3.7 && Cuts::eta < -1.7) &&
        Cuts::pT > 0.1*GeV), "VZERO2");
      declare(ChargedFinalState(Cuts::abseta < 1. && Cuts::pT > 0.15*GeV),
        "SPD");

      // Charged, primary particles with |eta| < 0.5 and pT > 50 MeV
      declare(ALICE::PrimaryParticles(Cuts::abseta < 0.5 &&
        Cuts::pT > 50*MeV && Cuts::abscharge > 0), "APRIM");

      // Access the HepMC heavy ion info
      declare(HepMCHeavyIon(), "HepMC");

      // Histograms and variables initialization
      book(_histNchVsCentr, 1, 1, 1);
      book(_histNpartVsCentr, 1, 1, 2);

    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {

      // Charged, primary particles with at least pT = 50 MeV
      // in eta range of |eta| < 0.5
      Particles chargedParticles =
        apply<ALICE::PrimaryParticles>(event,"APRIM").particles();

      // Trigger projections
      const ChargedFinalState& vz1 =
        apply<ChargedFinalState>(event,"VZERO1");
      const ChargedFinalState& vz2 =
        apply<ChargedFinalState>(event,"VZERO2");
      const ChargedFinalState& spd =
        apply<ChargedFinalState>(event,"SPD");
      int fwdTrig = (vz1.particles().size() > 0 ? 1 : 0);
      int bwdTrig = (vz2.particles().size() > 0 ? 1 : 0);
      int cTrig = (spd.particles().size() > 0 ? 1 : 0);

      if (fwdTrig + bwdTrig + cTrig < 2) vetoEvent;

      const CentralityProjection& centrProj =
        apply<CentralityProjection>(event, "V0M");
      double centr = centrProj();
      if (centr > 80) vetoEvent;
      // Calculate number of charged particles and fill histogram
      double nch = chargedParticles.size();
      _histNchVsCentr->fill(centr, nch);

      // Attempt to extract Npart form GenEvent.
      if (event.genEvent()->heavy_ion()) {
        const HepMCHeavyIon & hi = apply<HepMCHeavyIon>(event, "HepMC");
        _histNpartVsCentr->fill(centr, hi.Npart_proj() + hi.Npart_targ());
      }
    }


    /// Normalise histograms etc., after the run
    //void finalize() {   }

    /// @}


  private:

    /// @name Histograms
    /// @{
    Profile1DPtr _histNchVsCentr;
    Profile1DPtr _histNpartVsCentr;
    /// @}

  };


  RIVET_DECLARE_PLUGIN(ALICE_2010_I880049);

}