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ATLAS_2011_I929691

Jet fragmentation at 7 TeV
Experiment: ATLAS (LHC)
Inspire ID: 929691
Status: VALIDATED
Authors:
  • Stefan von Buddenbrock
References: Beams: p+ p+
Beam energies: (3500.0, 3500.0) GeV
Run details:
  • Inclusive jet production

The jet fragmentation function and transverse profile for jets with 25 GeV $< p_\text{T jet} < 500$ GeV and $|\eta_\text{jet}| < 1.2$ produced in proton-proton collisions with a center-of-mass energy of 7 TeV are presented. The measurement is performed using data with an integrated luminosity of 36 pb${}^{-1}$. Jets are reconstructed and their momentum measured using calorimetric information. The momenta of the charged particle constituents are measured using the tracking system. The distributions corrected for detector effects are compared with various Monte Carlo event generators and generator tunes. Several of these choices show good agreement with the measured fragmentation function. None of these choices reproduce both the transverse profile and fragmentation function over the full kinematic range of the measurement.

Source code: ATLAS_2011_I929691.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/ChargedFinalState.hh"

namespace Rivet {


  /// Jet fragmentation at 7 TeV
  class ATLAS_2011_I929691 : public Analysis {
  public:

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(ATLAS_2011_I929691);


    /// Initialisation
    void init() {
      const FinalState fs(Cuts::abseta < 2.0);

      FastJets antikt_06_jets(fs, FastJets::ANTIKT, 0.6, JetAlg::NO_MUONS, JetAlg::NO_INVISIBLES);
      declare(antikt_06_jets, "jets");

      ChargedFinalState tracks(Cuts::pT > 0.5*GeV && Cuts::abseta < 2.0);
      declare(tracks, "tracks");

      // Set up the histograms (each element is a binning in jet pT)
      for (size_t i = 0; i < 10; i++) {
        _p_F_z[i]     = bookProfile1D(i+1, 1, 1);
        _p_rho_r[i]   = bookProfile1D(i+11, 1, 1);
        _p_f_pTrel[i] = bookProfile1D(i+21, 1, 1);
      }

    }


    // Per-event analysis
    void analyze(const Event& event) {

      const Jets alljets = apply<FastJets>(event, "jets").jetsByPt(Cuts::absrap < 1.2);
      const Particles& tracks = apply<ChargedFinalState>(event, "tracks").particlesByPt();

      for (size_t i = 0; i < 10; ++i) {

        const Jets jets = filter_select(alljets, Cuts::pT > bedges[i] && Cuts::pT < bedges[i+1]);
        const int n_jets = jets.size();
        if (n_jets == 0) continue;

        // First... count the tracks
        Histo1D h_ntracks_z(*_p_F_z[i]), h_ntracks_r(*_p_rho_r[i]), h_ntracks_pTrel(*_p_f_pTrel[i]);

        for (const Jet& j : jets) {
          for (const Particle& p : tracks) {
            const double dr = deltaR(j, p, RAPIDITY);
            if (dr > 0.6) continue; // The paper uses pseudorapidity, but this is a requirement for filling the histogram
            h_ntracks_z.fill(z(j, p), 1.0/n_jets);
            h_ntracks_r.fill(dr, 1.0/n_jets);
            h_ntracks_pTrel.fill(pTrel(j, p), 1.0/n_jets);
          }
        }

        // Then... calculate the observable and fill the profiles
        const double weight = event.weight();
        for (const HistoBin1D& b : h_ntracks_z.bins())
          _p_F_z[i]->fill(b.xMid(), b.height(), weight);
        for (const HistoBin1D& b : h_ntracks_r.bins())
          _p_rho_r[i]->fill(b.xMid(), b.area()/annulus_area(b.xMin(), b.xMax()), weight);
        for (const HistoBin1D& b : h_ntracks_pTrel.bins())
          _p_f_pTrel[i]->fill(b.xMid(), b.height(), weight);

      }

    }


    double z (const Jet& jet, const Particle& ch) {
      return dot(jet.p3(), ch.p3()) / jet.p3().mod2();
    }

    double pTrel (const Jet& jet, const Particle& ch) {
      return (ch.p3().cross(jet.p3())).mod()/(jet.p3().mod());
    }

    // To calculate the area of the annulus in an r bin
    double annulus_area(double r1, double r2) {
      return M_PI*(sqr(r2) - sqr(r1));
    }


  private:

    Profile1DPtr _p_F_z[10], _p_rho_r[10], _p_f_pTrel[10];
    const vector<double> bedges = { 25., 40., 60., 80., 110., 160., 210., 260., 310., 400., 500. };

  };


  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(ATLAS_2011_I929691);


}