Rivet analyses

title: 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: - Expt page: ATLAS-TOPQ-2015-09 - Submitted to PLB - arXiv: 1606.02699

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

```c++

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/IdentifiedFinalState.hh"

include "Rivet/Projections/LeptonFinder.hh"

include "Rivet/Projections/PromptFinalState.hh"

namespace Rivet {

/// @brief dileptonic e-mu ttbar class ATLAS_2016_I1468168 : public Analysis { public:

RIVET_DEFAULT_ANALYSIS_CTOR(ATLAS_2016_I1468168);

void init() {
  // Eta ranges
  Cut eta_full = Cuts::abseta < 5.0 && Cuts::pT >= 1.0 * MeV;
  // Lepton cuts
  Cut lep_cuts = Cuts::abseta < 2.5 && Cuts::pT >= 25.0 * GeV;

  // All final state particles
  FinalState fs(eta_full);

  // Get photons to dress leptons
  IdentifiedFinalState photons(fs);
  photons.acceptIdPair(PID::PHOTON);

  // Projection to find the electrons
  IdentifiedFinalState el_id(fs);
  el_id.acceptIdPair(PID::ELECTRON);
  PromptFinalState electrons(el_id);
  electrons.acceptTauDecays(true);
  LeptonFinder dressedelectrons(electrons, photons, 0.1, lep_cuts);
  declare(dressedelectrons, "DressedElectrons");

  // Projection to find the muons
  IdentifiedFinalState mu_id(fs);
  mu_id.acceptIdPair(PID::MUON);
  PromptFinalState muons(mu_id);
  muons.acceptTauDecays(true);
  LeptonFinder dressedmuons(muons, photons, 0.1, lep_cuts);
  declare(dressedmuons, "DressedMuons");

  book(_h, 2, 1, 1);
}


void analyze(const Event& event) {

  // Get the selected objects, using the projections.
  const size_t num_es = apply<LeptonFinder>(event, "DressedElectrons").dressedLeptons().size();
  const size_t num_mus = apply<LeptonFinder>(event, "DressedMuons").dressedLeptons().size();

  // Evaluate basic event selection
  const bool pass_emu = num_es == 1 && num_mus == 1;
  if (!pass_emu) vetoEvent;

  // Fill histogram to measure the event acceptance
  _h->fill(13000);
}


void finalize() {
  // Normalize to cross-section
  scale(_h, crossSection() / picobarn / sumOfWeights());
}

private:

BinnedHistoPtr<int> _h;

};

RIVET_DECLARE_PLUGIN(ATLAS_2016_I1468168);

} ```