Rivet Analyses Reference

ATLAS_2020_I1803608

Electroweak Zjj at 13 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1803608
Status: VALIDATED
Authors:
  • Stephen Weber
  • Dag Gillberg
References:Beams: p+ p+
Beam energies: (6500.0, 6500.0) GeV
Run details:
  • pp -> Z [-> ee and mumu] + jets production at 13 TeV

Differential cross-section measurements are presented for the electroweak production of two jets in association with a $ZZ$ boson. These measurements are sensitive to the vector-boson fusion production mechanism and provide a fundamental test of the gauge structure of the Standard Model. The analysis is performed using proton-proton collision data collected by ATLAS at $\sqrt{s} = 13$ TeV and with an integrated luminosity of 139 fb$^{-1}$. The differential cross-sections are measured in the $Z\rightarrow \ell^+\ell^-$ decay channel ($\ell=e,\mu$) as a function of four observables: the dijet invariant mass, the rapidity interval spanned by the two jets, the signed azimuthal angle between the two jets, and the transverse momentum of the dilepton pair. The data are corrected for the effects of detector inefficiency and resolution and are sufficiently precise to distinguish between different state-of-the-art theoretical predictions calculated using Powheg+Pythia8, Herwig7+Vbfnlo and Sherpa 2.2. The differential cross-sections are used to search for anomalous weak-boson self-interactions using a dimension-six effective field theory. The measurement of the signed azimuthal angle between the two jets is found to be particularly sensitive to the interference between the Standard Model and dimension-six scattering amplitudes and provides a direct test of charge-conjugation and parity invariance in the weak-boson self-interactions. Note that the default entry point is for the inclusive Z+2jet selections. For the EW-only measurement use the option TYPE=EW_ONLY. In both cases, electron and muon channels are to be summed.

Source code: ATLAS_2020_I1803608.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/PromptFinalState.hh"
#include "Rivet/Projections/DressedLeptons.hh"
#include "Rivet/Projections/FastJets.hh"

namespace Rivet {
  

  /// VBFZ in pp at 13 TeV
  class ATLAS_2020_I1803608 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(ATLAS_2020_I1803608);


    /// @name Analysis methods
    /// @{

    /// Book histograms and initialise projections before the run
    void init() {
      FinalState fs(Cuts::abseta < 5.0);

      PromptFinalState photons(Cuts::abspid == PID::PHOTON);
      PromptFinalState electrons(Cuts::abspid == PID::ELECTRON);
      PromptFinalState muons(Cuts::abspid == PID::MUON);

      Cut cuts_el = (Cuts::pT > 25*GeV) && ( Cuts::abseta < 1.37 || (Cuts::abseta > 1.52 && Cuts::abseta < 2.47) );
      Cut cuts_mu = (Cuts::pT > 25*GeV) && (Cuts::abseta < 2.4);

      DressedLeptons dressed_electrons(photons, electrons, 0.1, cuts_el);
      declare(dressed_electrons, "DressedElectrons");

      DressedLeptons dressed_muons(photons, muons, 0.1, cuts_mu);
      declare(dressed_muons, "DressedMuons");

      FastJets jets(fs, FastJets::ANTIKT, 0.4, JetAlg::Muons::NONE, JetAlg::Invisibles::NONE);
      declare(jets, "Jets");

      _doControl = bool(getOption("TYPE") != "EW_ONLY");
      if (_doControl) {
        initialisePlots(SRplots, "SR");
        initialisePlots(CRAplots, "CRA");
        initialisePlots(CRBplots, "CRB");
        initialisePlots(CRCplots, "CRC");
      } else {
        initialisePlots(SRplots, "EW");
      }
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {

      // Access fiducial electrons and muons
      const Particle *l1 = nullptr, *l2 = nullptr;
      Particles muons = apply<DressedLeptons>(event, "DressedMuons").particles();
      Particles elecs = apply<DressedLeptons>(event, "DressedElectrons").particles();

      // Dilepton selection 1: =2 leptons of the same kind
      if (muons.size()+elecs.size() != 2) vetoEvent;
      if      (muons.size()==2) { l1=&muons[0]; l2=&muons[1]; }
      else if (elecs.size()==2) { l1=&elecs[0]; l2=&elecs[1]; }
      else vetoEvent;

      // Dilepton selection 2: oppostie-charge and in mass range
      if ( !oppCharge(*l1, *l2) )  vetoEvent;
      if ( !inRange((l1->mom()+l2->mom()).mass()/GeV, 81.0, 101.0) ) vetoEvent;

      // Electron-jet overlap removal (note: muons are not included in jet finding)
      // make sure jets do not overlap with an electron within DR<0.2
      Jets jets;
      for (const Jet& j : apply<FastJets>(event, "Jets").jetsByPt(Cuts::pT > 25*GeV && Cuts::absrap < 4.4)) {
        if (elecs.size() == 2 && (deltaR(j, *l1, RAPIDITY) < 0.2 || deltaR(j, *l2, RAPIDITY) < 0.2 )) {
          continue;
        }
        jets += j;
      }

      // Require 2 jets with pT > 85 and 80 GeV
      if (jets.size() < 2) vetoEvent;

      // Calculate the observables
      Variables vars(jets, l1, l2);

      // make sure neither lepton overlaps with a jet within 0.4
      for (const Jet& j : jets) {
        if (deltaR(j, *l1, RAPIDITY) < 0.4 || deltaR(j, *l2, RAPIDITY) < 0.4)  vetoEvent;
      }

      if (jets[0].pt() < 85*GeV || jets[1].pt() < 80*GeV ) vetoEvent;

      bool pass_VBFZtopo = (vars.mjj > 250*GeV && vars.Dyjj > 2.0 && vars.pTll > 20*GeV && vars.Zcent < 1.0 && vars.pTbal < 0.15);

      if (pass_VBFZtopo) {
        if      (_doControl && vars.Ngj  > 0 && vars.Zcent <  0.5) fillPlots(vars, CRAplots);
        else if (_doControl && vars.Ngj  > 0 && vars.Zcent >= 0.5) fillPlots(vars, CRBplots);
        else if (_doControl && vars.Ngj == 0 && vars.Zcent >= 0.5) fillPlots(vars, CRCplots);
        else {
          fillPlots(vars, SRplots);
        }
      }
    }


    void finalize() {
      const double xsec = crossSectionPerEvent()/femtobarn;
      scalePlots(SRplots, xsec);
      scalePlots(CRAplots, xsec);
      scalePlots(CRBplots, xsec);
      scalePlots(CRCplots, xsec);
    }

    /// @}


    /// @name Analysis helpers
    /// @{

    struct Variables {
      Variables(const vector<Jet>& jets, const Particle* l1, const Particle* l2) {
        // get the jets
        assert(jets.size()>=2);
        FourMomentum j1 = jets[0].mom(), j2 = jets[1].mom();
        pTj1 = j1.pT()/GeV; pTj2 = j2.pT()/GeV;
        assert(pTj1 >= pTj2);
        
        // build dilepton system
        FourMomentum ll = (l1->mom() + l2->mom());
        pTll = ll.pT(); mll = ll.mass();
        
        Nj = jets.size();
        Dyjj = std::abs(j1.rap() - j2.rap());
        mjj = (j1 + j2).mass();
        Dphijj = ( j1.rap() > j2.rap() ) ? mapAngleMPiToPi(j1.phi() - j2.phi()) : mapAngleMPiToPi(j2.phi() - j1.phi());
        
        Jets gjets = getGapJets(jets);
        Ngj = gjets.size();
        pTgj = Ngj? gjets[0].pT()/GeV : 0;
        
        FourMomentum vecSum = (j1 + j2 + l1->mom() + l2->mom());
        double HT = j1.pT() + j2.pT() + l1->pT() + l2->pT();
        if (Ngj) { 
          vecSum += gjets[0].mom(); 
          HT += pTgj;
        }
        pTbal = vecSum.pT() / HT;
        
        Zcent = std::abs(ll.rap() - (j1.rap() + j2.rap())/2) / Dyjj;
      }
      
      double Zcent, pTj1, pTj2, pTgj, pTll, mll, Dyjj, mjj, Dphijj, pTbal;
      size_t Nj, Ngj;

      Jets getGapJets(const Jets& jets) {
        Jets gjets;
        if (jets.size() <= 2)  return gjets;
        FourMomentum j1 = jets[0].mom(), j2 = jets[1].mom();
        double yFwd = j1.rap(), yBwd = j2.rap();
        if (yFwd < yBwd) std::swap(yFwd,yBwd);
        for (size_t i = 2; i < jets.size(); ++i)
         if (inRange(jets[i].rap(), yBwd, yFwd)) gjets += jets[i];
        return gjets;
      }

    }; // struct variables


    struct Plots {
      string label;
      Histo1DPtr m_jj, Dphi_jj, Dy_jj, pT_ll;
    };


    void initialisePlots(Plots& plots, const string& phase_space) {
      plots.label   = phase_space;
      size_t region = 0;
      if (phase_space == "SR")   region = 4;
      if (phase_space == "CRA")  region = 8;
      if (phase_space == "CRB")  region = 12;
      if (phase_space == "CRC")  region = 16;
      book(plots.m_jj,    1 + region, 1, 1);
      book(plots.Dy_jj,   2 + region, 1, 1);
      book(plots.pT_ll,   3 + region, 1, 1);
      book(plots.Dphi_jj, 4 + region, 1, 1);
    }


    void fillPlots(const Variables& vars, Plots& plots) {
      // The mjj plot extends down to 250 GeV
      plots.m_jj->fill(vars.mjj/GeV);
      if (vars.mjj > 1000*GeV) {
        plots.Dy_jj->fill(vars.Dyjj);
        plots.Dphi_jj->fill(vars.Dphijj);
        plots.pT_ll->fill(vars.pTll/GeV);
      }
    }


    void scalePlots(Plots& plots, const double xsec) {
      scale(plots.m_jj, xsec);
      scale(plots.Dy_jj, xsec);
      scale(plots.Dphi_jj, xsec);
      scale(plots.pT_ll, xsec);
    }

    /// @}


    private:

      Plots SRplots, CRAplots, CRBplots, CRCplots;
      bool _doControl;

  };


  
  RIVET_DECLARE_PLUGIN(ATLAS_2020_I1803608);

}

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