rivet is hosted by Hepforge, IPPP Durham

Rivet analyses reference

ALICE_2012_I1127497

Centrality dependence of charged particle production at large transverse momentum in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}} = 2.76$ TeV
Experiment: ALICE (LHC)
Inspire ID: 1127497
Status: VALIDATED
Authors:
  • Przemyslaw Karczmarczyk
  • Jan Fiete Grosse-Oetringhaus
  • Jochen Klein
References: Beams: p+ p+, 1000822080 1000822080
Beam energies: (1380.0, 1380.0); (287040.0, 287040.0) GeV
    No run details listed

The inclusive transverse momentum ($p_T$) distributions of primary charged particles are measured in the pseudo-rapidity range $|\eta| < 0.8$ as a function of event centrality in Pb--Pb collisions at $\sqrt{s_{nn}} = 2.76$ TeV with ALICE at the LHC. The data are presented in the $p_T$ range $0.15<p_T<50$ GeV/c for nine centrality intervals from 70-80% to 0-5%. The Pb-Pb spectra are presented in terms of the nuclear modification factor RAA using a pp reference spectrum measured at the same collision energy. We observe that the suppression of high-pT particles strongly depends on event centrality. In central collisions (0-5%) the yield is most suppressed with $R_{AA} \approx 0.13$ at $p_T= 6-7$ GeV/c. Above $p_T = 7$ GeV/c, there is a significant rise in the nuclear modification factor, which reaches $R_{AA} \approx 0.4$ for $p_T > 30$ GeV/c. In peripheral collisions (70-80%), the suppression is weaker with $R_{AA} \approx 0.7$ almost independently of $p_T$. The measured nuclear modification factors are compared to other measurements and model calculations.

Source code: ALICE_2012_I1127497.cc
  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/ChargedFinalState.hh"
#include "Rivet/Tools/Cuts.hh"
#include "Rivet/Projections/SingleValueProjection.hh"
#include "Rivet/Tools/AliceCommon.hh"
#include "Rivet/Projections/AliceCommon.hh"
#include <fstream>

#define _USE_MATH_DEFINES
#include <math.h>

namespace Rivet {

  /// @brief ALICE PbPb at 2.76 TeV R_AA analysis.
  class ALICE_2012_I1127497 : public Analysis {

  public:

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(ALICE_2012_I1127497);

    /// @name Analysis methods
    //@{

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

      // Declare centrality projection
      declareCentrality(ALICE::V0MMultiplicity(),
        "ALICE_2015_PBPBCentrality", "V0M", "V0M");

      // Charged, primary particles with |eta| < 0.5 and pT > 150 MeV
      declare(ALICE::PrimaryParticles(Cuts::abseta < 0.5 &&
        Cuts::pT > 150*MeV && Cuts::abscharge > 0), "APRIM");

      // Loop over all histograms
      for (size_t ihist = 0; ihist < NHISTOS; ++ihist) {

        // Initialize PbPb objects
        _histNch[PBPB][ihist] = bookHisto1D(ihist+1, 1, 1);

        std::string nameCounterPbPb = "counter.pbpb." + std::to_string(ihist);
        _counterSOW[PBPB][ihist] = bookCounter(nameCounterPbPb,
          "Sum of weights counter for PbPb");

        std::string nameCounterNcoll = "counter.ncoll." + std::to_string(ihist);
        _counterNcoll[ihist] = bookCounter(nameCounterNcoll,
          "Ncoll counter for PbPb");

        // Initialize pp objects. In principle, only one pp histogram would be
        // needed since centrality does not make any difference here. However,
        // in some cases in this analysis the binning differ from each other,
        // so this is easy-to-implement way to account for that.
        std::string namePP = _histNch[PBPB][ihist]->name() + "-pp";
        // The binning is taken from the reference data
        _histNch[PP][ihist] = bookHisto1D(namePP, refData(ihist+1, 1, 1));

        std::string nameCounterpp = "counter.pp." + std::to_string(ihist);
        _counterSOW[PP][ihist] = bookCounter(nameCounterpp,
          "Sum of weights counter for pp");

      }

      // Centrality regions keeping boundaries for a certain region.
      // Note, that some regions overlap with other regions.
      _centrRegions.clear();
      _centrRegions = {{0., 5.},   {5., 10.},  {10., 20.},
                       {20., 30.}, {30., 40.}, {40., 50.},
                       {50., 60.}, {60., 70.}, {70., 80.},
                       {0., 10.},  {0., 20.},  {20., 40.},
                       {40., 60.}, {40., 80.}, {60., 80.}};

    }


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

      const double weight = event.weight();

      // Charged, primary particles with at least pT = 150 MeV
      // in eta range of |eta| < 0.5
      Particles chargedParticles =
        applyProjection<ALICE::PrimaryParticles>(event,"APRIM").particlesByPt();

      // Check type of event. This may not be a perfect way to check for the
      // type of event as there might be some weird conditions hidden inside.
      // For example some HepMC versions check if number of hard collisions
      // is equal to 0 and assign 'false' in that case, which is usually wrong.
      // This might be changed in the future
      const HepMC::HeavyIon* hi = event.genEvent()->heavy_ion();
      if (hi && hi->is_valid()) {

        // Prepare centrality projection and value
        const CentralityProjection& centrProj =
          apply<CentralityProjection>(event, "V0M");
        double centr = centrProj();
        // Veto event for too large centralities since those are not used
        // in the analysis at all
        if ((centr < 0.) || (centr > 80.))
          vetoEvent;

        // Fill PbPb histograms and add weights based on centrality value
        for (size_t ihist = 0; ihist < NHISTOS; ++ihist) {
          if (inRange(centr, _centrRegions[ihist].first, _centrRegions[ihist].second)) {
            _counterSOW[PBPB][ihist]->fill(weight);
            _counterNcoll[ihist]->fill(event.genEvent()->heavy_ion()->Ncoll(), weight);
            foreach (const Particle& p, chargedParticles) {
              float pT = p.pT()/GeV;
              if (pT < 50.) {
                double pTAtBinCenter = _histNch[PBPB][ihist]->binAt(pT).xMid();
                _histNch[PBPB][ihist]->fill(pT, weight/pTAtBinCenter);
              }
            }
          }
        }

      }
      else {

        // Fill all pp histograms and add weights
        for (size_t ihist = 0; ihist < NHISTOS; ++ihist) {
          _counterSOW[PP][ihist]->fill(weight);
          foreach (const Particle& p, chargedParticles) {
            float pT = p.pT()/GeV;
            if (pT < 50.) {
              double pTAtBinCenter = _histNch[PP][ihist]->binAt(pT).xMid();
              _histNch[PP][ihist]->fill(pT, weight/pTAtBinCenter);
            }
          }
        }

      }

    }


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

      // Right scaling of the histograms with their individual weights.
      for (size_t itype = 0; itype < EVENT_TYPES; ++itype ) {
        for (size_t ihist = 0; ihist < NHISTOS; ++ihist) {
          if (_counterSOW[itype][ihist]->sumW() > 0.) {
            scale(_histNch[itype][ihist],
              (1. / _counterSOW[itype][ihist]->sumW() / 2. / M_PI));
          }
        }
      }

      // Postprocessing of the histograms
      for (size_t ihist = 0; ihist < NHISTOS; ++ihist) {
        // If there are entires in histograms for both beam types
        if (_histNch[PP][ihist]->numEntries() > 0 && _histNch[PBPB][ihist]->numEntries() > 0) {
          // Initialize and fill R_AA histograms
          _histRAA[ihist] = bookScatter2D(ihist+16, 1, 1);
          divide(_histNch[PBPB][ihist], _histNch[PP][ihist], _histRAA[ihist]);
          // Scale by Ncoll. Unfortunately some generators does not provide
          // Ncoll value (eg. JEWEL), so the following scaling will be done
          // only if there are entries in the counters
          double ncoll = _counterNcoll[ihist]->sumW();
          double sow = _counterSOW[PBPB][ihist]->sumW();
          if (ncoll > 1e-6 && sow > 1e-6)
            _histRAA[ihist]->scaleY(1. / (ncoll / sow));

        }
      }

    }

    //@}

  private:

    static const int NHISTOS = 15;
    static const int EVENT_TYPES = 2;
    static const int PP = 0;
    static const int PBPB = 1;

    /// @name Histograms
    //@{
    Histo1DPtr _histNch[EVENT_TYPES][NHISTOS];
    CounterPtr _counterSOW[EVENT_TYPES][NHISTOS];
    CounterPtr _counterNcoll[NHISTOS];
    Scatter2DPtr _histRAA[NHISTOS];
    //@}

    std::vector<std::pair<double, double>> _centrRegions;

  };

  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(ALICE_2012_I1127497);


}