rivet is hosted by Hepforge, IPPP Durham

Rivet analyses reference

ATLAS_2018_I1711114

g \to bb at 13 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1711114
Status: VALIDATED
Authors:
  • Ben Nachman
  • Zihao Jiang
  • Christian Gutschow
References: Beams: p+ p+
Beam energies: (6500.0, 6500.0) GeV
Run details:
  • Boosted g \to b bbar at 13 TeV

The fragmentation of high-energy gluons at small opening angles is largely unconstrained by present measurements. Gluon splitting to $b$-quark pairs is a unique probe into the properties of gluon fragmentation because identified $b$-tagged jets provide a proxy for the quark daughters of the initial gluon. In this study, key differential distributions related to the $g\to b\bar{b}$ process are measured using 33fb$^{-1}$ of $\sqrt{s}=13$ TeV $pp$ collision data recorded by the ATLAS experiment at the LHC in 2016. Jets constructed from charged-particle tracks, clustered with the anti-$k_\text{t}$ jet algorithm with radius parameter $R=0.2$, are used to probe angular scales below the $R=0.4$ jet radius. The observables are unfolded to particle level in order to facilitate direct comparisons with predictions from present and future simulations. Multiple significant differences are observed between the data and parton shower Monte Carlo predictions, providing input to improve these predictions of the main source of background events in analyses involving boosted Higgs bosons decaying into $b$-quarks.

Source code: ATLAS_2018_I1711114.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
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/ChargedFinalState.hh"
#include "Rivet/Projections/FastJets.hh"

namespace Rivet {


  /// g -> bb at 13 TeV
  class ATLAS_2018_I1711114: public Analysis {
  public:

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(ATLAS_2018_I1711114);


    /// Book cuts and projections
    void init() {
      // All final state particles
      FinalState fs(Cuts::abseta<5.0);

      ChargedFinalState cfs(Cuts::pT > 0.5*GeV && Cuts::abseta < 2.5);
      FastJets smallR_jets(cfs, FastJets::ANTIKT, 0.2, JetAlg::NO_MUONS, JetAlg::NO_INVISIBLES);
      declare(smallR_jets, "track_jets");

      FastJets largeR_jets(fs, FastJets::ANTIKT, 1.0, JetAlg::NO_MUONS, JetAlg::NO_INVISIBLES);
      declare(largeR_jets, "largeR_jets");

      _h_R   = bookHisto1D(1,1,1);
      _h_phi = bookHisto1D(2,1,1);
      _h_z   = bookHisto1D(3,1,1);
      _h_rho = bookHisto1D(4,1,1);
    }


    void analyze(const Event& event) {

      const double weight = event.weight();

      const PseudoJets& myJets = apply<FastJets>(event, "largeR_jets").pseudoJetsByPt(450*GeV);
      if (myJets.empty()) vetoEvent;

      fastjet::Filter trimmer(fastjet::JetDefinition(fastjet::kt_algorithm, 0.2), fastjet::SelectorPtFractionMin(0.05));
      Jets myTrimmedJets;
      for (const Jet& jet : myJets)  myTrimmedJets += Jet(trimmer(jet));
      std::sort(myTrimmedJets.begin(), myTrimmedJets.end(), cmpMomByPt);
      if (myTrimmedJets[0].pT() < 450*GeV) vetoEvent;

      const Jets& myJets_charged = apply<FastJets>(event, "track_jets").jetsByPt(Cuts::pT > 10*GeV);
      PseudoJets pjs;
      for (auto tj : myTrimmedJets[0].pseudojet().constituents()) {
        fastjet::PseudoJet pj = tj;
        pj.set_user_index(-1); // dummmy
        pjs.push_back(pj);
      }
      for (size_t i = 0; i < myJets_charged.size(); ++i) {
        fastjet::PseudoJet pj = myJets_charged[i];
        pj *= 1e-20; // ghostify momentum
        pj.set_user_index(i);
        pjs.push_back(pj);
      }
      fastjet::ClusterSequence tagged_seq(pjs, fastjet::JetDefinition(fastjet::antikt_algorithm, 1.0));
      PseudoJets GAfatjet = fastjet::sorted_by_pt(tagged_seq.inclusive_jets(50.0));
      Jets associated_trackjets;
      for (auto pj : GAfatjet[0].constituents()) {
        if (pj.user_index() >= 0)  associated_trackjets += myJets_charged[pj.user_index()];
      }
      if (associated_trackjets.size() < 2) vetoEvent;
      std::sort(associated_trackjets.begin(), associated_trackjets.end(), cmpMomByPt);

      size_t nbtags = 0;
      for (unsigned int ij = 0; ij < 2; ++ij) {
        if (associated_trackjets[ij].bTagged(Cuts::pT > 5*GeV))  ++nbtags;
      }
      if (nbtags != 2) vetoEvent;

      // Now, time to compute the observables!
      const Vector3 b1v = associated_trackjets[0].p3();
      const Vector3 b2v = associated_trackjets[1].p3();
      const Vector3 plane1 = b1v.cross(b2v);
      const Vector3 gv = b1v + b2v;
      const Vector3 beam = Vector3(0,0,1);
      const Vector3 plane2 = beam.cross(gv);

      const double fTz = associated_trackjets[1].pT() / (associated_trackjets[0].pT()+associated_trackjets[1].pT());
      const double fTR = deltaR(associated_trackjets[0], associated_trackjets[1], RapScheme::YRAP); //N.B. this uses y and not eta
      const FourMomentum dijet = associated_trackjets[0].mom() + associated_trackjets[1].mom();
      const double fTrho = log(dijet.mass() / dijet.pT());
      const double fTphi = (plane1).angle(plane2)/M_PI;

      _h_R->fill(  fTR,   weight);
      _h_phi->fill(fTphi, weight);
      _h_z->fill(  fTz,   weight);
      _h_rho->fill(fTrho, weight);
    }


    /// Scale histos
    void finalize() {
      _h_R->normalize(  1.0, false);
      _h_phi->normalize(1.0, false);
      _h_z->normalize(  1.0, false);
      _h_rho->normalize(1.0, false);
    }


  private:

    /// Histograms
    Histo1DPtr _h_z, _h_R, _h_rho, _h_phi;

  };


  DECLARE_RIVET_PLUGIN(ATLAS_2018_I1711114);

}