Rivet analyses
title: ALICE_2012_I1127497
Centrality dependence of charged particle production at large transverse momentum in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 2.76$ TeV
Experiment: ALICE (LHC)
Inspire ID: 1127497
Status: VALIDATED
Authors: - Przemyslaw Karczmarczyk - Jan Fiete Grosse-Oetringhaus - Jochen Klein
References: - arXiv: 1208.2711
Beams: p+ p+, 1000822080 1000822080
Beam energies: (1380.0, 1380.0); (287040.0, 287040.0)GeV
Run details: none 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
```c++ // -- C++ --
include "Rivet/Analysis.hh"
include "Rivet/Analyses/AliceCommon.hh"
include "Rivet/Projections/Beam.hh"
include "Rivet/Projections/ChargedFinalState.hh"
include "Rivet/Projections/HepMCHeavyIon.hh"
include "Rivet/Projections/SingleValueProjection.hh"
include "Rivet/Tools/Cuts.hh"
namespace Rivet {
/// @brief ALICE PbPb at 2.76 TeV R_AA analysis. class ALICE_2012_I1127497 : public Analysis {
public:
/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ALICE_2012_I1127497);
/// @name Analysis methods
/// @{
/// Book histograms and initialise projections before the run
void init() {
// Access the HepMC heavy ion info
declare(HepMCHeavyIon(), "HepMC");
// Declare centrality projection
declareCentrality(ALICE::V0MMultiplicity(), "ALICE_2015_CENT_PBPB", "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
book(_histNch[PBPB][ihist], ihist + 1, 1, 1);
std::string nameCounterPbPb = "counter.pbpb." + std::to_string(ihist);
book(_counterSOW[PBPB][ihist], nameCounterPbPb); // Sum of weights counter for PbPb
std::string nameCounterNcoll = "counter.ncoll." + std::to_string(ihist);
book(_counterNcoll[ihist], 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 = mkAxisCode(ihist + 1, 1, 1) + "-pp";
// The binning is taken from the reference data
book(_histNch[PP][ihist], namePP, refData(ihist + 1, 1, 1));
std::string nameCounterpp = "counter.pp." + std::to_string(ihist);
book(_counterSOW[PP][ihist], nameCounterpp); // Sum of weights counter for pp
// Book ratios, to be used in finalize
book(_histRAA[ihist], ihist + 16, 1, 1);
}
// 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.}};
// Find out the beam type, also specified from option.
string beamOpt = getOption<string>("beam", "NONE");
if (beamOpt != "NONE") {
MSG_WARNING("You are using a specified beam type, instead of using what"
"is provided by the generator. "
"Only do this if you are completely sure what you are doing.");
if (beamOpt == "PP")
isHI = false;
else if (beamOpt == "HI")
isHI = true;
else {
MSG_ERROR("Beam error (option)!");
return;
}
}
else {
const ParticlePair& beam = beams();
if (beam.first.pid() == PID::PROTON && beam.second.pid() == PID::PROTON) {
isHI = false;
}
else if (beam.first.pid() == PID::LEAD && beam.second.pid() == PID::LEAD) {
isHI = true;
}
else {
MSG_ERROR("Beam error (found)!");
return;
}
}
}
/// Perform the per-event analysis
void analyze(const Event& event) {
// Charged, primary particles with at least pT = 150 MeV
// in eta range of |eta| < 0.5
Particles chargedParticles = apply<ALICE::PrimaryParticles>(event, "APRIM").particlesByPt();
// Check type of event.
if (isHI) {
const HepMCHeavyIon& hi = apply<HepMCHeavyIon>(event, "HepMC");
if (!hi.ok()) {
MSG_WARNING("HEPMC Heavy ion container needed for this analysis, but not "
"found for this event. Skipping.");
vetoEvent;
}
// 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();
_counterNcoll[ihist]->fill(hi.Ncoll());
for (const Particle& p : chargedParticles) {
double pT = p.pT() / GeV;
if (pT < 50.) {
const double pTAtBinCenter = _histNch[PBPB][ihist]->binAt(pT).xMid();
_histNch[PBPB][ihist]->fill(pT, 1 / pTAtBinCenter);
}
}
}
}
}
else {
// Fill all pp histograms and add weights
for (size_t ihist = 0; ihist < NHISTOS; ++ihist) {
_counterSOW[PP][ihist]->fill();
for (const Particle& p : chargedParticles) {
double pT = p.pT() / GeV;
if (pT < 50.) {
const double pTAtBinCenter = _histNch[PP][ihist]->binAt(pT).xMid();
_histNch[PP][ihist]->fill(pT, 1 / 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
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]->scale(1. / (ncoll / sow));
}
}
}
/// @}
private:
bool isHI;
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];
Estimate1DPtr _histRAA[NHISTOS];
/// @}
std::vector<std::pair<double, double>> _centrRegions;
};
RIVET_DECLARE_PLUGIN(ALICE_2012_I1127497);
} ```