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ATLAS_2016_I1468168

Dileptonic $e \mu$ $t\bar{t}$ early cross-section measurement at 13 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1468168
Status: VALIDATED
Authors:
  • Tom Neep
  • Barbara Alvarez Gonzalez
  • Yang Qin
References: Beams: p+ p+
Beam energies: (6500.0, 6500.0) GeV
Run details:
  • $t\bar{t}$ production

This paper describes a measurement of the inclusive top quark pair production cross-section ($\sigma_{t\bar{t}}$) with a data sample of 3.2 fb${}^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV, collected in 2015 by the ATLAS detector at the LHC. This measurement uses events with an opposite-charge electron-muon pair in the final state. Jets containing $b$-quarks are tagged using an algorithm based on track impact parameters and reconstructed secondary vertices. The numbers of events with exactly one and exactly two $b$-tagged jets are counted and used to determine simultaneously $\sigma_{t\bar{t}}$ and the efficiency to reconstruct and $b$-tag a jet from a top quark decay, thereby minimising the associated systematic uncertainties. The cross-section is measured to be: $\sigma_{t\bar{t}} = 818 \pm 8$ (stat) $\pm 27$ (syst) $\pm 19$ (lumi) $\pm 12$ (beam) pb, where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, the integrated luminosity and the LHC beam energy, giving a total relative uncertainty of 4.4\%. The result is consistent with theoretical QCD calculations at next-to-next-to-leading order. A fiducial measurement corresponding to the experimental acceptance of the leptons is also presented.

Source code: ATLAS_2016_I1468168.cc
 1#include "Rivet/Analysis.hh"
 2#include "Rivet/Projections/FinalState.hh"
 3#include "Rivet/Projections/IdentifiedFinalState.hh"
 4#include "Rivet/Projections/PromptFinalState.hh"
 5#include "Rivet/Projections/DressedLeptons.hh"
 6
 7namespace Rivet {
 8
 9
10  /// @brief dileptonic e-mu ttbar
11  class ATLAS_2016_I1468168 : public Analysis {
12  public:
13
14    RIVET_DEFAULT_ANALYSIS_CTOR(ATLAS_2016_I1468168);
15
16    void init() {
17      // Eta ranges
18      Cut eta_full = Cuts::abseta < 5.0 && Cuts::pT >= 1.0*MeV;
19      // Lepton cuts
20      Cut lep_cuts = Cuts::abseta < 2.5 && Cuts::pT >= 25.0*GeV;
21
22      // All final state particles
23      FinalState fs(eta_full);
24
25      // Get photons to dress leptons
26      IdentifiedFinalState photons(fs);
27      photons.acceptIdPair(PID::PHOTON);
28
29      // Projection to find the electrons
30      IdentifiedFinalState el_id(fs);
31      el_id.acceptIdPair(PID::ELECTRON);
32      PromptFinalState electrons(el_id);
33      electrons.acceptTauDecays(true);
34      DressedLeptons dressedelectrons(photons, electrons, 0.1, lep_cuts, true);
35      declare(dressedelectrons, "DressedElectrons");
36
37      // Projection to find the muons
38      IdentifiedFinalState mu_id(fs);
39      mu_id.acceptIdPair(PID::MUON);
40      PromptFinalState muons(mu_id);
41      muons.acceptTauDecays(true);
42      DressedLeptons dressedmuons(photons, muons, 0.1, lep_cuts, true);
43      declare(dressedmuons, "DressedMuons");
44
45      book(_s , 2, 1 ,1);
46      book(_c , "_counter");
47    }
48
49
50    void analyze(const Event& event) {
51
52      // Get the selected objects, using the projections.
53      const size_t num_es = apply<DressedLeptons>(event, "DressedElectrons").dressedLeptons().size();
54      const size_t num_mus = apply<DressedLeptons>(event, "DressedMuons").dressedLeptons().size();
55
56      // Evaluate basic event selection
57      const bool pass_emu = num_es == 1 && num_mus == 1;
58      if (!pass_emu) vetoEvent;
59
60      // Fill histogram to measure the event acceptance
61      _c->fill();
62    }
63
64
65    void finalize() {
66      // Normalize to cross-section
67      scale(_c, crossSection() / sumOfWeights());
68
69      double err = _c->err();
70      _s->addPoint(13*TeV, _c->val(), make_pair(0.5, 0.5), make_pair(err,err));
71
72    }
73
74
75   private:
76
77    CounterPtr _c;
78    Scatter2DPtr _s;
79
80  };
81
82
83  // The hook for the plugin system
84  RIVET_DECLARE_PLUGIN(ATLAS_2016_I1468168);
85
86}